CAPTURING ADDITIONAL WATER IN THE TUCSON AREA
By The Rillito Creek Hydrologic Research Committee of The University of Arizona and The U. S. Geological Survey
June 1959
Open-file Report CONTENTS Page Abstract...... 1 Introduction...... 3 Metropolitan development...... 3 Physical characteristics of Tucson basin...... 4 Surficial water supplies...... 8 Precipitation...... 8 Runoff...... 12 Sediment content of floodwaters...... 21 Quality of floodwaters...... 23 Soils...... 23 Evaporation...... 24 Vegetation...... 28 Planned control of vegetation...... 31 Areas of phreatophytes...... 31 Natural recharge of ground water...... 31 Subsurface water supplies...... 34 Geology...... 34 Rock units...... 34 Crystalline complex...... 34 Pantano beds...... 34 Alluvial deposits of the Tucson basin...... 35 Alluvial fans...... 37 Inner-valley fill...... 37 Structure...... 37 Physiography...... 38 Ground water...... 39 Occurrence and movement...... 39 Quality of water...... 43 Storage and yield...... 46 Volumetric analyses...... 48 Subsurface storage of water by artificial means...... 50 Quality of water...... 51 Physical properties...... 51 Chemical properties...... 51 Microbial activity...... 52 Location of storage areas...... 53 Subsurface distribution and ultimate recovery of water in storage...... 54 Summary...... a 55 Investigations essential to the capture of additional water in the Tucson area...... 55 Committee...... 56 References cited...... 58
ii ILLUSTRATIONS Page Figure 1. Flood on Rillito Creek, August 3, 1955...... 6 2. Block diagram of the Tucson basin area...... in pocket 3. Geologic map of the Tucson basin area...... in pocket 4. Mean monthly precipitation in the Tucson area and at Mount Lemmon...... 9 5. Rain gages in the Tucson basin...... 11 6. Streamflow depletion between Sabino Creek and Rillito Creek at Oracle Road bridge...... 18 7. Surface inflow and outflow of Rillito Creek drainage...... 20 8. Annual floods of Rillito Creek near Tucson...... 22 9. Sediment rating curve, San Pedro River at Charleston...... 25 10. Index map of soil surveys, Tucson basin...... 26 11. Relation of evaporation to temperature, Tucson, 1928-58...... 29 12. Precipitation and potential evapotranspiration at Tucson and Mount Lemmon...... 30 13. Areas of phreatophytes with shallow water table along Rillito Creek...... 32 14. Conglomerate in Pantano beds...... 36 15. Ground-water contour map of Tucson basin, spring * of 1959...... 40 16. Hydrograph of water levels in two representative wells 1916-59...... 44 17. Quality of ground waters Rillito Creek project area...... 47 18. Ground-water lowering in Tucson basin 1947-59...... 49
iii TABLES Page Table 1. Cooperative observers, U. S. Weather Bureau...... 12 2. Monthly and annual discharge, in acre-feet, of Rillito Creek near Tucson...... 14 3. Summary of streamflow records available in Rillito Creek drainage...... 16 4. Tributaries and their estimated inflow into Rillito Creek drainage...... 19 5. Vegetation in the lower Rillito drainage area...... 28 6. Chemical analyses of ground water in the Tucson basin area...... 43
iv CAPTURING ADDITIONAL WATER IN THE TUCSON AREA
By The Rillito Creek Hydrologic Research Committee of the University of Arizona and U. S. Geological Survey
ABSTRACT
This report represents a pre crystalline complex is one of the liminary study on the possibilities principal problems requiring inves of increasing available water sup tigation . plies within the Tucson basin to meet the anticipated water demand Until disturbed by man, the a- resulting from the rapid increase in mount of water in storage in the population. The study was made by sediments of the Tucson basin re the University of Arizona and the mained almost constant. In recent U. S. Geological Survey. years, however, water has been with drawn from the basin much faster The Tucson basin is a depressed than it has been replenished by structural block between the sur rainfall and runoff; consequently, rounding mountain masses. The im static water levels have declined as permeable Pantano beds and the crys much as 35 feet, and net ground- talline complex compose the mountain water storage loss has been estimat masses and form the margin and floor ed as 250,000 acre-feet during the of the ground-water basin. The sedi past 12 years. It is apparent that ments which constitute the actual ground-water supplies will become ground-water reservoir are of three depleted unless measures are taken principal types: (l) alluvial depos to replenish the amount in storage. its of the Tucson basin,which under lie most of the broad,virtually flat The runoff potential in Rillito floor of the valley; (2) the inner- Creek is equal to 80 percent of the valley fill, which underlies the amount of water used in the greater flood plains of the major washes; Tucson area. Much of this incoming and (3) the alluvial-fan deposits water is lost before it can be used, along the mountain fronts. The as the potential evaporation is thickness and general configuration about nine times the annual precipi of the inner-valley fill and the fan tation, and vegetation along the deposits are fairly well known; how stream channels uses an estimated ever, the thickness of the deposits 2,500 acre-feet per year. Moreover, of the Tucson basin, the main source runoff leaving the Rillito Creek of ground water, is not certain, and basin averages about 12,000 acre- their relationship to the adjacent feet per year. and underlying Pantano beds and It is believed that additional of salient features of the Tucson water, which is now lost by evapora basin indicates the need for further tion or outflow from the basin via studies along several lines the Rillito Creek during the rainy sea pattern of precipitation throughout son, could be captured. Salvage of the basin, amounts and distribution this water would ease the pressure of runoff, quality of both surface on the ground-water reserves,through and ground water, amount of water transfer into the distribution sys lost by evaporation and transpira tem and recharge into the subsur tion, amount of ground water in stor face, where the ground-water reser age and its movement within the ba voir has been partially depleted. sin, and the feasibility of artifi cial recharge of the ground-water A review of present knowledge reservoir. INTRODUCTION
The problem of the availability water or soil surfaces or from foli of water to meet the future demand age of vegetation. Virtually all of the greater Tucson area motivated water that falls as precipitation the University of Arizona and the eventually returns to the atmosphere U. S. Geological Survey to initiate as vapor. preliminary studies on the possibil ities of increasing available water From this continuous circula supplies. The basic purpose of this tion in the hydrologic system, man report is to determine the feasibil obtains water for his needs in agri ity of detailed investigations of culture, industry, and domestic use. methods for capturing additional wa On a continuing basis, water cannot ter in the Tucson area to supplement be withdrawn from this hydrologic municipal supplies. As metropolitan system at a rate that exceeds 're Tucson is the fastest growing commu plenishment from rainfall and run nity in the Southwest, owing to its off. Withdrawals at greater rates climatic appeal and commodious liv can be made only at the expense of ing, the population in the next 10 depleting the amount in ground-water or 15 years may be expected to in or soil-water storage. crease to more than half a million persons; however, the magnitude of These conditions may be ex such growth and expansion will de pressed in common business words. pend on the availability of adequate The water in ground-water and soil- water. Preliminary appraisal indi water storage is the basin's capital cates that it will be necessary to assetj precipitation is the gross capture additional water within the water income. Interception, evapo Tucson basin or import it from else ration, and transpiration are na where. As there are many unknown ture's water income tax. Thus, the factors relating to the practicabil net water income is runoff, plus ity of capturing additional water, that amount that can be recovered or it is necessary to make a quantita salvaged from the portion that con tive appraisal of the pertinent com stitutes taxes. The basin's water ponents of the hydrologic system in assets cannot remain "in the black" the Tucson area. if these assets are depleted and there is no restoration. In order Water in the hydrologic system to establish a business account on moves in an ever-continuing cycle. the hydrologic system, man must be Its circulation speeds up and slows informed fully on all its compo down repeatedly and may vary from nents. Such information can be year to year, but over the years the gained from intelligent and unprej system or cycle remains in approxi udiced research, statistical water mate balance. In effect, no water records on income and outgo, and a- is added to or lost from the hydro- nalysis of these factors over a pe logic system by natural processes in riod of years. Only when such a a given region. Water is precipi commonsense approach is used will it tated from atmospheric vapor as rain be possible for man not to be taken or snow. Part becomes surface run unaware by a serious water shortage. off, from which a portion is stored in the soil or in ground-water res Metropolitan Development ervoirs for varying periods of time. A large part returns to the atmos Early inhabitants in Tucson phere as vapor, evaporating from settled along the Santa Cruz River where there were small amounts of governing factor in the ultimate surface flow or where water could growth of the greater Tucson area. be obtained from shallow wells. As Thus it is necessary to know the the community grew, the water demand rate at which water can be withdrawn was met by the development of addi from storage and whether additional tional wells along the Santa Cruz water can be captured for beneficial River and in the area between the use. Santa Cruz River and Rillito Creek. After World War II the community ex On the basis of the assumption perienced rapid growth; the popula that it is feasible to capture addi tion increased sixfold in about 15 tional water which is otherwise lost years. Business and construction to the atmosphere, the problem re investments presently run into hun duces to the question, "How could dreds of millions of dollars. The this captured additional water be maintenance of these values and stored or used?" There are several the creation of additional wealth ways of utilizing the water after it through investment in line with the is captured: (l) direct transfer of projected increases in population water into the distribution system; require an adequate water supply. (2) recharge of water into the sub surface by natural infiltration and Moreover, although it is ex percolation into the ground-water pected that irrigated acreage in the reservoir; and (3) storing of water valley will decrease from the pres artificially in areas where the ent level, it will be replaced by ground-water reservoir has been par urban development and new industry tially depleted. which is being attracted to the area by the favorable climate and an ade Arid lands are characterized by quate labor supply. The present in a shortage of water and if man wish dustry, largely concentrated in elec es to occupy arid lands, such as the tronics and aircraft, requires mod Tucson area, his survival depends erate amounts of water for sanita upon adequate water supplies. Even tion, air conditioning, and land more important, there is a moisture scaping. Heavy industry would in deficiency as the evaporation poten crease the water demands even fur tial is about nine times the annual ther. precipitation. Almost every drop of water exposed to solar radiation is Metropolitan supplies presently quickly changed to vapor. Conse are obtained from ground-water re quently, man has several choices serves stored in the sediments un when his water demand exceeds the derlying the area. Undoubtedly there natural perennial replenishment. He are large amounts of water in stor can (l) transport water into the age, but the rate of withdrawal far area; (2) capture additional liquid exceeds annual replenishment. It is water from the hydrologic system; or quite apparent that this vast stor (3) move to areas that have ample age reservoir will become depleted water supplies. unless measures are taken to replen ish the storage. The rapid decline Physical Characteristics of in water levels after World War II Tucson Basin has been documented by Schwalen and Shaw (1957). This trend not only Rillito Creek and its tribu will continue but will be of even taries drain the northern and east greater magnitude. The amount of ern parts of the Tucson basin and water that is available perennially the adjacent Santa Catalina, Tanque for municipal demand will be the Verde, and Rincon Mountains. Rillito Creek is tributary to the Santa Cruz head in the fan material beyond the River, joining it above the narrows base of the mountains. Some of where both surface and subsurface these channels are deeply entrenched water leave the Tucson basin. The in the fan material; some may have principal tributary of Rillito Creek become obliterated by agricultural is Pantano Wash, which, with its and urban development where they tributaries, drains the area east of originally crossed the valley floor. the Tucson basin between the Rincon, The lowlands are drained through Santa Rita, and Whetstone Mountains. relatively shallow channels that The total drainage area of Rillito feed the larger tributaries or the Creek is 918 square miles. main stem of Rillito Creek. There seems to be a definite relationship The Tucson basin is an inter- between the size, slope, and other montane trough typical of the arid geometric features of these channels Southwest. Such troughs, although and the amounts of water discharged they may contain a through-flowing by them. These relationships have drainage system, were not carved by not yet been established. streams flowing through them, but represent structural basins between Climate in the Tucson basin is mountain ranges. They are partially typical of that in an arid region. filled with fan, lake, or flood- Summer temperatures frequently ex plain deposits shed from surrounding ceed 105° on the valley floor, and mountajns or brought in from areas winter temperatures seldom drop be upstream. low freezing. The dominant features of the rainfall in the lower alti The valley floor slopes gradu tudes are scantiness and extreme var ally upward, away from the stream iability from one year to the next. channel toward the mountain blocks. At the higher altitudes temperatures These slopes are broken locally by are characteristically lower and shallow steps or terraces. At the precipitation is greater than on the base of the fan material the slopes valley floor. become steeper, and above the base of the mountains they may be pre With one exception, there have cipitous. The downvalley slope of been no unusual, excessively damag the Rillito Creek channel near Tuc ing, floods in Tucson basin in re son is more than 20 feet per mile, cent years. With the passage of becoming greater at the higher alti years, memories of past events be tudes . come hazy and people tend to assume that because floods have not oc The stream channel near the curred recently they cannot happen. confluence of Rillito Creek with the Areas along the stream channels that Santa Cruz River is about 2,300 feet have been flooded in past years be above msl (mean sea level). The gin to appear desirable for residen summits of the Santa Catalina, Rin tial construction and pressures con, and Santa Rita Mountains extend build up for residential zoning of more than 9,000 feet above msl, and these areas. Failure to resist these about 220 square miles of the drain pressures is an invitation to dis age area is more than 5,000 feet aster. Are things bad already? They above msl. are in Albuquerque where people build on arroyos. The mountain slopes are drained through a series of ravines and can Figure 1 shows Rillito Creek at yons, some of which discharge water the Campbell Avenue crossing in Tuc most of the year. Other channels son on August 3, 1955. The discharge Figure 1.