Number 63 Issue Paper February 2019 Aquifer Depletion and Potential Impacts on Long-term Irrigated Agricultural Productivity

When developing policies that regulate groundwater systems that are being depleted, the potential consequences of groundwater depletion need to be fully assessed to determine the trade-offs that exist between the undesired impacts of groundwater depletion and whether these impacts outweigh the benefits associated with groundwater use. (Photo from Ed Hennigan/Shutterstock.)

is even higher in arid and semi-arid ar- the Mississippi Embayment Aquifer Abstract eas, where the only consistent source of (Mississippi River lowlands bordering Groundwater is the Earth’s most irrigation water is groundwater. In these Arkansas and Mississippi), aquifers in extracted raw material, with almost regions, however, the use of groundwa- southern Arizona, and smaller aquifers 1,000 cubic kilometers per year (800 ter typically far exceeds the rate at which in many western states. million acre-feet per year) of ground- it is naturally replenished, indicating The groundwater resource with the water pumped from aquifers around the that these critical groundwater resources greatest long-term depletion is the High world. Approximately 70% of ground- are being slowly depleted. Within the Plains (Ogallala) aquifer in the Great water withdrawals worldwide are used to United States, groundwater depletion has Plains Region of the United States, support agricultural production systems, occurred in many important agricultural where groundwater levels have declined and within the United States, about 71% production regions, including the Great by more than 50 meters (150 feet) of groundwater withdrawals are used for Plains Region (Nebraska, Colorado, in some areas. The Central Valley of irrigating croplands. This percentage of Oklahoma, New Mexico, and northern California, however, is experiencing the groundwater used to support agriculture Texas), the Central Valley of California, highest groundwater depletion intensity CAST Issue Paper 63 Task Force Members

Authors Dana Osborne Porter, Department of Ocean, and Atmospheric Sciences, Biological and Agricultural Engineering, Oregon State University, Corvallis John Tracy, Chair, Texas A&M Texas A&M University, Lubbock Jay Lund, Department of Civil and AgriLife Research—Texas Water Zhuping Sheng, Texas A&M AgriLife Environmental Engineering, Univer- Resources Institute and Department Research Center at El Paso and Depart- sity of California–Davis of Civil Engineering, Texas A&M ment of Biological and Agricultural Engi- University, College Station C. G. Olson, U.S. Geological Survey, neering, Texas A&M University, El Paso Reston, Virginia (Emeritus Scientist) Jennifer Johnson, River and Reser- Steve Sibray, Conservation and Survey voir Operations, Bureau of Reclama- Division, School of Natural Resources, CAST Liaison tion, Boise, Idaho Institute of Agriculture and Natural Re- Leonard Konikow, U.S. Geological sources, University of Nebraska–Lincoln David Baltensperger, Department of Survey, Reston, Virginia (retired) Soil and Crop Sciences, Texas A&M Gretchen Miller, Department of Civil Reviewers University, College Station Engineering, Texas A&M University, College Station Michael Campana, College of Earth,