--Flood on Rillito Creek, August 3, 1955. at the time the picture was taken They, together with the Pantano was 6,000 cfs (cubic feet per sec beds, form the margins, and at some ond). For Rillito Creek, this means unknown depth, the floor of the Tuc that a flood of this magnitude or son ground-water basin (fig. 2). larger will recur on the average once every 2,33 years. It should be The Pantano beds are a sequence noted that this flood is almost bank of conglomerate, siltstone, sand full and that any significant in stone, and claystone, originally de crease in flow would cause the posited as basin-fill material. That stream to spill over its banks. it was not deposited in the surfi- cial basin is shown by the fact that During 50 years of record, it is tilted and broken by faults, floods have occurred with peak dis and that beds believed to be correl charge of four times the amount of ative with it form part of the actu water shown in the photo. The excep al mountain masses, as along the top tion noted previously was a flood of of Redington Pass. The Pantano beds about 40,000 cfs that occurred on are tightly cemented and of low per Pantano Wash near Vail. The peak of meability, a pertinent fact in ap this flood was reduced greatly be praising the water potential for fore it reached Rillito Creek, but metropolitan Tucson. there seems to be no logical reason why a flood of equal size or larger The sediments in the basin con should not have occurred in Rillito stitute the actual ground-water res Creek near Tucson. ervoir and are of three principal types, as follows: (l) deposits of The mountains surrounding the the Tucson basin, which underlie Tucson basin are composed of rocks most of the broad, virtually flat of several types and have a complex floor of the trough between the sur history. The Catalina-Tanque Verde- rounding mountains; (2) inner-valley Rincon Mountains mass is largely a fill, which underlies the present metamorphic complex of gneiss, but channels and flood plains of the sedimentary, igneous, and several Santa Cruz River and Rillito Creek other types of metamorphic rocks are and their tributaries; and (3) al exposed on the north, east,and south luvial-fan deposits along the moun sides of the mountains, and in a few tain fronts (fig. 3). places within the Rillito drainage area itself. The mountainous area The thickness and general con drained by Pantano Wash consists of figuration of the inner-valley fill various types of igneous, sedimen and the alluvial fans are fairly tary, and metamorphic rocks. The well known,as they can be determined Santa Cruz River on the west side of by observation and from the records the Tucson basin flows near the base of shallow wells. The thickness of of the Tucson Mountains, which are the deposits of the Tucson basin, composed largely of volcanic rocks. however, is not certain, and their The basal rocks may be thought of as nature and relation to the adjacent a crystalline complex, of some im and underlying crystalline complex portance in the present study be and Pantano beds form one of the cause they are of low permeability principal problems requiring inves and yield little or no ground water. tigation in the present study. SURFICIAL WATER SUPPLIES
Precipitation As in all arid or semiarid re is associated with very warm, moist, gions of the world, the dominant and unstable air which has swept a- features of the rainfall in most of round the southern margins of the Arizona are scantiness and extreme Atlantic Ocean high-pressure cell variability from one year to the and advanced into Arizona from the next. On the desert floor in the Gulf of Mexico. This air, in pass vicinity of Tucson, at a mean alti ing over the strongly heated land tude of about 2,500 feet, the aver masses, is made even more unstable, age precipitation is only about 10 and when it is forced to ascend over inches a year. Forty percent of the numerous mountain ranges of this occurs in the two months of southern Arizona copious showers re July and August (fig. 4), whereas a sult. These showers have a very large part of the remaining 60 per marked diurnal variation, being most cent falls as heavy showers scat intense over the mountains during tered almost randomly through the the midafternoon when surface heat rest of the year. ing and the general convergence of air associated with the upslope Although the same rainfall re mountain winds are at a maximum. In gime is present at higher altitudes, the valleys the heaviest summer amounts are characteristically rains usually do not occur until the greater. For example, the Mount Lem- late afternoon or early evening, at mon rain gage, at slightly over which time the desert floor is con 7,500 feet, receives an average of siderably warmer than the surround about 30 inches of precipitation ing cloud-covered mountains. each year -- almost exactly three times that falling on the desert Not all the warm-season rain floor. However, monthly amounts fall is the result of simple convec- (fig. 4) are extremely variable from tive activity of the type described one year to the next, perhaps even above. A small, but important, part more so than at lower altitudes. is associated with tropical disturb On Mount Lemmon 9.55 inches of pre ances which form off the west coast cipitation was recorded in March of Mexico at about 15°N. latitude. 1954, but the very next year, in These storms usually dissipate as March 1955, none at all fell. The they move northward into middle lat same pattern holds for almost every itudes, but they are normally still other month for example, August intense and extensive enough when rainfall has ranged from 0.55 inch they reach the 30th parallel to pro (1958) to 11.71 inches (1955). Only duce heavy rainfall in southern Ari in May, which is normally dry, and zona. This type of rainfall differs July, which is normally wet, does from the normal convective type in the average really have much mean several respects. It is more wide ing. spread, has a lesser intensity but longer duration, and is only rarely Most of the rainfall in south associated with thunder and light ern Arizona can be attributed to one ning. Some of the heaviest rain of four sources, depending mainly on falls on record,particularly in Sep the season of the year. A large per tember, are associated with these centage of the summer thundershowers tropical disturbances. 7-
6-
Mt. Lemmon 5-
OD 3 a0 4- 1
o 3- o
2-
1-
Tucson area
F M * I i 6 Month
Figure 4. Mean monthly precipitation in the Tucson area and at Mount Lemmon 10
Winter, or cool-season, rains ment of the Atlantic high-pressure are generally less intense but more cell and its attendant moist un widespread than those of summer. stable airmass. On the other hand, They also show a smaller variation winter precipitation is encouraged with ground elevation, sometimes be by a southward displacement of the ing heavier on the desert floor than middle-latitude westerlies, in which in the mountains. Part of this pre at that time of year are well- cipitation is associated with the developed cyclonic storms. middle-latitude stormbelt, which oc casionally moves far enough toward The rain-gage network in the the equator in winter for its south area for which long-term published ern margins to affect Arizona. It records exist is that of Cooperative is only when these cyclonic storms Observers for the U. S. Weather Bu move in directly from the Pacific reau. The rainfall data from these Ocean across the northern and cen observers are published in "Clima- tral parts of the country that meas tological Data, Arizona" and "Hourly urable amounts of rain can occur. Precipitation Data, Arizona," month When the path of the storm is more ly publications of the U. S. Weather nearly north to south, east of the Bureau. Past records of precipita 105th meridian,about all that south tion were published in Bulletin W, ern Arizona can expect is plenty of "Climatic Summary of the United wind and subnormal temperatures. States, Southern Arizona," which tabulated precipitation data to Probably the heaviest rains of 1930, and the "Supplement to Bulle winter are associated with the so- tin W, Arizona," which contains the called "Kona" storms or "cold lows" data for 1931 through 1952. of the subtropical Pacific Ocean. These intense disturbances form in Table 1 lists the U. S. Weather the vicinity of the Hawaiian Islands Bureau Cooperative Observer sta and move very slowly eastward to the tions, which are indicated on the coast of southern California. In map by a four-number designation. this region or slightly Inland they Those stations listed under Hourly often remain stationary for several Precipitation Data have recording days. But once they get caught in gages, and hourly values of rainfall the strong upper-level westerlies, are published as noted above. The they move rapidly northeastward a- record of one station, University of cross the United States. As these Arizona, Tucson, has been put on IBM storms normally pass directly over punch cards and is available from Arizona, frequently advancing very the Institute of Atmospheric Physics slowly, and as they retain most of (IAP in map explanation). their moisture supply while moving In from the Pacific, they can pro There is an excellent small- duce several days of moderate to scale network in the area which can heavy precipitation, which is often be denoted as the Atterbury Reser accompanied by lightning and thun voir Drainage Area network (fig. 5). der. This network is operated by the De partment of Agricultural Engineer In conclusion,it might be stat ing of the University of Arizona. ed that, as a general rule, the at The area covered, indicated on fig mospheric conditions most conducive ure 3, is between the Benson Highway to summer precipitation in Arizona and Pantano Wash, northwest of Vail are a northward displacement of the and southeast of Davis-Monthan Air upper-level middle-latitude westerly Force Base. It covers about 18 wind belt and a westward displace square miles. FINAL CO. N. R.IIE CO.
R- I6E-. RILLITO STATION Soldier's Comp
,)£ SAGUARO - NATIONAL
USWB Cooperative Observers USWB Recording Gages IAP Cooperative Observers ARS Cooperative Observers 6 MILES Atterbury Reservoir Gages (recording gages) Atterbury Reservoir Watershed
Figure 5. --Rain gages in the Tucson basin. 12
Table 1.--Cooperative Observer, U. S. Weather Bureau
Period of record*** Number Name (as of 11/58) 2159 *Cortaro, 3 SW 1945 to present 5732 Mount Lemmon Inn 10/58 to present Mount Lemmon Summit 1957 to 1958 Mount Lemmon 1950 to 1957 5908 *N-Lazy-H Ranch 1941 to present 7355 *Sabino Canyon 1941 to present 7403 Sahuarita, 2 NW 1956 to present 8796 Tucson Campbell Expt. Farm 1949 to present 8800 *Tucson Magnetic Observatory 1912-16; 1934 to present 8805 *Tucson Mountain Park 1948 to 2/56 8815 **Tucson, U. of Arizona 1867 to present 8820 Tucson, W. B., Airport 1940 to present Hourly Precipitation Data 8810 Tucson Nursery 1948 to present 8820 Tucson, W. B., Airport 1940 to present
* Records in Supplement, Bulletin W ** Records in Bulletin W and Supplement *** Occasional short breaks may exist in these periods This network was established by In general, reports are received the Department of Agricultural En each month from these Cooperative gineering to study the relationship Observers, and are kept at the In between rainfall and runoff. Data stitute. Most of these observers have been collected since 1956 and use a small plastic wedge-shaped it is expected that the network will gage. The records, extending for remain in operation. It consists of about 3-| years, are fairly complete, 30 standard rain gages, of which 3 and the observation can be classed are recording. Plans exist for the as fair to good. These data have placement of additional Larsen-type not been analyzed or tabulated in rain gages in selected small areas. any routine manner. The gages are regularly maintained, and rainfall is measured after each Another group on campus col storm. Complete records for the pe lecting rainfall data from individ riod of operation are kept by the uals is the Agricultural Research Department of Agricultural Engineer Service of the U. S. Department of ing. These data have been analyzed Agriculture. Four gages in the area and studied intensively and are in of interest are designated on fig usable form. ure 5. The Institute of Atmospheric Runoff Physics has been collecting rainfall records from private individuals in The longest continuous record the general area of interest. These of streamflow in the Tucson basin is stations are designated on figure 5. that of Rillito Creek near Tucson. 13
The gaging station was established Bourke (1891) called a "sand wash." in 1908 by the Agricultural Engi Cottonwood, alder, and sycamore grew neering Department, University of along the watercourse; large mes- Arizona. In January 1926, operation quites inhabited the bottoms. On of the station was assumed by the the mesas above the creek small U. S. Geological Survey with cooper stunted mesquite,sage brush, cactus, ative financing by the University and "excellent grama and sacatone and later by the State Land Depart (sic) grasses" prevailed. ment. The records from this gaging station show the streamflow in Ril- The Rillito's largest tribu lito Creek,and the extreme variabil tary, Pantano Wash, evidently used ity is seen in the tabulation of to be dry along most of its course. monthly discharge (table 2). Two marshes, probably perennial, possibly seasonal, existed along the The average discharge of Rilli- middle reaches. One, below the en to Creek near Tucson is 12,330 acre- trance of Davidson Canyon, occurred feet per year for the 50 years of in conjunction with a spring which record. This average has been sharp one traveler described as flowing "a ly raised by a few wet years,partic hogshead per minute." The other ularly 1915 and 1916. Although marsh, a favorite stopping place, factual records are not available was located about where the Pantano prior to 1908, it is important to station of the Southern Pacific understand general conditions of Railroad now stands. this earlier period. Much informa tion is available from historical Various travelers commented on documents, and the following account the tall sacaton grass around the has been prepared from some of cienegas. As late as 1887 a propos these. al was made to tap the surplus water by means of a ditch and convey it to The changes in ecology and hy Tucson for irrigation use. On the drology experienced during the uplands, as well as the bottomlands 1880 f s by the drainage basin of Ril- along Pantano, mesquite seems to lito Creek parallel rather closely have existed well before 1880. those taking place at the same time throughout most of the rest of According to customary usage, southeastern Arizona. Rillito Creek "Pantano Wash gives way to "Cien- used to flow in "an insignificant ega Creek" above the Junction of bed" through a "pretty and well- the main stream with Mescal Arroyo. cultivated little valley" past the "Cienega Creek" prior to 1880 flowed site of New Port Lowell. Rothrock for most of its length above the (1875) states that the creek sup point where its valley widens, some plied enough water both for the use 7 miles south of Mescal Creek. Then, of the post and for the irrigation as now, the quantities of native of some small fields. Drinking water grasses made the Empire Valley a came from wells, which in 1875 "stockman's paradise." The tradi struck water only 25 to 33 feet be tion persists locally that there has low the surface. been a marked mesquite invasion in the valley in recent years. Histor West of the Port, sometimes a ical documentation is too inade mile away, sometimes at its Junction quate either to refute or to support with the "dry bed of the Santa the contention. Cruz," the Rillito ceased to flow, its bed became enlarged, and it took Even under the conditions that on the characteristics of what existed prior to 1880 flooding must Table 2. Monthly and annual discharge, in acre-feet, of Rillito Creek near Tucson, Ariz,
Water The year Oct. Nov. Dec. Jan. Fet>. Mar. Apr. May June July Aug. Sept. year 1909 0 0 5,760 168 204 128 0 0 0 6,000 10,220 5,520 28,000 1910 0 0 0 110 0 0 0 0 100 1,100 3,150 150 4,610 1911 0 0 0 2,020 0 300 0 0 0 1,650 4,290 3,030 11,290 1912 0 0 0 0 0 3,740 25 0 0 3,000 5,000 0 11,760 1913 300 0 0 0 420 650 0 0 0 50 200 30 1,650 1914 0 0 0 1.2 821 0 0 0 12 2,470 2,910 2,590 8,800 1915 500 1,360 60,000 21,370 25,450 10,090 1,210 4,240 0 5.0 0 0 120,000 1916 0 0 0 37,060 2,200 3,600 58 0 0 910 7,770 686 52,280 1917 28 0 0 900 272 7.9 0 0 0 5,070 2,850 638 9,770 1918 0 0 0 0 6.9 7,760 0 1,000 483 144 6.7 0 9,4oo 1919 38 7.9 4.0 12 827 329 662 0 0 30,750 4,120 462 37,210 1920 0 2,350 2,750 4,430 11,630 2,280 555 0 0 0 2,000 30 26,020 1921 0 0 0 0 0 0 0 0 0 25,980 16,150 365 42,500 1922 18 0 0 232 0 0 0 0 105 208 1,870 595 3,030 1923 0 0 0 0 0 0 0 0 0 2,510 4,100 61 6,670 1924 0 335 5,160 87 0 2.0 177 0 0 0 0 0 5,760 1925 0 0 0 0 0 0 0 0 0 708 1,510 2,500 4,720 1926 0 2.0 0 0 0 192 5«3 0 0 141 52 974 1,940 1927 32 0 71 194 1,490 1,260 0 0 0 38 198 1,300 4,580 1928 0 0 0 0 0 0 0 0 0 397 837 50 1,280 1929 10 0 0 0 0 0 0 0 40 1,840 6,980 17,950 26,820 1930 0 0 0 0 0 3,350 0 0 153 3,150 3,250 688 10,590 1931 0 0 0 0 5,490 109 0 0 18 32 6,280 125 12,050 1932 79 2,390 1,280 403 5,230 1,660 0 0 0 3,600 192 0 14,830 1933 Ut8 0 0 2.0 228 137 0 0 0 40 127 666 1,650 1934 282 0 0 0 0 0 0 0 0 662 895 260 2,100 1935 0 0 16 1,440 3,560 793 0 0 0 373 7,940 4,150 18,270 1936 0 24 0 258 902 0 0 0 0 272 2,130 16 3,600 1937 0 0 2.0 0 3,040 383 0 0 0 184 770 67 4,450 1938 0 0 0 0 0 1,880 0 0 60 83 470 4.0 2,500 1939 0 0 0 0 0 0 0 0 0 1,740 5,090 48 6,880 1940 1* 2 0 2 263 0 0 0 446 24 7,100 516 8,360 1941 0 161 12,700 2,740 4,430 6,980 0 0 0 159 1,850 645 29,740 1942 0 2.0 734 468 331 248 0 0 0 0 250 139 2,170 1943 0 0 0 0 0 609 0 2 0 0 1,720 270 2,600 1944 0 0 0 0 0 0 12 0 0 734 2,360 83 3,190 1945 28 56 0 0 73 305 0 0 0 450 2,970 6.0 3,890 1946 71 0 0 4.0 0 0 0 0 0 1-22 2,470 69 3,040 1947 135 65 0 0 0 0 0 0 0 2.0 3,770 147 4,120 1948 0 14 0 0 0 0 0 0 0 244 427 274 959 1949 0 0 524 1,290 1.8 7.9 .4 0 0 64 259 770 2,920 1950 0 0 0 0 0 0 0 0 579 6,340 339 0 7,260 1951 0 0 0 0 0 0 0 0 0 2,260 1,880 0 4,140 1952 26k 1,180 528 2,080 0 1,860 115 6.9 0 53 65 0 6,160 1953 0 0 0 0 0 13 0 0 0 1,730 0 0 1,740 1954 0 0 0 0 0 6,420 0 0 37 3,670 1,760 1,150 13,040 1955 0 0 0 0 0 0 0 0 0 3,870 8,430 0 12,300 1956 18 0 0 26 0 0 0 0 0 257 14 0 315 1957 0 0 0 2,760 186 54 0 12 0 45 1,020 137 4,210 1958 282 230 36 0 79 6,580 278 0 0 1,320 2,430 29 11,260 15
have occurred as part of the normal course was indefinite and lin regime of the Tucson basin. Because ed by an almost continuous no well-clefined channel existed,how growth of cottonwood,ash, wal ever, the water evidently spread out nut and willow trees. These in a shallow sheet across the valley conditions continued until floor, doing relatively little dam 1872, when the United States age. Army post was moved from Tuc son to Port Lowell on the Ril The first disastrous recorded lito largely because natural flood came in August 1880, when Pan- grass could be cut for hay. tano Wash destroyed several sections Thereafter the Rillito cut a of the Southern Pacific Railroad wide channel from ten to fif track. In 1885 a similar flood teen feet deep. wrecked 6 miles of track, and the Rillito for a time became unford- The adjectives "unbroken", "in able. In 1887 a flood "fully fif definite", and "almost continuous" teen feet in depth" coursed down Ci- are perhaps questionable. Otherwise, enega Creek, drowning numerous cat the descriptive material can be con tle. Through July and August of firmed at least generally by his that year intermittent flooding con torical evidence. However, any di tinued, culminating September 11, rect relationship as that postulated 1887, in a torrent which destroyed between the relocation of Lowell and bridges across the Rillito and caus the trenching of Rillito Creek must ed water to stand "two miles wide" be strongly questioned. Old Fort in the valley north of Tucson. Lowell, 7 miles away, also had hors es to feed and grass to be cut. The These floods on Rillito Creek move, at best, would have affected and its tributaries, like those on only a few additional of the 918 the Santa Cruz River during the same square miles in the Rillito drain years,seem to have indicated a tran age area. sition between past and present con ditions. The runoff pattern had Even more important, strikingly evidently changed; at the same time, similar processes of change were go river channels had not yet accom ing on at the same time in the more modated themselves to the new loads general areas of the Santa Cruz ba by trenching. The earliest channel sin and, with some modifications, in cutting that can be documented oc the San Pedro basin. Far from being curred on the Rillito immediately merely local, the fundamental deter prior to August 5, 1890. The paral minants of erosion and ecological lel to conditions along the Santa change along the Rillito Creek ap Cruz is striking; the channel trench pear to have been wider perhaps along the latter stream began form even regional in extent. ing August 4, 1890. A summary of all available Kirk Bryan (1925),drawing heav streamflow records in Rillito Creek ily on a study by Smith (1910;, sum is presented in table 3. Average marizes the story as follows: annual runoff at the various sta tions was computed for the periods The valley of Rillito shown to provide comparisons between Creek...was...an unbroken for records for comparable periods. The est of mesquite in 1858, when discharge from 35 square miles of the first settlement was made. Sabino Canyon drainage area is a Between the trees was a good little more than the discharge of growth of grass and the river Rillito Creek from its entire 918 M CD
Table 3. Summary of streamflow records available in Rillito Creek drainage
Drainage Runoff, in acre-feet area Records Avg. 50 yrs. Avg. 26 yrs. Avg. 6 yrs. Total for Gaging station (sq.mi.) available 1908-58_____1952-58 1952 -58____period
Rillito Creek near Tucson 918 Oct. 1908-Sept.l958 12,310 6,180 7,150
Sabino Creek near Tucson 35.5 July 1904-June 1912 July 1932-Sept.l958 6,230 7,530
Sabino Creek near Mt. Lemmon 3.19 May 1951-Sept.1958 941
Rincon Creek near Tucson 44.8 Oct. 1952-Sept.l958 2,950
Atterbury Reservoir near Tucson 18 Jan. 1956-Dec. 1958 807
Rillito Creek near Wrightstown 221 June 1940-Dec. 1946
Pantano Wash near Tucson 602 June 1940-June 1941 6,760 17
square miles. Records from other Creek is 212 acre-feet per square stations within the basin tend to mile. These records cover the ex confirm that the basin potential is tremes of runoff that may be expect greater than shown by the records of ed in the basin. By interpolating Rillito Creek near Tucson. between these unit runoff values and applying values so determined to the Most of this discharge is drainage area of each section, it is floodwater produced by summer cloud possible to estimate the total in bursts causing high flows of short flow to Rillito Creek. As shown in duration. Only a part of it is the table 4, this inflow for the period result of snowmelt from the high al 1952-58 averaged 38,220 acre-feet titudes. Flood peaks resulting from annually. During the same period storms in the upper part of the ba only 7,150 acre-feet discharged past sin may be wholly or partially dis the Rillito Creek near Tucson gaging sipated before they reach the mouth station as surface flow out of the of Rillito Creek. The reduction in basin. The areas listed in table 4 peak discharge is caused by tempo are delineated in figure 7. rary channel storage or by channel retention. Channel storage results It must be emphasized that the from filling of the channel with wa figures of inflow are estimates and ter. It causes a lag of time in the as such are subject to large error. travel of the flood wave, but it This stresses the need for addition does not create a loss of total al gaging-station records to provide flow. Channel retention, which is a more accurate determination of the caused by sponging up of water by basin potential. However, it is cer porous material in the channel,caus tain that the basin potential of es a reduction in total flow. Rillito Creek is considerably great er than is shown by records of out A striking example of reduction flow from the basin. of flood peaks occurred during the flood of August 12, 1958, on Pan- Most of the water in Rillito tano Wash near Vail. The peak of Creek results from heavy storms over 40,000 cfs was reduced to 8,930 cfs the basin. Although these flood- at the Rillito Creek gaging station, waters could supplement the ground- 29 miles downstream. Another example water reserves if they could be cap is illustrated by figure 6. A peak tured and utilized, they also pre of 2,250 cfs and total runoff of sent a definite hazard in their 226 acre-feet passed the Sabino present uncontrolled state. Since Creek gaging station on July 7,1950. 1915, the greatest flood on Rillito The same rise with presumably no Creek near Tucson was 24,000 cfs on tributary inflow reached the Rillito September 23,1929. How this compares Creek gage 5 hours later. The peak with some of the earlier floods of had been reduced to 310 cfs and run 1880, 1885, and 1887 is not known. off for the ensuing 13 hours was on The description of water standing ly 42 acre-feet. "two miles wide" north of Tucson at the culmination of the 1887 flood It is possible to use available indicates that it must have exceeded discharge records to approximate the the 1929 flood considerably. With unit runoff from various parts of out control, such a flood can recur the basin. Records from Atterbury and, with encroachment of the flood Wash show annual runoff from the ba plain by residential development and sin floor as 16 acre-feet per square as a result of deterioration of the mile. Rincon Creek has average an channel, recurrence of a flood of nual runoff of 66 acre-feet per similar magnitude to that of 1887 square mile, while that of Sabino could create a disaster. 18
2,300 -
Explanation 2,000 - Total runoff Sabino Creek 226 acre-feet Rillito Creek ...... 42 acre-feet (Time interval of 5 hours applied)
"g 1,500 o o 0) DO
2L Flood of July 7-8, 1950
§ O
o> 1,000 bo
S
500 -
250 -
0 - I I I I I I 6 7 10 II 12 13 Hours
Figure 6. Streamflow depletion between Sabino Creek and Rillito Creek at Oracle Road bridge. Table 4. Tributaries and their estimated inflow into Rillito Creek drainage
Average runoff Average runoff in acre-ft. of tributaries Tributary Drainage per sq. mi. in acre-ft. area 1952-58 1952-58 (sq.mi.) (estimated) (estimated) S. Slope of Catalina Mts. 60.8 50 3,040 Sabino Creek at gage 35.5 a/212 a/7,530 Bear Canyon at mouth 16.6 200 2,490 Sabino Creek below Bear Canyon 20.0 30 600 Agua Caliente & Tanque Verde W. 140.0 50 7,000 East Bank of Pantano Wash No. 1 22.4 16 360 S. Slope Tanque Verde Ridge 25.4 16 410 Rincon Creek at gage 44.8 a/66 a/2,960 East Bank of Pantano Wash No. 2 32.0 16 510 Agua Verde Creek at mouth 37.9 50 1,900 Tucson Urban 25.0 25 620 Cienega Creek near Vail 418.6 30 10,460 West bank of Pantano No. 3 b/21.0 16 340 Total contribution from tributaries 918.0 38,220
Rillito Creek at gage 918 a/7.8 a/7,150
ay Based on gaging station records. b/ Does not include 18 sq.mi. comprising Atterbury Reservoir drainage area. R.I2E. <^ <--jg.l3E. " ~"~~ ~ ~JQr "* ~ \SJLs. ~ R.I IE. W / *$l / 5
South slope of Catalina Mountains T. 12 S. §r ^;p^^^ Agua Caliente and Basin outflow Tanque Verde Wash
East bank of Pantano Wash
,South slope of Tanque Verde Ridge
East bank of Pantano Wash West bank of Pantano
Agua Verde Creek at mouth
Rincon Creek at gage EXPLANATION Cienega Creek near Vail
Existing gaging station 0 6 MILES Direction of flow I ..... I
Figure 7. --Surface inflow and outflow of Rillito Creek drainage. 21
Through the use of Gumbel plot made that will provide sufficient ting, a device used by many hydrolo- information to permit an appraisal gists, the maximum annual floods of engineering feasibility and prob since 1915 are portrayed in figure able cost of the treatment of flood- 8. The mean annual flood is deter waters for recharge, or to deter mined as 6,000 cfs. According to mine the approximate life of a stor this plotting, a flood of 24,000 cfs age reservoir. can be expected to recur on the av erage of about once every 50 years. The sediment content of flood- waters resulting from the short, Sediment Content of Floodwaters intense summer rainfall on desert areas is known to be relatively The capture of water in arid high. This is particularly true lands requires that knowledge be ob when stream-bank undercutting and tained on the transport of sediment headward erosion of the stream chan by floodwaters. Salvage of present nels occur. Floodflows from Pantano ly wasted floodwaters would involve Wash, the principal tributary of the problem of sediment removal. Rillito Creek, have long been dis Information concerning the sediment tinctive for their dark color and load carried by Rillito Creek and high silt content as compared to the its tributaries is almost completely flow from other tributaries. lacking. However, observation of floodflows reveals that there is Open-bottle samples from three wide variation in the amount and the floods with estimated discharge of physical character of sediment 400 to 1,000 cfs in the Pantano Wash transported. The following factors ranged in silt content from 3.9 to are particularly important in their 5.4 percent, and averaged 4.2 per effect upon the sediment load: (l) cent (j. E. Fletcher, 1959, oral geologic and textural character of communication). The estimated max the soil surface in the source area; imum velocity was slightly more than (2) amount and type of vegetative 9 feet per second. Samples col cover in the source area; (3) physi lected in a similar manner from ographic and topographic character floodflows in the Santa Cruz River of the source area; (4) stream chan had sediment contents of as much as nel condition and character of 4 percent. In comparison with the streambed material; (5) storm char- relatively high sediment content in act eristics ---in tensity, duration, these two streams, the following re and areal extent; and (6) discharge sults were obtained from samples peaks and variation in flow as shown collected from a floodflow eminating by the shape of flood hydrographs. from Sabino Canyon on March 22, 1958: It is important that studies be
Locat ion of sampling Percent of sediment Sabino Read Bridge on Rillito Creek 0.0004 Dodge Blvd. Bridge on Rillito Creek 0.023 Oracle Road Bridge on Rillito Creek 0.084 Cortaro Road Bridge over Santa Cruz River 0.100 CO CO
Drainage area 918 square miles. Period 1915-58
20,000
-o d 0 o
& 15,000 a
§ o a £ 10,000 bo
5,000
1.01 1.1 1.3 1.5 4 6 8 10 20 30 40 50 100 200 Recurrence interval, in years
Figure 8. --Annual floods of Rillito Creek near Tucson, Ariz. 23