because of increased use over the last economic incentives to encourage water extraction and availability and that much several decades. The most obvious conservation. All of these methods should extraction is from geologic formations consequences of depleting groundwater be considered when developing plans without significant recharge since mean- resources are the loss of a long-term to address the long-term consequences ingful groundwater pumpage to support water supply and the increased costs of of groundwater depletion. In addition, agricultural production has occurred (i.e., pumping groundwater as the water table when developing policies that regulate within the last 60 years). In addition, in declines further below the ground sur- groundwater systems that are being many areas of the world, including the face. There are many other consequences depleted, the potential consequences of United States, groundwater extraction is associated with groundwater deple- groundwater depletion need to be fully concentrated in semi-arid to arid regions, tion, however, including the loss of the assessed to determine the trade-offs that where the age of groundwater can range productivity of groundwater production exist between the undesired impacts of from hundreds to thousands of years, sug- wells (possibly requiring the construc- groundwater depletion and whether these gesting that this groundwater is not being tion of new wells); the depletion of the impacts outweigh the benefits associated replenished at the rate that it is being flow of water in rivers, creeks, and lakes with groundwater use. extracted and that these groundwater re- when they are hydrologically connected sources are being depleted. Consequences to underlying aquifers; the shifting and of the long-term depletion of groundwa- subsidence of land surfaces that can oc- Introduction ter resources include the direct impacts of cur when groundwater is extracted from Groundwater is the Earth’s most depleting the resource, which can reduce aquifers; and the intrusion of high saline, extracted raw material, with almost 1,000 water availability for local and regional or poor quality, water from other subsur- cubic kilometers per year (km3/yr) (800 societies, economies, and ecosystems, face formations. million acre-feet per year) of groundwa- and global impacts of groundwater being The most effective approaches for ter used to support agricultural, munici- released to the atmosphere and oceans addressing groundwater depletion focus pal, commercial, industrial, and energy once it is brought above ground, contrib- on reducing or eliminating the imbal- production (Margat and van der Gun uting approximately 0.3 to 0.4 millime- ance between the inflow and outflow of 2013). Approximately 70% of ground- ters (mm)/yr of sea level rise since 2000 water to an aquifer. Methods that focus water withdrawals are used for irrigated (Döll et al. 2014; Konikow 2011). on increasing the inflow to groundwater agriculture (71% in the United States) This issue paper reviews the causes resources include the development of (Margat and van der Gun 2013). The and consequences of groundwater deple- managed aquifer recharge systems and annual groundwater use is only a small tion1, with a focus on impacts to agricul- altering land-use practices to increase the fraction of total economically available ture as the largest sector of groundwater infiltration of water below the land sur- groundwater, estimated to be 0.00041%/ use. This understanding can aid in devel- face. Methods that focus on decreasing yr (Gleeson et al. 2016). Although this oping effective policies and practices for groundwater use include the implementa- appears to indicate that groundwater use tion of more efficient irrigation systems, could be sustained at its current levels 1 Italicized terms (except genus/species names and the development of agricultural crops for centuries to come, it does not reflect published material titles) are defined in the Glos- that require less water, and the creation of the spatial distribution of groundwater sary.

2 COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY groundwater development, use, and man- factors, including the well diameter, hydraulic gradient, the flow in an aquifer agement. Before groundwater depletion water level depth below the land surface, with a hydraulic conductivity of 10.0 can be addressed, however, a basic under- ease of water flow through soil or rock feet/day (ft/d) will be ten times the flow standing of the principles of groundwater near the well, and power of the pump as in an aquifer with the same hydraulic occurrence and behavior is necessary. used to extract the groundwater. gradient but in which the hydraulic con- Groundwater is rarely static, and it ductivity is 1.0 ft/d. In natural geologic Basic Principles of is generally in motion with natural flow materials, the hydraulic conductivity can rates (or flow velocities) being highly vary by more than ten orders of mag- Groundwater Occurrence variable and ranging from several tens of nitude (with clays, shales, and granitic and Behavior in Aquifers feet per day or more to less than several rocks having very low values and gravels Understanding Groundwater feet per century, depending on the type and some porous limestones and basalts Groundwater occurs almost every- of soil or rock that the groundwater is having very high values, for example). where, and it is generally defined as the flowing through, the proximity to surface In natural groundwater flow systems, occurrence of water in the soil, sedi- water bodies, and well depth. Groundwa- before people drilled wells and developed ment, and rock at pressures at or greater ter flow is always in three dimensions, groundwater for their water supply, there than atmospheric pressure. Practically and the magnitude and direction of the was always a dynamic balance between speaking, when a well is being drilled, flow depend on the hydraulic properties water entering the aquifer (recharge) and groundwater is encountered when water of the soil it is flowing through and the water leaving the aquifer (discharge). starts filling the well from surrounding steepness of the potentiometric surface, Although the water levels in the aquifer soil and rock. The depth at which this referred to as the hydraulic gradient. The (and the water table elevation, represent- occurs can be considered the water table, hydraulic gradient is the difference in ing the top of the subsurface zone satura- which is the top of the zone in which the potential energy over a given distance tion) can vary over time as precipitation subsurface is saturated with water, and it and can be characterized simply as the varies on a daily, seasonal, or decadal can vary greatly—generally being deeper difference in water elevations between scale, thereby affecting recharge, the in arid climates and near the land surface two wells along a flow path (Figure 1). aquifer systems would remain in a more in humid climates, near surface water Steeper hydraulic gradients bring faster or less long-term equilibrium condition bodies, or in areas where human activi- groundwater. in which there were no persistent changes ties have applied water on the landscape. The ease with which water can move in the average amount of water in the Groundwater can be withdrawn from the through the subsurface (including soils; aquifer (stored in pores of soil and rocks) subsurface through a well that extends unconsolidated sands, clays, or gravels; and discharged to streams and springs— below the water table or through hori- and geologic formations of bedrock) is unless long-term climatic changes created zontal channels or tunnels that intersect characterized by its hydraulic conductiv- permanent changes in the recharge (or the water table. Water production from ity (or similar parameters such as perme- boundary conditions) for the aquifer. groundwater wells depends on several ability or transmissivity). For a given When water is pumped from a well, it causes water levels in the well and adjacent aquifer to decline, referred to as “drawdown.” This drawdown in turn creates a hydraulic gradient that draws water from the aquifer toward and into the well—to replace water the pump re- moves from the well. This drawdown and water removal also disturbs the natural equilibrium in the aquifer and the balance between natural recharge and natural discharge. Lowering the water level in the aquifer also means that some water has been removed from “storage” within the aquifer’s pore spaces. Groundwater and surface water are two components of the overall intercon- nected water resource (Winter et al. 1998). In places or at times when the water table is relatively high, ground- water flow can discharge to streams, lakes, springs, and wetlands, providing base flow and long-term persistence to Figure 1. Depiction of groundwater hydraulic gradient. wetland streams (Figure 2A). Where the

COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY 3 An important distinction between unconfined and confined aquifers is the nature of their storage properties—that is, how much water they yield per unit draw- down (or decline in water level). In an unconfined aquifer, water level declines cause drainage (or dewatering) of the pore spaces as the material between the original water table level and the new Figure 2. The connection between groundwater and surface water can occur as lower water table position transitions either a groundwater discharge (A) or a groundwater recharge (B) from saturated to unsaturated conditions. situation. (Source: Winter et al. [1998].) In a confined aquifer, however, draw- down does not cause dewatering (except in extreme cases); instead, the reduction water table is below local surface water, logical and geochemical processes that in storage comes from a combination however, streams or lakes can lose water affected the rock and soil material after of compression of the aquifer (and pore by seepage into the subsurface, thereby its deposition. Many shallow aquifers are spaces) and expansion of water under the recharging underlying aquifers (Figure composed largely of unconsolidated sedi- decreased pressure from pumping. There- 2B). Because of the interconnection ments, including permeable sands and fore, in a confined aquifer the reduction between groundwater and surface water, gravels. These include alluvial aquifers in mass of water per unit volume of aqui- large-scale development of groundwater that parallel rivers and basin-fill aquifers fer per unit volume of water removed by through well pumping may disturb the that contain erosion products from nearby pumping a well is much smaller than for existing groundwater flow system and mountains. Examples of prolific aquifers an unconfined aquifer; consequently, in a affect local or regional surface water composed largely of unconsolidated confined aquifer the effects of pumping resources—generally diminishing surface sediments include the Ogallala aquifer, on water levels will spread much farther water availability. the Central Valley aquifer in Califor- and faster than in an unconfined aquifer nia, the alluvial basins of Arizona, and having a similar permeability. Understanding Aquifers the Mississippi Embayment aquifer in An aquifer is generally accepted as be- Arkansas, Mississippi, and Louisiana. In ing a geologic formation, group of forma- such aquifers, permeability is primarily The Relationship of Drought to tions, or part of a formation that contains from the interconnectedness and size of Groundwater Use sufficient saturated permeable material the pore spaces. Groundwater use has grown signifi- to yield significant quantities of water to Other aquifer systems occur in hard- cantly across the United States over the wells or springs (Lohman 1972). Aquifers rock (or bedrock) formations. In such last century, especially to supply irrigated typically consist of a dominant type of systems the permeability can arise from agriculture. Many factors have led to this rock material. In some aquifers, the top a combination of relatively high pri- increased reliance on groundwater. Tech- of the saturated zone is a free surface at mary (intergranular) porosity coupled nologically, advances in pump technol- atmospheric pressure—the water table. with secondary porosity features related ogy allowed for submersible pumps that These aquifers are called “unconfined” to fractures or openings caused by the can efficiently extract large quantities of or water table aquifers. Other aquifers— dissolution of the rock material as water groundwater from deep below the ground typically deeper ones—are overlain (or flows through these openings. Examples surface. This technology began to be “confined”) by low-permeability forma- include consolidated sandstones with deployed extensively across the United tions, such as clays or shales, and the well-connected pore spaces, such as the States in the 1950s, which coincided with entire aquifer is saturated. Water in such a Dakota Sandstone in South Dakota and rural electrification across the nation that confined aquifer (or “artesian” aquifer) is adjacent states; limestone and other car- facilitated use of submersible pumps. under pressures greater than atmospheric, bonate rock aquifers, such as the Floridan This supported cost-effective use of and the water level in a well drilled into aquifer, which underlies much of Florida groundwater for water supply, especially a confined aquifer can rise to a height and some adjacent parts of Georgia for irrigated agriculture. A second factor above the top of the aquifer. If the pres- and derives its high permeability from increasing groundwater use has been sure is great enough for the water level primary porosity enhanced by intercon- long-term regional droughts, especially in the well to rise above the land surface, nected openings caused by dissolution of in regions with large agricultural sectors. the well can flow freely without a pump. the limestone; and volcanic rock aquifers, Additional factors include over-allocation An aquifer’s ability to yield significant such as the Columbia Plateau and the of surface water and local availability of quantities of water to a well depends Snake River Plain aquifers in the north- groundwater as a “point-of-use” resource on its porosity and permeability, which west, which derive their permeability not requiring expensive distribution depend on the geologic origin of the from both primary porosity and fractures infrastructure. material, the rock type, and the geo- in rock formations. Prior to the 1930s, agricultural produc-