The samples indicate that as sources, 1957). the flow, which amounted to about 750 cfs, issued from the canyon Quality of Floodwaters mouth the silt content was negli gible but that it progressively in Although floodwaters from moun creased as a sediment load was pick tain canyons have not been analyzed ed up from the stream channel of chemically, it is certain that those Rlllito Creek and the Santa Cruz in granitic or gneissic areas have River. an extremely low content of soluble salts and should be classed as soft A comparatively large number of waters. open-bottle silt samples have been collected from the San Pedro River Floodwaters from rainfall on at Charleston over a period of years the valley slopes within the Tucson by the U. S. Geological Survey for basin, and those entering the area the Agricultural Engineering Depart in Pantano Wash, may be expected to ment, University of Arizona. A cor have a salt content of several hun relation of sediment content with dred parts per million. No chemical discharge is shown in figure 9. The analyses of these floodwaters are points show rather wide variation available, but the waters may be from the curve but represent aver comparable in quality to those sam age conditions. The sediment con pled from the San Pedro River at tent increases with discharge in the Charleston. These waters average lower range and levels off at about about 300 ppm (parts per million) 5 percent. It is believed that sed in total soluble salts and have an iment contents of floodflows from average hardness of about 130 ppm. Pantano Wash will show an equally A few samples have been collected wide variation in relation to dis from the Atterbury Reservoir and charge, as conditions in the drain stock ponds in this drainage area. age area are somewhat comparable to They have an average soluble salt those on the San Pedro River above content of about 160 ppm and a hard Charleston. Mechanical analysis of ness of 98 ppm. suspended sediment samples and cor relation with streambed and channel A sample representative of the conditions should permit an estimate base flow of Cienega Creek at a of the total sediment load. point where it enters the Tucson ba sin had a total soluble salt content The computation of the dry of 875 ppm and a hardness of 368 ppm. weights of the sediment collected or This is effluent ground water from stored in reservoirs vary consider an area with considerable limestone ably, depending upon the mechanical and gypsum deposits. analysis of the sediments and even upon the composition of the clay Soils fraction, as well as the conditions under which the storage takes place The available soils data on the that is, under water continuously, Tucson basin and vicinity are as or alternately submersed and ex follows: posed to the atmosphere. For exam ple, the density of the Lake Mead 1. A detailed soil survey of sediment has been determined to be the Tucson area was made by the Bu 65 Ib. per cu. ft., and those in the reau of Chemistry and Soils, U. S. Roosevelt and San Carlos Reservoirs Department of Agriculture (Youngs estimated at 70 Ib. per cu. ft. (in- and others, 1931). The report covers teragency Committee on Water Re parts of the Santa Cruz River and 24
Rillito Creek, the area designated noting land-use practices, estimat "A" on figure 10. ing slopes and amounts of erosion, and noting vegetative cover. Detailed descriptions of the soils in the area mapped are given Range site and condition sur in the above report. In general, the veys are being continued by the Soil soils represent two broad groups: Conservation Service upon requests (l) the older upland soils, which from ranchers. However, there are have a very definite accumulation of no surveys being made or pending at lime or caliche in the subsoil, such the present time in the Rillito as the soils of the Final series; drainage basin above Vail. and (2) those on the more recently deposited stream-bottom lands or Evaporation lower alluvial fans, such as the soils in the Gila and Pima series, As the Tucson basin study is most of which are mellow and friable chiefly concerned with the total wa throughout and lack a very definite ter budget, evaporation from sur horizon of lime accumulation. In the faces of soil (including stream- Pima series, however, the subsoil is beds), water, vegetation, snow, and rather heavy in texture and some ice must be considered. In fact, what compact and tough in places. evaporation plays an important role in the hydrologic cycle in that it The stony or gravelly alluvial generally accounts for a large part fans, which have been badly cut by of the water lost, especially in erosion, are underlain by subsoil semiarid and arid regions. material that is highly calcareous and more or less firmly cemented. The factors controlling evapor ation are known, but an accurate 2. Data were obtained from de quantitative analysis of the rela tailed soil surveys of areas desig tive effectiveness of each is diffi nated "B" on figure 10. The data cult because of their interrelations from these surveys made by the Soil (Linsley, Kohler, and Paulhus, Conservation Service, U. S. Depart 1949). The following factors have ment of Agriculture, since 1931 are to be considered: available in its Tucson office. 1. Vapor-pressure differences. 3. Data from a 1936 soil survey The rate of evaporation depends on made by the Soil Conservation Ser the difference between the vapor vice also are available at its Tuc pressure of the water and the sat son office. The areas covered by uration vapor pressure in the air this survey are designated "C" on above the water surface. figure 10. 2. Temperature. The rate of 4. Data from a range site and emission of molecules from liquid condition survey, collected by the water is a function of the tempera Soil Conservation Service, are ture the higher the temperature, available at its Tucson office. the greater the rate of emission. The areas covered by these surveys (which are complete only for small 3. Wind. There is a relation parts of the Rillito drainage basin) between evaporation and wind move are designated "D" on figure 10. ment, but its exact nature has not This type of survey includes taking been determined. sufficient soil borings to determine surface and subsurface soil types, 4. Atmospheric pressure. Evap- 1958 A 1957 D 1956 10 X 1955 A 1954 + 1953 O 1952 1951
1.0
. 1
0) CO
.01
001 1.0 10 100 1,000 10,ooo Discharge, in cubic feet per second
Figure 9. --Sediment rating curve, San Pedro River at Charleston, Ariz. 27 oration decreases as atmospheric 1. The amount of water lost pressure increases. from a container, whether it be a lake or a pan, is measured. As 5. Quality of water. Evapora evaporation from free-water surfaces tion decreases as the specific grav in pans is greater than from adja ity increases. cent water bodies, an adjustment is required to estimate evaporation The above factors affecting from lakes on the basis of nearby evaporation as discussed apply pan measurements. chiefly to a free-water surface. Evaporation from soil, vegetation, Attempts have been made to de snow, and ice is affected by these termine evaporation from natural same factors but requires special soils and from snow by exposing sam consideration. ples in small pans and determining their loss in weight. Such exposures An important factor affecting are probably no better index to the volume of evaporation from a evaporation from snow or soil in soil surface is the evaporation op place than the measured loss from an portunity, or the availability of exposed water surface. water. As long as the soil surface is saturated, the evaporation rates 2. The vapor-pressure gradient are probably not greatly different for the determination of the flow of from those which would be observed moisture through a layer of the at from a water surface at the same mosphere above the evaporating sur temperature. However, If the soil face is measured. The above basic surface is not saturated, the rate data and wind velocities are used in of evaporation is limited by the formulas for computing evaporation. rate at which moisture is trans ferred to the surface from below, 3. Heat-budget analysis, which even though existing meteorological requires a measurement of net radia conditions might favor a greater tion, heat transfer by soil conduc rate. tion, and air temperature and vapor- pressure gradients above the sur A part of all precipitation is face, will account for the fraction temporarily retained on the exposed of solar energy used hourly or daily surfaces of vegetation. The water for evaporation or evapotranspira thus retained is returned to the at tion. Hence the water loss at the mosphere by evaporation. Like evap earth's surface can be estimated oration from the soil, this loss is with good accuracy from the disposi greatly dependent on the evaporation tion of energy at the earth-air in opportunity. terface (Suomi and Tanner, 1958). The evaporation opportunity 4. The changes of moisture in from both snow and ice is practical soils and streambeds following rain ly 100 percent, and the rate of fall on the area or changes of flow evaporation is substantially the in the stream channels are measured. same as the evaporation from shallow These changes in soil moisture can water. be measured by weighing soil samples before and after heating; or by us A number of different methods ing bouyoueos blocks in the lower have been utilized for measuring ranges of moisture and tensiometers water transfer to the atmosphere in the higher ranges near the field (Bernard arid others, 1949). These capacity; or by the neutron method, may be grouped into the following in which a measurement is made of four distinct approaches to the the number of hydrogen nuclei pre problem. sent per unit volume of soil. 28
For an estimate of the evapora basin ranges from Sonoran desert tion from free-water surfaces in the flora at low altitudes on the valley basin, the evaporation data from the floor to pine-fir forested areas on U. S. Weather Bureau Class A Land the mountain tops. Prior to develop Pan at Tucson could be used. The ment by pumping, mesquite forests average yearly evaporation for the and cottonwood groves, together with period 1928-58 at this station was batamote, were the predominant types 87.8 inches; this, of course, far of vegetation on the bottom lands exceeds the average rainfall of 10.8 adjacent to stream channels. Creo inches at this same station. Figure sote bush, cacti, paloverde, mes 11 shows the relation of evaporation quite, and desert shrubs with some to temperature for the station at grasslands are found on the valley Tucson, and figure 12 shows the slopes. Oak, Juniper, pirion pine, moisture deficiency of the basin and grasslands occupy the lower created by the evaporation poten mountain slopes and pine and fir are tial. at the higher altitudes. There are no data available for Vegetation types are closely the Tucson basin regarding evapora correlated with temperature and pre tion from surfaces of snow, ice, and cipitation, and within the drainage vegetation; however, available data basin these are directly related to from other areas could possibly be altitude. Table 5 lists the predom adapted for use in making rough inant types, the altitudes in which estimates of the evaporation from they are commonly found, and the ap the basin. proximate acreages of each in that part of the drainage basin north of Vegetation Cienega Creek. Vegetation within the Tucson
Table 5. Vegetation in the lower Rillito drainage area
Altitude Area Per- Type (feet) sq. miles cent Creosote bush, cacti, desert shrubs, and grasses; mes quite, cottonwood, and other trees along stream channels and on bottom lands...... 2,000-3,000 148 32 Cacti, paloverde, desert shrubs, and grasses...... 3,000-4,000 152 33 Grasses, and some chaparral, 4,000-5,000 69 15 Oak, pirion pine, Juniper, and grasses...... 5,000-6,300 37 8 Arizona pine and Douglas fir, 6,300-9,000 55 12 106 29 15
CQ 0) ,4 14 - o 5 CQ 13 - June
12 - July May 11 -
10-
Q) 9- April
6- March 5- o 4- a cd February [ovember 3- January ^December so 2- Q) 1- 0- 5'7 s's 45 49 53 65 69 73 77 ! 93 Mean monthly temperature (°F) Figure 11. --Relation of evaporation to temperature, Tucson, Ariz., 1928-58. (Based on U. S. Weather Bureau data) 30 8 TUCSON, ARIZONA 7 - UNIVERSITY OF ARIZONA ELEV. 2410 1951-53 (INCL.) 6- POTENTIAL EVAPO 5- TRANSPIRATION 4- 3 2 0 POTENTIAL EVAPOTRANSPIRATION MI LEMMON (AFTER THORNTHWAITE) O O ELEV. 7690 PRECIPITATION - (NEAR SUMMER HAVEN) SOIL MOISTURE 1951-53 (INCL) STORAGE RUNOFF OR SURPLUS SOIL MOISTURE USE POTENTIAL EVAPO SOIL MOISTURE DEFICIT TRANSPIRATION 0 Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec Figure 12.--Precipitation and potential evapotranspiration at Tucson and Mount Lemmon. 31 The average annual rainfall Figure 13 shows the cleared and ir ranges from about 10 inches at Tuc- rigated acreages, overflow areas, son to about 35 inches in the Cata- and phreatophytic areas within the lina Mountains at an altitude of basin. The total acreage in each of 8,000 feet (Schwalen, 1942). The these classifications is as follows: small water yield from the drainage cleared or cultivated, 1,850 acres; area is due to the large excess of sandy overflow areas, 250 acres; and potential evapotranspiration over phreatophytes, 2,750 acres. precipitation (Thornthwaite, 1948). In general, the water table Planned control of vegetation within the areas shown on the map (fig. 13) ranges in depth from some The increase in water yield what less than 10 feet during wet that could be expected of vegetation periods to a maximum of 30 feet at manipulation on the watershed (phre- the end of dry periods. The average atophytes excepted), assuming such water level is about 20 feet. The manipulation feasible for an area of direct draft by phreatophytes from high recreational value, probably the ground-water reservoir probably would be small. In the lower areas does not average much more than 1 the potential evapotranspiration- acre-foot per year.. The total a- precipitation ratio is very high and mount of water which might possibly little gain in available water could be salvaged from this area is fur be expected from changes in density ther limited by the rather intensive and composition of vegetation. The suburban development in that area, greatest gain could be expected in and the fact that the native vegeta the pine-fir zone, but the area is tion in the form of mesquite growth, small, the stands are relatively cottonwood trees, and sycamores in open, and the soil is thin and the lower canyon reaches is consid coarse textured. In addition,much of ered by many to be highly benefi the area is extremely steep and cial. The present draft upon the rocky. These conditions suggest ground-water reservoir in this area that (l) the surface runoff-soil by phreatophytes may be estimated at moisture-storage ratio would be high roughly 2,500 acre-feet per year. and (2) the total soil moisture- storage capacity would be low. Under Natural Recharge of Ground Water such conditions, changes in vegeta tion probably would have a relative It is evident from the effects ly small effect on water available of pumping draft upon the Tucson ba for streamflow. sin ground-water reservoir that only a small percentage of the rainfall Areas of phreatophytes on the drainage area is recharged to ground water. By far the greater The shallow water-table areas part of it is lost by evaporation within the drainage basin are limit from the soil and vegetative sur ed to the bottom land along Rillito faces or, subsequently, by evapo Creek upstream from about 2 miles transpiration. That part of the west of the junction with Pantano precipitation which finally becomes Wash, the Tanque Verde Wash and nar a part of the main body of ground row strips along lower Sabino and water comes principally from the Esperero Cfnyons, and the Agua Cali- following immediate sources (Schwal- ente Wash, Esperero Canyon and Agua en and Shaw, 1957): (l) direct in Caliente Wash maintain free water- filtration from rainfall;(2) ground- table conditions only after wet per water movement into the basin as un iods, and. function as phreatophytic derflow; (3) seepage from irrigated areas only during these periods. lands; and (4) seepage from stream uo t- CO R.I4E. S R.I5E. 1 / ojiX N 7 . 10 II 12 8 12 7 9 / 10s---' Ofi^ ii \ «:! f \ ' |\ / ' o-. / R.I6E. i \ 15 14 13 >a 17 16 u l ' 15 14 13 18 ; 17 \ ,*u' f . 1 ,' " ~ en I / f) ; , \ ^ '; ^^'^ i H $ I ^ ^'20 22 23 24 19 21 ^ £ t; 22 23 24 V 21 22 V /jf , &',iyy V) 'f r l, R^LL r ^? 4 A y/ ^1 °.£ESK 30 ,JA 29 28 /I !" 27 26 25 29 28 27 tffiwffiffiffii* --i^^^ (£$' 3$f ^^tep ZZZ3 A\////'r4(^ iSffefe $/j/>i%IJi^ v/////////^;."'A $/.$. ^ifel|l8 ^^^*5 .;< ""**"fj*" ** r vfs/fy'T/frZj'i'1 \S '£':$' 34 36 3'6*-v. ^^m^ 34 35 31 32 33 34 31 ;>;^i ^ && \ ^/VT~ &" ** 'i »'"' GRANT ROAD ~He§^ y/ J '» ' Y/j /% ^2^ $f* ^$ Pjjfa ., f?*.^^ ^rr. /- 3 2 1 6 V^ 5 4 :$j/''^m' a W^Qi, ^&yzffi« ^"^v^P (T^^^^j **'Vr. ^Igy** \..- ***^ *"*A**^^iij 'Zjjj?' 0 0 SPEEDWAY <£ \\ ^ N a: EAST SPEEDWAY « S?5^^^ ^"M^t§; \ \^-\ 0 ci f** FIGURE 13. AREAS OF PHREATOPHYTES WITH SHALLOW WATER TABLE !=£fia*B ALONGAl /^M/^ RILLITODM 1 ITf\ GREEK.r*OCCIS bi^dK-Mv.M COTTONWOODPHRtATOrnYl tb-AND MOSTLY SYCAMORE. MtDUUM C. C/.1. 1. 1 . V.'.'.'i C A MDY OVERFLOW AREAS- MOSTLY AGRICULTURAL ENGINEERING DEPARTMENT UNIVERSITY OF ARIZONA r"r*."i ii'tl DM FAMOTE AND SMALL GROWTH. 1959 W%%h CLEARED OR CULTIVATED AREAS. 1 O 1 2 1.1 i i Scole in Miles 33 channels. rock on bottom and sides, has been placed across a narrow gorge. This Direct recharge to the water barrier causes essentially all the table from precipitation is of underflow to come to the surface no importance (Turner and others, where it is measured. The underflow 1943). An exceptional rain of 2 at this point, water recharged from inches upon the normally dry desert 460 square miles of surface area, floor and slopes is sufficient to amounts to less than 1,000 acre-feet wet the soil only to a depth of a per year. foot or two. At the end of a long rainy period, and then only under Seepage or deep percolation the most favorable conditions for losses from irrigated land and infiltration, the soil will be wet ditches is actually the recircula- only to a depth of a few feet. Rare tion of ground water, but it is a ly does rainfall upon the valley source of return flow in the immedi floor penetrate to a depth below the ate area of use. It may amount to as root zone of native vegetation or much as 25 percent of total pumpage, wet the soil beyond the depth from and in permeable soils may be even which it will be captured by surface more. It must be considered in any vegetation. ground-water inventory, but it does not increase the available water Significant, but limited re supply by its full amount because of charge from rainfall does occur gradual deterioration in quality. through the fractures in the rocks at the base of the mountains and Infiltration from stream chan also in some areas of coarse, open nels during periods of flow is the detrital outwash adjacent to the major source of recharge in Rillito mountain base. The generally imper Creek. During the winter months meable character of the mountain some streams such as Sabino Creek rock formations precludes any appre and Tanque Verde Wash may flow for ciable movement of ground water long periods of time. Much of this from the mountain areas down to the water is retained by the sandy ground-water reservoir. stream channels after leaving the crystalline complex, and wells along The determination of ground- lower Rillito Creek may reflect the water movement into the basin as un recharge. However, this is a shal derflow is normally difficult. For low ground-water reservoir and some tuitously, an accurate measurement of this recharge must be considered of underflow into Pantano Wash near temporary, as it will be evaporated Vail has been accomplished. Here a or used by native plants. concrete arch dam, anchored to bed 34 SUBSURFACE WATER SUPPLIES Geology The deposits of the Tucson ba particularly to the fact that Tuc sin, particularly in the Rillito son basin deposits and Pantano beds drainage area, do not extend to the are similar enough in origin and foot of the adjacent mountain front. lithology to make it difficult to The area between the Pantano-Rillito distinguish between them in well drainage and the mountain front is, logs. The critical geologic fea for the most part, an erosion sur tures have been discussed by Moore, face cut on indurated rocks and and others (1941), Smith (1938} thinly covered by alluvial outwash Turner and others (1943), Johnson on the ridges, by inner-valley fill (in Halpenny. 1952), Voelger (1953), within tributary arroyos, and by al Kidwai (1957), Brennan (1957), and luvial fans adjacent to the moun Schwalen and Shaw (1957). Schwalen tains (fig. 2). The alluvial out- and Shaw in particular have outlined wash may be a part of the fans, and the general geologic features and it is included with them on the map. their relations to ground-water The rocks underlying the Catalina hydrology. foothills area, north of Rillito Creek, are mostly the Pantano beds. Rock Units Between Pantano Wash and the Rincon- Tanque Verde front, the rocks in Crystalline complex clude gneiss, tightly indurated sed imentary rocks of either Paleozoic The Catalina, Rincon, and Tan- or Cretaceous age, Cenozoic volcanic que Verde Mountains consist chiefly rocks, and, in some places, Pantano of banded, granitic gneiss, and beds. smaller bodies of granite and sedi mentary rocks. Other rocks of the Between the mountain front and mountain mass represent deposits the area occupied by the main part laid down when Precambrian, Pale of Tucson, the surface of the crys ozoic, and possibly Cretaceous seas talline rocks slopes downward so covered the area, and some of them that the Tucson basin deposits are have been converted by metamorphism at least several hundreds of feet into new types of rocks. Locally, thick. The relationship between the the crystalline complex is fractured crystalline complex under the Tucson and small quantities of water have basin and the shelf along the moun been produced from the fractured tain front is not clearly known. zones. For the most part, however, The transition may be a gradual the crystalline complex forms an im slope, but it is more likely to be permeable barrier to movement of an abrupt dropoff. If the transi ground water. Rain and snowmelt tion is abrupt, the exact position form runoff on the mountain slopes, of it is not known, but is a domi but do not enter the subsurface un nant feature of tremendous influence til the streams enter areas under on the occurrence, movement, and lain by permeable inner-valley fill. volume of water in the Tucson basin. Paucity of information on this crit Pantano beds ical feature is due partly to the lack of detailed study, but more The Pantano beds (Moore and 35 others, 1941) consist of several tatoc Wash, showing slight tilting thousand feet of tightly cemented and faulting of the beds. The most conglomerate, sandstone, siltstone, common rock in this area Is red and claystone, deposited in a basin claystone, but all three of the mem or basins of unknown extent, which bers recognized by Voelger are pre predate the modern topography. Voel- sent. The beds are so well Indurat ger (1953) assigned three members to ed and Impermeable that they do not the Pantano beds, separated by un yield water to wells in any appreci conformities and characterized by able quantities. different compositions. The lower unit contains fragments of Paleozoic The Pantano beds in the Catali limestone and granite, but no Cata- na foothills and along the west and lina gneiss. The middle unit con south sides of the Rlncon Mountains tains some granite and limestone have been steeply tilted and involv fragments, and Catallna gneiss. The ed In thrust faulting. This means upper unit contains a high percent that the beds are related to the age of Catallna gneiss, and is simi crystalline complex of the mountain lar in composition to the alluvium blocks, as far as the structural, in modern streams leaving the moun eroslonal, and deposltional history tains, and to the fans along the of the area is concerned. The in mountain front. clination and relationships of the Pantano beds under the center of the Voelger's study implies that Tucson basin are not known. The beds the lower part of the Pantano beds might be present In complicated was deposited when the ancestral fault blocks along with other units Catalina Mountains were uplifted. of the crystalline complex. On the At that time the cover consisted of other hand, the floor under the pre Paleozoic sedimentary rocks, and the sent Tucson basin might coincide in Catallna gneiss was not exposed to part with the floor of some part of erosion. The presence of rocks sim the original Pantano basin of depo ilar to the Pantano in the areas sition, and the Pantano beds might south and east of the Rlncon Moun overlie the older units of the cry tains, and the composition of these stalline complex with only slight rocks, suggest that the Pantano was dips. Schwalen and Shaw (1957) noted deposited in a basin or basins the presence of Pantano beds at a flanked by mountains which did not depth of 550 feet In a well south of necessarily or entirely coincide Davls-Monthan alrbase. with the existing ranges. Alluvial deposits of the Tucaon Brennan (1957) concluded that basin at least 8,000 feet or more of Pan tano beds were deposited In the area The Tucson basin is filled with south of the Rlncon Mountains. As an unknown thickness of gravel, these beds have had older rocks, sand, silt, and clay. Most of this such as Paleozoic limestone and material was probably brought into gneiss, thrust on top of them, the the basin by streams and slope wash Pantano probably predates the last from the adjacent mountains and by major structural activity in the through-flowing streams that en area. tered the basin from the south and east. The material was deposited Pantano beds are exposed in the largely in flood plains, but some of Catallna foothills north of Rillito the clay was possibly laid down In Creek. Figure 14 shows a typical shallow and temporary lakes. The exposure of the Pantano beds in Pon- lithology of the material varies U) Figure 14. --Conglomerate in Pantano beds. Exposure in Pontatoc Wash, Catalina Foothills, showing faulting and slight tilting. 37 considerably, both vertically and mountain fronts and at the mouths of horizontally, and correlation be canyons are often considered to be tween units over any large areas, important as channels for recharge except in the most general way, has of mountain runoff into the ground- not been possible. Further work in water basin (fig. 3). It is obvious interpreting well logs and samples, that most of the fans along the Cat however, may result in interpreta alina and Rincon fronts do not pro tions of facies relationships that vide such avenues of recharge be will be useful in establishing the cause they are not directly con geologic geometry, both vertically nected with the Tucson basin depos and horizontally. These parameters its . The fans at the mouths of can can then be translated into degrees yons in the foothills area, however, of permeability of various parts of probably do serve to channel water the sediments. into the inner-valley fill of the major streams. Most of the wells in the Tucson area yield water from the Tucson ba Inner-valley fill sin deposits, which are surpassed in permeability only by the younger in The inner-valley fill is the ner-valley fill along the present material underlying the channels and streams. flood plains of existing streams. The more important areas of exposure Alluvial fans are along the Santa Cruz and Rillito channels. The thickness of the fill Alluvial fans along the margins in most places is a few tens of of the mountains, particularly at feet, or 100 to 200 at most. The the mouths of several canyons, have lateral extent of the fill is shown been formed by coarse material shed by the flood plains occupied by the from the mountains. For the most streams within historic times. The part, the fans in the Rillito Creek inner-valley fill consists of sand, drainage area are not thick, but silt, and gravel, which has not been remnants of extensive fans occur in greatly cemented by minerals nor parts of the Catalina foothills greatly compacted. The fill forms area, particularly at the western the most permeable unit of the area. end, and in isolated patches east of Shallow wells of relatively large Sabino Canyon road. capacity have been developed for do mestic and agricultural uses along Outwash from the fan deposits the flood plains of both the Santa has contributed to the thin layer of Cruz River and Rillito Creek. Water gravel and sand that forms a veneer from such wells is derived mainly on most of the ridges in the foot from recharge from the respective hill area. The rather abrupt slope streams, and the direct relationship from Rillito Creek northward to the of this water to precipitation and Catalina front might imply that the runoff is clearly shown by the hy entire foothill area is underlain by dro graph of the well at University a wedge of coarse, permeable fan ma farm (fig. 16). terial, but exposures in the washes and drill records show conclusively Structure that the slope is an erosion surface cut on the tilted Pantano beds and The Tucson basin represents a covered with the thin layer of al depressed block between the sur luvium. rounding mountain masses, which have been elevated. The extent of struc Alluvial-fan deposits along the tural relief formed by faulting, 38 folding, or a combination of both, of the Tucson basin are expressed in is not known. The Catalina Mountains the structural, depositional, and appear to be a great domal upwarp or erosional features of the area. fold, and their present elevation There is insufficient information to may be due in part to this doming. explain all the details, but a gen Boundary faults, however, surround eral outline may be given. the Catalina-Rincon Mountain mass, and some of the uplift is probably The present mountain ranges of due to faulting (fig. 2). the Tucson area were formed by a crustal disturbance which followed The boundary faults trend ap deposition of several thousand feet proximately north-south and east- of Pantano beds. Since this distur west in the Rillito Creek drainage bance the mountains have been modi area. A conspicuous fault zone along fied by erosion and perhaps by some the south face of the Catalinas sep additional uplift. The basins have arates the crystalline complex from been filled to various depths by Pantano beds. The angulate pattern sediments, and, in some places, by of the boundary faults, and the fact volcanic ash and lava flows. that the Pantano beds are locally tilted, fractured, and faulted, in Some of the mountain fronts dicate that some of the boundary that originally formed bold, steep faults may extend into the basin. A faces, called fault scarps, have re block mountain topography, on a treated during erosion so that they smaller scale than the modern topog may now be some miles from the orig raphy visible, may be present on the inal faults bounding the basins, and floor of the basin where it is cov have been worn down to gentle slopes ered by the basin deposits. called fault-line scarps. A typical example is the Sierrita Mountains In particular, an east-west mass southwest of Tucson. Such ero fault might approximately parallel sion or cutting back of the front the course of lower Rillito Creek leaves an erosional surface on the and line up with the boundary fault bedrock which slopes from the valley south of Agua Caliente Hill. The in up toward the mountain. This sur ferred fault near Sabino Canyon face, which is called a "pediment," might also be projected southward is covered with a thin layer of al into the basin along a line just luvial debris, only a few feet or a east of Wilmot Road. If a fault- few tens of feet thick, carried block pattern of topography does ex from the mountains to the valley by ist within the basin, the blocks streams and sheet wash during floods. were probably modified by erosion The surface of this pediment cap before the old surface was com commonly merges imperceptibly with pletely buried by Tucson basin de the valley floor. The distinction posits. The presence of such a between a pediment surface and the rugged topography might have an In valley floor proper, or an alluvial fluence locally on the capacity of fan, is important because the mate the basin deposits to store and rial overlying the pediment is not transmit water, and certainly would thick enough to store or transmit control the thickness of the fill any appreciable amount of water. from place to place. The south and west fronts of Physiography the Catalina and Rincon Mountains are flanked by pediments, extending The relationships of the geolo to about the positions of Rillito gic units that control the hydrology Creek and Pantano Wash. The present 39 mountain fronts are steep and bold, plains. These flood plains occupy suggesting that they are fresh fault troughs cut into the older Tucson scarps rather than eroslonal fault- basin deposits, and are filled with line scarps. The pediments may rep a few tens of feet of material de resent erosion of foothill blocks posited in Recent time. that were uplifted in the original mountain building to elevations in Ground Water termediate between those of the ba sin block and the higher mountain The Tucson area obtains its blocks, In a steplike arrangement. water supply from the ground-water If this Is so, the basinward side of reserves within the Tucson basin. the pediments should be bounded by Thus, it is essential to know the faults, now concealed by fill, and amount of water in storage, how much the transition from the foothill can be withdrawn, and for how long. areas to the basin should be a fair Such determinations will be diffi ly abrupt dropoff. cult; however, it is believed that the amount in storage can be calcu While the mountains were being lated and the more pertinent fac eroded and the pediments were being tors, such as the amount which can formed, the basin was filled by de be withdrawn and the character of tritus shed from the mountains and the withdrawal response, can be as was brought into the Tucson basin by certained. In order to determine the Santa Cruz River and Pantano these factors, certain geologic in Wash. The basin deposits probably formation and water records are extended to a greater height than needed to make the ultimate quanti the present valley surface during tative analysis. Much of this infor the last stages of filling, and all mation is already at hand and a re or parts of the foothill pediments sume of the ground-water conditions may have been buried. Pantano Wash in the Tucson basin follows. probably contributed a major portion of the deposits in the area south Occurrence and movement east of Tucson, in effect building a delta or fanlike deposit across the The ground-water basin, for the Santa Cruz Valley. The course of most part, is the area between the Pantano Wash probably swung back Rillito-Pantano drainage and the and forth over the surface as depos Santa Cruz River, and the Tucson ba its built up, blocking each course sin deposits constitute the major in turn. This sort of deposition, aquifer in the area. As the rock of course, causes great irregulari units immediately north of Rillito ties in the character of deposits, Creek and east of Pantano Wash (fig. both Vertically and laterally. 3) form the outer margins of the water basin, there Is little hope After the basin was filled to that any significant amounts of wa the present surface or slightly ter could be obtained from these above, the streams began to cut down less permeable rocks. The thickness rather than, to build up. The reason of the Tucson basin deposits is not for this change is not known. It may definitely known, but available data have been caused by climatic changes indicate that it may range from 500 or by regional tilting of the area, to 800 feet throughout most of the or by both. In any event, a series area. Underlying these deposits of erosional pulses carved out some are a series of beds of fine mate of the original fill, leaving a se rial, for the most part the Pantano ries of terraced surfaces stepping beds, and although they may contain down to. the present valley flood water their yield may be relatively small. FINAL CO. N. CO. 6 MILES FIGURE 15. GROUND-WATER CONTOUR MAP OF TUCSON BASIN, SPRING 1959. AGRICULTURAL ENGINEERING DEPARTMENT UNIVERSITY OF ARIZONA JUNE 6, 1959 41 A water-table map provides the recharge reaches the ground-water hydrologist with an excellent docu reservoir. Along Rillito Creek the ment to determine the character of water table is shallow, thus ena the water reservoir. The distribu bling only a part of the floodflow tion and the shape of the water- to enter the subsurface. There are table contours indicate to the hy similar conditions in parts of the drologist many factors such as the Santa Cruz River where surface water permeability of the rocks, direction enters the subsurface via the inner- of movement, effects of natural re valley fill. Little or no recharge charge, and depletion effects of enters the ground-water basin from withdrawal. Schwalen and Shaw (1957) direct precipitation on the land have documented much of the informa surface. Only after long periods of tion on these factors and the sustained streamflow do any signifi changes that have occurred over the cant amounts of recharge take place, past 40 years. even along the inner-valley stream- beds . Such conditions existed during The ground-water contour map the winter months of 1958, when a (fig. 15) shows the water-table con sustained flow of about 6 weeks ditions as of spring 1959. The water caused the water table to rise sev table in the Tucson basin ranges eral feet along the Rillito Creek from about 10 feet to about 600 feet flood plain. below the land surface. In greater Tucson the water level ranges from As the amount of annual re about 10 feet to about 300 feet be charge into the ground-water reser low the land surface. As water moves voir is small, it is necessary to at right angles to the contours, the remember that the ground-water re general movement is from the south serves in Arizona's alluvial basins east to the northwest; however, in were emplaced over a period of many the area along Rillito Creek the centuries, even thousands of years. movement is from east to west. Also, in the past centuries there may have been more rainfall and run In the area between the Cata- off available for recharge into the lina Mountains and Tanque Verde ground-water reservoirs. The amount Ridge, the contours are more closely of net recharge to the basins today spaced, indicating a barrier condi is only a small fraction of the tion, possibly due to faulting, that amount that is being withdrawn; accounts for a sharp drop in the consequently, there is a decline of water table and may be related to a the water table and depletion of buried pediment surface. In general, ground-water reserves. where the contours are closely spaced the rocks are less permeable Water movement in the subsur than in those areas where they are face is little understood by man, in more openly spaced. spite of all his technological know ledge gained in the field of hydrol The configuration of water- ogy. The exact nature of how water table contours also indicates areas moves in the subsurface is directly that receive recharge from the land related to the character of the rock surface. The shape of the contours materials. As man dwells on the along Rillito Creek clearly shows land surface, he is able to observe that recharge occurs along the and understand surficial phenomena inner-valley fill, whereas the con better than those in the subsurface. tours in the Pantano Wash area Subterranean observations are nec clearly indicate that little or no essary in order to understand the 42 ground-water system and these are that movement in the saturated zone difficult to obtain. However, there is very slow under the best of con are several basic physical laws and ditions. In general, the character fundamentals that apply to ground- of the materials in the subsurface water movement, but they are appli indicates that the velocity of move cable only in isotropic media. As ment in the saturated zone is only the subsurface materials never real several hundred feet per year. It ly occur under such conditions, the might require several centuries for behavior of water is modified by the a drop of water to move from the character of the materials. southeastern part of the basin to downtown Tucson at the prevailing Movement of water in the unsat- hydraulic gradient. However, when urated zone, that part between the the hydraulic gradient is increased, water table and the land surface, is as by depression cones of with due primarily to the force of grav drawal, the velocity increases pro ity. The dominant vector of move portionately. The velocity of move ment is directly downward. However, ment in the unsaturated zone is also impermeable layers cause the water related to the character of the rock to move obliquely to the vertical material, but is several hundred component in certain areas; but it times greater than in the saturated seeks a straight downward path. zone. The effect of water moving The movement in the saturated zone vertically downward in the stream is in the direction of maximum hy channel material, such as in Rillito draulic gradient and the velocity is Creek and the Santa Cruz River, is directly proportional to the magni well known and the water level rises tude of the gradient. The hydraulic very quickly, in terms of weeks and gradient is expressed as the ratio even days. But it is necessary to of the vertical difference between keep in mind that the rate of move any two points of the water-table ment in these areas is not compara surface to the flow distance between ble to the velocity in the saturated the points. In the Tucson ground- zone. water basin the hydraulic gradient, in general, ranges from about 10 to A good illustration of movement 15 feet per mile. Such gradients are is shown by the hydrographs (Schwal- common throughout the alluvial ba en and Shaw, 1957 fig. 10) which sins in the arid Southwest. Exami document the water level from 1916 nation of the water-table contour through 1959 (fig. 16). One well is map (fig. 15) shows a variance of located in the inner-valley fill of the hydraulic gradient along the Rillito Creek at the University eastern side of the basin, which Farm on Campbell Avenue. The other reflects differences in the perme well is located on the campus of the ability. If the subsurface were a University. The hydrographs show the homogeneous mass, the contours would effect of withdrawal of ground water be uniformly spaced under natural and the effect of recharge from conditions. Thus, the contour map surface-water infiltration. The Farm clearly reflects the inhomogeneous well hydrograph fluctuates widely, nature of the subsurface rocks. showing relationships between with drawal and recharge to the well The rate of movement of water field. The recharge effects corres in the subsurface is one of specula pond very well with the years which tion by many persons, even by hy- had above-normal precipitation, par drologists. However, an analysis of ticularly 1941 and 1952. An exami some basic fundamental laws and nation of the Campus hydrograph, arithmetical calculations indicate however, shows no effect of this re- 43 charge, as the new water did not ground water within the Tucson basin move this far southward. The hydro- deposits has been fairly well estab graph clearly shows that the con lished from numerous chemical analy tinued decline of the water level is ses. In the water underlying the due to withdrawal from storage. The major part of the area the total geologic framework corroborates soluble salt content is less than these conditions, as there is a dif 500 ppm and the hardness is less ference in the materials in the than 1.70 ppm. Small areas are in inner-valley fill and the materials dicated along Rillito Creek and underlying the University campus. It Tanque Verde Wash in which the hard is unlikely that recharge effects ness is less than 1.35 ppm and the would extend from the inner-valley total soluble salts are between 200 fill to the Campus well, a distance and 300 ppm (fig. 17). A similar of a little more than 2 miles, in situation exists in the Canada del such a short period of time. There Oro area, but the total soluble salt is a noticeable increase in the rate content is less than 200 ppm. These of decline of the water level in the are the waters of best quality found Campus well after 1946. within the Tucson area, and typical analyses of them are given in table Quality of water 6. The general character of the Table 6. Chemical analyses of ground water in the Tucson basin area Rillito Creek Tanque Verde Creek Canada del Oro Section 25 Section 5 Section 14 T.13S., R.14E T.14S., R.16E. T.12S., R.13E. (ppm) (ppm) (ppm) Total soluble salt 265 233 156 Calcium 30 15 22 Magnesium 0 8 4 Sodium 44 40 14 Chloride 20 12 14 Sulfate 25 30 T Carbonate 0 0 0 Bicarbonate 146 127 102 Pluoride 0.4 0.9 Hardness as Ca 74 72 72 Analyses from three wells, where the total soluble salt content rather widely spaced in the central is less than 500 ppm, but in which part of the area, have been selected the hardness is between 85 and 170 to show the quality of the water ppm. The analyses follow. u UNIX^ERSITY FARM-CAMPBELL AVENUE W ELL w/ > v S E U*,S E I/4,6«C. 19, T.I3 S., R.4E. 10 10 TT^ ^ N*. ;N~ ^ /V M. ^ ^ ^^ "*~ ^^^W- r^^X, r~ fj * _^ . J> " n/ y^ *-v_ / ' -v... %/ <** 20 -*^s -X/V-... >/* » --^r "^ »« / 20 30 30 40 40 50 50 UNIVERS ITY CAMPUS -AGRICULTURAL BUILDIN G WELL 60 SE Ui,NW]£,8«C.7, T.I4S..R.I4E. 60 70 70 03 ~- -" * - k=J - ^d-" «v-^ ^-v^N- 80 *- .. .. '*+^* - v^- -x^- 80 > v^~ t ~"»^>.» -^«-^^- -^^A> /\^_^. *" ^y" cfl 90 90 100 IOO 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 iO i 0) u V -4-» UNIVERSI'"Y FARM-CAMPBELL AVENUE W Fll 10 r* 10 (!) |/ V vft_ S E '/4,S E >/4.S«C 19. T.I 3 S. . R .4 E . ^~^- ^^ ^, 20 "^v^_ 20 S«.^x^ 'V/ f ^^v^- 30 -v7V_ ^s/*- 30 "^N^^ ^ ^ "^^_- ^N t 40 X^y\ A. «. 4O X. f* *^*S ^^ l\ \J\r\ V * r v_ 50 \J V. 50 v_ / s^ / 60 ^ ~n _ if" ^^ 60 70 70 80 80 ^ "c - UNIVERS ITY CAMPUS rAGRI CULTURALB JILDIN G WELL ~^ ^tai 90 "~- - ^***^ ^N i S E '>4,N W '/4,8«C.7,T. I4S..R .14E. 90 ^_^- -Xx~- IOO ^^ IOO > ta>^*_ ^^s' no "*^^»_^_ HO ^~~^ "^ v^ j -*w^ 120 ^V ~\^. 120 t -M*. ^^^-^ r"vv^ "\^ j >^S 130 -* r ^-* ^-^ "* 130 140 140 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 Figure 16. --Hydrograph of water levels in two representative wells, 1916-59. 45 Rillito Creek Tanque Verde Creek Canada del Oro Section 7 Section 18 Section 13 T.13S., R.14E. T.14S., R.15E. T.13S., R.13E. (ppm) (ppm) (ppm) Total soluble salt 304 324 370 Calcium 30 46 35 Magnesium 4 10 8 Sodium 50 23 62 Chloride 14 10 36 Sulfate 40 10 70 Carbonate 0 0 0 Bicarbonate 166 225 159 Pluoride 0.4 0.2 0.3 Hardness as Ca 003 92 155 121 South of the Southern Pacific hardness exceeds (1.70 ppm). Typical Railroad is a zone of ground water analyses of waters from wells in In which the total soluble salt con this area follow. tent is more than 500 ppm and the Section 4 Section 13 Section 12 T.16S., R.15E. T.15S., R.14E. T.14S., R.13E (ppm) (ppm) (ppm) Total soluble salt 870 585 821 Calcium 127 94 74 Magnesium 7 13 4 Sodium 110 54 170 Chloride 32 22 44 Sulfate 372 214 339 Carbonate 0 0 0 Bicarbonate 220 186 190 Pluoride 0.3 0.4 Hardness as Ca 348 288 202 Along the west edge of the val ppm, and with the typical high sul- ley and more or less parallel to the fate content of the waters of Santa Santa Cruz River are waters with a Cruz Valley. The following analyses salt content of more than 500 ppm, are typical of the waters in this and in some cases more than 1,000 area. Section 17 Section 15 Section 19 T.13S., R.13E. T.15S., R.13E. T.16S., R.14E, (ppm) (ppm) Total soluble salt 686 717 784 Calcium 120 60 130 Magnesium 4 8 36 Sodium 60 145 34 Chloride 103 70 38 Sulfate 204 180 206 Carbonate 0 0 0 Bicarbonate 195 254 275 Pluoride Hardness as Ca 003 317 185 473 46 North of Rillito Creek in the available water. foothills of the Santa Catalina Mountains is an area of Pantano The amount of water that is in beds, buried in some places and ex storage in the Tucson basin is di posed in others. Dry holes or wells rectly related to the areal extent of extremely poor yield have been and thickness of the deposits, and constructed in this formation, but to the character of the materials. in many places they will not produce The Tucson basin is bounded on the sufficient water for a single home, north, east, and west sides by im or water of satisfactory quality for permeable rock barriers, as shown on household purposes. This may be figure 2. Underflow into the basin termed a questionable area, as to comes from the Pantano Wash and the both quantity and quality of ground Santa Cruz River. However, the a- water. mounts are small compared to the amounts being withdrawn and the The availability of a water water moves at an extremely slow supply in the foothill area adjacent pace. The natural underground out to Tanque Verde Ridge and along the flow from the basin northwest of base of the Rincon Mountains is Tucson also is small and moves very questionable, except along the bot slowly. Under natural conditions in tom land adjacent to Rincon Creek. flow is approximately equal to out Wells along the creek, in general, flow; thus, no water is gained or have a total salt content of less lost in the subsurface system. than 300 ppm, and a hardness of slightly more than 85 ppm. To obtain quantitative data on storage and yield, the character of Practically all ground waters the materials must be translated in within the area contain small a- to hydrologic terms, such as perme mounts of fluoride, which do not ex ability and specific capacity. Per ceed the allowable limit of 1.5 ppm meability may be expressed in terms except in the area north of Tanque of gallons per day moving through a Verde Wash. In this area all well cross section of 1 square foot under water should be checked for fluoride unit hydraulic gradient under pre where young children are to use the vailing field conditions. This co water for drinking. Some water has efficient is a measure of the abili been found with a fluoride content ty of the sediments to transmit wa in excess of 10 ppm. ter. Storage and yield Also important is the matter of specific yield, which is defined as In Arizona the ground-water re the ratio of the volume of water serves in the arid alluvial basins that will drain by gravity from a have been used to meet the water de saturated rock to the total volume mand, except where available surface of the rock. In a number of Ari supplies have been developed. Since zona's alluvial basins, the specific the early existence of the Tucson yield ranges from 10 to 20 percent, community its supply has been ob which is considerably less than the tained from ground water. Tucson actual porosity of the materials. might not have grown to its present Perhaps half the amount of water in size if these ground-water reserves storage drains to wells. Schwalen had not been available. (Turner and and Shaw have stated that the spe others, 1943). The growth of numer cific yield in the Tucson basin is ous communities throughout the State about 10 or 12 percent, which indi is limited because of the lack of cates that the materials are less N. R.I IE 6 Miles t J oy ^§^^j^ AREA TOTAL SOLUBLE SALTS -500 PPM .INE Cl yiOiNO WATERS + IOGR /&AL HARDNESS FIGURE IT-QUALITY OF GROUNDWATER IN RILLITO CREEK PROJECT AREA. AGRICULTURAL ENGINEERING DEPARTMENT UNIVERSITY OF ARIZONA 48 permeable than in some of the other will be pumped from the Tucson basin alluvial basins. This yield appears will generally be determined by the to be corroborated by the geologic demand for water in the future; how evidence at hand. In the more im ever, the nature of the rocks in the permeable layers, at depths below subsurface will be a dominant factor 500 to 800 feet, the specific yield in the increased costs and the ulti may be something less than 1 per mate specific yield. The magnitude cent, which means only limited quan of the hydraulic gradient necessary tities of ground water are available to cause water to flow into the for withdrawal. wells to meet the demand will be de termined by the permeability of the An examination of the ground- deposits. The ultimate question, water depletion, as shown by the map then, is "How long will it be pos of water-table decline (fig. 18), sible to pump water at a rate to documents and corroborates the pre meet the demand?" In part this will vious statements. The contours be determined by the quantity of wa clearly show a cone of depression in ter in storage. Analysis of the the area, and the withdrawal of ground-water reserves indicates that ground water is directly related to the quantity is more or less fixed, the population density and expansion and the rate of withdrawal will be of metropolitan Tucson. More than determined by demand. 35 feet of decline has taken place in the last 12 years beneath the The quantity of water in re central part of the city. The cone serve in the Tucson basin could be is spreading outward in all direc determined logically by a flow-net tions, and this trend will continue analysis. The Agricultural Engi until it reaches the hard-rock neering Department of the University boundary areas, when conditions will of Arizona has collected consider worsen because the decline will in able hydrologic data which would crease at even a greater rate. serve as an excellent base with which to make such an analysis. How In the northeastern part of ever, further data are needed on the basin along the Pantano Wash the exact amounts of withdrawal near the confluence of Rillito over certain periods of time, and Creek, the decline contours are al these must be correlated with the ready closely spaced, indicating the decline of the water table over the presence of a barrier in this par same period. The character of the ticular area. Demands for additional rocks in the subsurface must be amounts of water will greatly ac known to make a flow-net analysis. celerate the depletion and a further A number of drillers' logs and sam decline of the water level in this ples have been collected over the area is inevitable. It has been es past years during development of timated that in the past 12 years wells. Systematic studies of these the Tucson basin has suffered a net data may provide adequate informa loss of 250,000 acre-feet of water. tion on the rock character, which As the basin's assets are being de could then be translated into per pleted and the amount of renewal or meability parameters and also be replenishment is small, there is used for construction of the geolo reason to be concerned about how gic geometry of the basin. The long these assets will last. availability of such basin geologic information would enable evaluation Volumetric analyses of the water-table-decline data to determine the specific yield of the The rate at which ground water ground-water reservoir. Mf Lemrnon R.I6E. Soldier's Comp FIGURE 18. GROUND-WATER LOWERING IN TUCSON BASIN 1947- AGRICULTURAL ENGINEERING DEPARTMENT UNIVERSITY OF ARIZONA JUNE 6, 1959 VO 50 The use of an electric analog California (Baumann, 1953, Schiff, computer would greatly speed the 1955; Muckel, 1959; see also Todd, transformation of the geologic data 1959). into hydrologic parameters and would enable the analyst to predict the The method used most commonly withdrawal response for any given in California is ponding or spread year in the future. The demand of ing water in surface basins. Water water for any future year would be is also spread in natural stream in accordance with the projected channels, in furrows and ditches, population of the area. Thus it and in abandoned gravel pits. Water would be possible to know reasonably is injected directly into the satu well the position of the water table rated zone by means of pits, shafts, in any particular year. and wells. Subsurface Storage of The principal advantages of Water by Artificial Means subsurface storage are that the ca pacity of the reservoir is very It has been estimated that the large and that water in subsurface annual total inflow to Rillito Creek storage is not depleted by evapora basin may be about 40,000 acre-feet tion losses. On the other hand, (table 4). A large proportion of several technical questions arise the inflow occurs during relatively regarding the quality and quantity short flood periods. Therefore, it of the water to be stored. In order is unlikely that much of it could be to store it most efficiently, much diverted for direct use of any kind; information also must be at hand re if it is to be used, it will be ne garding the physical processes in cessary to detain the water during volved in water movement. Some of high stages of flow and put it into the essential facts are known, temporary storage for future use. whereas others remain to be deter mined. The types of information in As reservoir facilities are not volved are described below. available for surface storage of the water, consideration may be given to The source of much of the flood storing increments of it in the sub runoff in Rillito Creek during the surface reservoir. Much of the wa summer is from thundershowers of ter pumped from wells is withdrawn high intensity, short duration, and from storage; recharging water to relatively small areal extent. Many the ground-water reservoir by arti of these occur in the upper parts of ficial means thereby augments the the drainage area of Pantano Wash, amount in storage. along the Santa Rita, Empire, or southern Rincon Mountains; others Many methods of recharging wa occur in the upper tributary drain ter to subsurface storage reservoirs age areas of Rillito Creek, on the have been practiced in other areas. north side of the Tanque Verde Ridge Water spreading was started in Ger or southern slopes of the Catalina many more than a century ago, and in Mountains. This summer flash-flood the United States in 1889. In more runoff must first be controlled be recent years much work has been done fore being stored or used. This in several other countries and might be done best by structures states. Descriptions of the methods built in the upstream parts of the and of technical points involved main stream channels, near the moun with them have been published, for tain fronts, and on washes that con example, in Texas (Sundstrom, 1952; tribute tributary inflow. Prom these Moulder and Prazor, 1957) and in structures the water could be re- 51 leased through conduits at con that its velocity is reduced, much trolled rates to downstream areas of.the sediment drops to the bottom. for storage or use. Fine particles remain in suspension, commonly in quantities of a few hun Runoff during the late winter dred parts per million. Water of or spring is commonly not torrential this quality can be induced to in but of longer duration, resulting filtrate into surface recharge from snowmelt and winter rains in areas, provided the intake areas the higher mountains, such as in the consist of sediments of suitable upper Sabino and Bear Canyon drain permeability, and provided they are age areas. This winter runoff, with periodically cleaned of silt and characteristics of more steady flow, clay by some means such as suction, longer duration, and lighter silt sand replacement, or sluicing. load, is believed to be the source of a large part of the natural re If such water is to be re charge to the Rillito Creek area. charged to the subsurface by means The part of this runoff that is not of wells, it must have low silt con naturally recharged would be most tent, as silt in the water tends to suitable for artificial recharge, clog the aquifer adjacent to the because of these characteristicsj well bore. Intermittent back- however, the quantities of water pumping removes some of the silt, that may be so utilized have not but it is not known whether this been determined, and further studies would maintain permeability over a will be needed on this point. long period of time. If the recharge well is not back-pumped the water Quality of Water must be treated, by coagulation and filtration, so that the silt content The physical and chemical char is reduced to a few parts per mil acter of the water captured for lion or even a fraction of a part subsurface storage is of utmost im per million. It becomes apparent, portance in planning methods of then, that water treated sufficient storing. If the water is not suit ly for well injection will likely be able in quality, it must be treated. of good enough physical quality for use in a domestic or municipal sup Physical properties ply, as well as for other purposes, and may be used directly if the ex The most significant physical isting demand can absorb the quan property of the water to be consid tity available. When the supply of ered is its content of suspended such water temporarily exceeds the sediment. The relatively steady current demand, however, it can be flow of winter runoff is commonly of stored underground temporarily and low enough sediment content for in pumped back later to meet peak de filtration, as considerable natural mands . recharge of this water takes place, The flash-flood runoff, however, may Chemical properties contain as much as several thousand parts per million of sediment, and Water to be used for recharge little if any of it could be re to subsurface reservoirs should be charged either naturally or artifi analyzed chemically and compared cially without treatment, as it with the Boil chemistry of the in would seal the recharge surfaces. take surface as well as the chemical quality of the ground water with If floodwater is temporarily which it will come into contact, in detained in a sedimentation basin so order to avoid a chemical combina- 52 tion which would form precipitates organisms, which are relatively dor such as the relatively insoluble mant or in a state of equilibrium carbonates. The effect of using wa with their environment. If surface ters of different chemical quality water is injected in these strata, is illustrated by tests in Kern it is likely to contain oxygen as County, Calif. (Muckel, 1959, p.42). well as an abundance of organic ma The infiltration rate achieved by terial, by which the native organ using canal water was about half isms are stimulated to activity and that obtained by using well water, growth. Of particular concern are which contained about double the the colonial "slime-forming" bacte amount of dissolved solids, on the ria, which secrete pectinlike Jelly same intake surface. The well water (slime) which adheres to the aquifer had a conductivity (K x 1C-6) of 646, particles and thus reduces permea and the canal water, 239; however, bility (van der Qoot and others, the well water had a slightly lower 1955). pH and a considerably lower percent sodium, both of which are likely to During well injection experi affect infiltration rates. ments in the West Coast Basin near Los Angeles, chlorination of the re Although the choice of water charge water has been useful in in for subsurface storage is based not hibiting bacterial growth and main so much on its chemical quality as taining injection rates. Chlorine on its source and availability, its was added at rates ranging from 1.5 chemistry should be determined so to 20 ppm. It was concluded that a that proper chemical treatment may constant dosage of 8 to 10 ppm was be considered. sufficient to control the bacteria, although initial and periodic "slug" Microbial activity treatments of 20 ppm were recom mended to remove accumulations of When water is spread over a slime (van der Qoot and others, soil surface for a period of many 1957, p. 60). By February 1959 the days or weeks, the rate of infiltra dosage in one well had been reduced tion commonly decreases with time. to 5, then to 3 ppm, without impair This probably is due largely to bio ing the intake rate (John Mitchell, logical activity in the soil (Muc Los Angeles County Flood Control kel, 1959, p. 25). Comparative tests District, 1959, oral communication). have shown that more nearly constant It was also found that a dosage of infiltration rates can be maintained as much as 12 ppm "appeared to im in sterile soil. It appears from pose no special hazard of corrosion" this that in ordinary soils the to well casing. pores become partially clogged by the products of microbial growth, The required chlorination rate and that permeability reduction is at a given recharge or storage site due to partial disintegration of in the Tucson basin would have to be soil aggregates by the attack of mi determined empirically, as each en croorganisms on organic materials in vironment is likely to be different the aggregates. from others that have been studied. The criteria for determining chlo Clogging of aquifer pore spaces rine dosage should include a rate by bacterial growth is also a prob low enough to avoid corrosion in the lem in water injection through well, but high enough to control the wells. The sands or other water bacteria and maintain a relatively bearing alluvial strata in the sub constant specific intake of the re surface contain many types of micro charge well. 53 Location of Storage Areas storage. If the permeable sequence of strata extends upward to the land The selection of sites for sub surface, water may be spread di surface storage of water Is deter rectly over the reservoir and allow mined by several physical factors, ed to percolate downward; if it is which may be grouped as follows: (l) overlain by relatively impermeable source, quantity, and quality of wa strata, however, water would have to ter to be stored; (2) geologic fea be Injected to the reservoir through tures of the ground-water reservoir; shafts or wells. and (3) hydraulic characteristics of the ground-water reservoir. The hydraulic properties of the ground-water reservoir also are to The source, quantity, and qual be considered in choosing storage ity of water that is potentially sites. Principal among these are the available for storage have been de depth to water, the configuration of scribed briefly in preceding sec the water surface or pressure sur tions. The channels of Rlllito Creek face, and the ability of the reser and Its main tributaries carry most voir to transmit and store water. If of the water from its source; by the water level Is too close to the natural processes of seepage some of land surface, there is not enough It Is stored in the alluvium beneath space in the unsaturated zone to the channels, and the quantity thus store additional water without the stored may be increased by control danger of losing it by evapotrans- ling the flow and treating the chan piratlon. Thus the water level nels. Other storage areas may be should be deep enough so that if located adjacent to these channels, more Is added it still will be sev If part of the streamflow is divert eral feet below ground in order to ed to nearby spreading grounds. Fi avoid evaporation loss. Or, if phre- nally, portions of the streamflow atophytes grow In the area, it may be diverted by canal or pipe to should be perhaps 20, 30, or even 50 storage sites more remote from the feet, according to the plant type, streams but closer to an area of de so that the water will not be pumped sired use, such as areas of pumpage up and transpired by the plants. On for the Tucson municipal system. the other hand, if the water table Thus the storage area must be some is very deep,there may be relatively where between the point of availa Impermeable layers between the sur bility and the point of eventual face and the saturated zone, and the use, the exact location depending use of shafts or wells rather than upon geologic and hydraulic consid surface spreading areas may be nec erations . essary. The Important geologic features The shape of the piezometric that relate to subsurface storage surface or water table, which is re are llthology, structure, and extent lated to the rate and direction of of the ground-water reservoir. The subsurface water movement, should be potential reservoirs in the Tucson considered in planning storage. If basin are composed of alluvial sed water is recharged to a reservoir in imentary rocks, in which permeabil which the saturated zone has a uni ity is related to grain size, as formly sloping surface, the water sortment, and sedimentary structure. table or pressure surface forms a A permeable rock unit must also have mound or ridge on the former sur a structural attitude, thickness, face. If recharge continues for a and areal extent such that it will long period of time, the mound be contain a large volume of water in comes elongated in a downgradient 54 direction. Observations of this water-level observations in nearby movement should be made in order to wells, in order to plan the most ef determine the best locations for re ficient withdrawal of the water from covering the water by pumping after storage when needed. Water that has a given time Interval. been recharged by infiltration from the surface, either by natural pro The effects of previous with cesses or by spreading, can be pump drawals by pumping may also be evi ed from wells in the downgradlent dent by the shape of the piesometric direction; water injected through surface, and may have a bearing on wells may be pumped back through the the location of storage sites. A same wells. For example, in Amaril- prolonged pumping draft in excess of 10, Tex., brine was injected period natural recharge in several places ically into about 90 million gallons has created a depressed area in the of recharge water, and the water water table, such as the trough that later pumped back was tested for extends from southeast Tucson to chloride content to determine the ward Rillito Narrows (figs. 15, 18). rate of recovery of stored water. Storage of excess water in such an After 90 million gallons had been area of depression would seem highly pumped back, the recovery of in desirable from at least two stand jected water was between 78 and 90 points; (l) The dewatered sediments percent (Moulder and Frazor, 1957, have been saturated in the past, so p. 22). An experiment in El Paso that the wetting requirement is rel showed that almost all water in atively low, and a large part of the jected can be recovered (Sundstrom, water injected into them would re 1952). place water removed by pumping; and (2) the decline in water levels is In summary, the feasibility of evidence that pumping lifts in the storing water underground in the area have increased, and possibly Tucson basin by artificial means can that specific capacities of wells presently be viewed from a theoreti have decreased so that water stored cal standpoint. The process has there artificially would represent been proved feasible in other local replenishment in a place where it is ities, and experience gained there badly needed. is useful in directing further re search locally; but certain assump Finally, the properties of wa tions must now be made regarding ter transmission and storage in the some of the variable factors at par reservoir should be determined. ticular locations in the Tucson ba Aquifer tests at a potential site sin. Actual quantitative evaluations yield information on the recharge of such operations must be derived rates that may be anticipated for a empirically through closely control given cross-sectional area of water led experimental work under local bearing material, and on the quanti conditions. The results of such work ties of water that may be stored and would provide water-management agen recovered in a given volume of rock. cies the technical data needed for planning or considering actual un Subsurface Distribution and derground storage operations. Ultimate Recovery of Water in Storage Research and experimentation on this subject should include consid Wherever water is recharged to eration of all the pertinent physi the ground-water reservoir, its dis cal factors and processes mentioned position underground should be in the above section source of wa studied and recorded, by means of ter, its quantity and quality, meth- 55 ods of treatment, location of stor the life of the water reserves. The age sites, methods of storage, and exact methods and operations need to efficient means of recovery and be determined, as to whether the re beneficial use. charge should be accomplished by in duced infiltration or through con SUMMARY duits and wells into the subsurface. Experiments in recharge indicate This compilation and analysis that virtually all recharged water of data relating to the capture of can be recovered. additional water in the Tucson basin provides preliminary Information on 4. Because the Tucson area is many components of the hydrologlc in the arid Southwest it is experi system in the area. The report encing an explosive population in shows, however, that there is a lack crease and industrial expansion, and of much needed information in numer as a result water demands are in ous fields. It is believed that creasing at alarming rates. Addi further intensive research through tional water supplies must be made the coordinated efforts of the sev available in order to sustain prop eral groups which made this study erty values and the economy as a will provide quantitative answers to whole. As figure 16 shows, the rate questions that must be answered be of decline in water levels has mark fore any program to capture addi edly Increased since 1946. tional water can be undertaken. Con clusions reached by this report are 5. Even though considerable as follows: information has been assembled on the hydrologic system in the Tucson 1. Although precipitation and area, there is still much to be runoff in the Tucson basin are ex known and understood about its com tremely variable, the basin poten plexities . The physical processes tial represents a replenishable re and the interrelationship of the source that at present is largely various components must be known in lost. This potential amounts to ap order to bring about the efficient proximately 40,000 acre-feet per capture of water for beneficial use year, which is about 80 percent of in the Tucson area. the amount of water used by greater Tucson today. INVESTIGATIONS ESSENTIAL TO THE CAPTURE OF ADDITIONAL WATER 2. Although there are large IN THE TUCSON AREA quantities of ground water in stor age, these supplies are assets, or Before capture and recharge of water reserves, of the basin and surface water can be accomplished, are definitely limited. The ulti it will be necessary to investigate mate amount of water that can be further certain fundamental prob withdrawn from this storage is con lems. Among them are the .following: trolled by the character and distri bution of the sedimentary rocks in 1. Pattern of precipitation the subsurface. A quantitative anal throughout the basin. ysis of the basin's water assets must be made and from this the life 2. Amount and distribution of of the reserves can be estimated. runoff at critical points. 3. Additional surface waters 3. Quality of surface water could be captured and recharged into and the amount of sediment it con the ground-water basin to prolong tains . 56 4. Quality of ground water, University of Arizona: particularly from the deep aquifers. Agricultural Experiment Station Richard K. Prevert 5. Water loss by evaporation Agricultural Engineering Depart and transpiration. ment Harold C. Schwalen 6. Geologic framework with Civil Engineering Department particular reference to the thick Gene M. Nordby ness and distribution of the dif Department of Geology ferent rocks and their structural John F. Lance attitude. Willard D. Pye Institute of Atmospheric Physics 7. Amount of ground water in A. Richard Kassander storage and its movement within the Institute of Water Utilization basin. Sol D. Resnick K. James DeCook 8. Amount of natural recharge, and the feasibility and techniques U. S. Geological Survey of the best areas for artificial re Surface Water Branch charge . Douglas D. Lewis Ground Water Branch The desired studies can best be John W. Harshbarger, Chairman carried out in an integrated program among the several organizations at In addition, many persons have the University of Arizona and the provided substantial assistance to U. S. Geological Survey. The results the principal participants. As there of a comprehensive hydrologic inves was a considerable amount of work in tigation in the Rillito Creek area compiling and analyzing the data and also would provide useful guidance in preparing illustrations and and information for water management tables, the committee gratefully ac in other parts of greater Tucson. knowledges the following contribu The Santa Cruz River has a drainage tions: area of 2,000 square miles and it is quite reasonable to believe that UNIVERSITY OF ARIZONA many factors would be applicable to ward the possibility of capturing Agricultural Engineering Department additional water from this drainage. The Committee believes that the re Richard J. Shaw compiled data sults and objectives stemming from for the water-table-contour map and this report deserve serious consid the water-table-decline map. David eration by the people living in the Fonken supplied much information on Tucson basin. the rainfall-infiltration data from the Atterbury Wash study and on COMMITTEE evaporation from stream channels. The principal persons who par Civil Engineering Department ticipated in preparing "The feasi bility of capturing additional water Henry H. Miles compiled infor in the Tucson basin" include the mation on the natural recharge that members of the Rillito Creek basin might be effective in the Rillito research project committee, as fol Creek basin. lows : 57 Department of Geology graphs and other illustrations re lating to surface-water aspects. Robert Streitz and George E. Maddox compiled considerable infor Ground Water Branch mation from well logs and drilling samples, in order to compile the E. Fred Pashley, Jr., compiled subsurface geologic sections. the data for the geologic map and prepared the subsurface geologic il Institute of Atmospheric Physics lustrations from available data. William P. Hardt supplied advice and William D. Sellars prepared the suggestions on the ground-water section on rainfall characteristics, volumetric analysis. Leopold A. Clayton H. Reitan compiled much of Heindl supplied much first-hand the precipitation data, and James R. knowledge of the geology of the Tuc- Hastings made the analysis of flood son basin. history and channel trenching. U. S. DEPARTMENT OP AGRICULTURE Institute of Water Utilization Agricultural Research Service George R. Catron assisted in preparation of the soils map. Joel E. Pletcher assisted in making data available for the re U. S. GEOLOGICAL SURVEY port. Surface Water Branch In addition, many other persons in the various groups are to be com Roy B. Sanderson, Louis P. mended for their diligent efforts in Denis, and George R. Dempster com the preparation of illustrations and piled much of the surface-water data typing of manuscripts in order to on floods and prepared the hydro- meet the completion date. 58 REFERENCES CITED Baumann, P., 1953, Experiments with fresh-water "barrier to prevent sea water intrusion: Assoc. Jour. Am. Water Works, v. 45, no. 5, p. 521-534. Bernard, M., and others, 1949, Hydrology handbook: Am. Soc. Civil Engineers, Manuals of Eng. Practice No. 28. Bourke, J. G., 1891, On the border with Crook: New York, Charles Scribner's Sons. Brennan, D. J., 1957, Geological reconnaissance of Cienega Valley, Pima County, Arizona: Univ. Arizona Ph. D. thesis, 53 p. Bryan, Kirk, 1925, Date of channel trenching (arroyo cutting) in the arid Southwest: Science, new ser., v. 62, (Oct. 16, 1925), p. 338-344. Halpenny, L. C., and others, 1952, Ground water in the Gila River basin and adjacent areas, Arizona a summary: U. S. 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