4 COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY tion in the High Plains regions of Texas, levels because of the increased pumping by 2013. In the southern part of the High Oklahoma, and Kansas was mostly to supplement diminished surface water Plains aquifer, water levels have declined dryland farming. Irrigation development supplies (Matthai 1979). more than 50 meters (m) (150 feet) in in the Texas High Plains began during places (Figure 3), resulting in a loss of a major drought in the 1930s, in which more than half of the predevelopment a substantial increase in crop yield was Understanding saturated thickness of the aquifer in some noted in response to irrigation (Musick Groundwater Depletion places (McGuire 2014). Similar problems et al. 1990). The significant expansion of are pervasive in many aquifers across irrigation across the High Plains, how- Groundwater Depletion across the United States and globally. A map ever, occurred in response to the drought the United States of long-term (1900–2008) groundwater of the 1950s, which affected much of Several large aquifer systems in the depletion in major aquifers across the this region and the southwestern United United States are experiencing substantial nation (Figure 4) shows large volumetric States. Advances in drilling and pump- problems from the depletion of ground- losses also occur in the Central Valley of ing technologies, along with financial water. The U.S. aquifer system with the California, the Mississippi Embayment resources made available after World greatest long-term groundwater storage aquifer, the alluvial basins of southern War II, allowed access to a seemingly depletion is the Ogallala aquifer in the Arizona, and numerous smaller aquifer endless supply of water from the Ogallala Great Plains region of the United States, systems—especially in the arid western aquifer that underlies much of the region with nearly 400 km3 (325 million acre-ft) states. (Colaizzi et al. 2009; Musick et al. 1990). Thus many farms converted from dryland (rainfed) to irrigated agriculture with extractions from the High Plains aquifer. The use of water from the High Plains aquifer for irrigation allowed sustained agricultural productivity through the Texas “Drought of Record” (Dethloff and Nall 2018; D. Marble, Personal communication). This pattern repeated itself in the western United States in the 1970s. The 1976–1977 drought impacted large por- tions of the United States, especially the western states. Even though California had constructed extensive water supply reservoirs and canal systems to mitigate the effects of interannual droughts, the drought of 1976–1977 resulted in signifi- cant curtailment of water deliveries to irrigated farmland, especially in the San Joaquin Valley. This drought in California had wide-reaching impacts from the loss of riparian habitat to decreased recreation activities, such as rafting and skiing, that bring revenue to the state. Agriculture losses were estimated to be $510 mil- lion, mostly from nonirrigated acres. As a result of this drought, the state developed a water management plan to address the potential of the drought continuing into 1977 and beyond. This included a conservation plan that directed all users with access to both surface water and groundwater to increase their reliance on groundwater (California Department of Water Resources 1977). The drought resulted in increased groundwater usage Figure 3. Changes in water levels in the High Plains aquifer, predevelopment and ultimately decreased groundwater (about 1950) to 2013. (Source: McGuire [2014].)

COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY 5 level changes in groundwater wells in some of the most severely depleted aqui- fers showed that a large number of the wells (approximately 80%) had declines in water levels from the 1970s through the year 2000, and only a small fraction of the wells (6%) had rising water tables (Tillman and Leake 2010). After the year 2000, however, well after enactment of the GMA (2000 to 2008), about half of the monitored wells continued to have falling water levels, whereas 35% showed rising water levels. This supports the idea that measures Arizona used to remedi- ate the impacts of groundwater depletion worked in some of the AMAs, although increased importation of Colorado River water under the Central Arizona Proj- ect in recent decades complicates the analysis. There is a large variability in the Figure 4. Cumulative long-term volumetric groundwater depletion in the United ability to stop or reverse the depletion of States during 1900–2008 in km3 (modified from Konikow [2013]). groundwater resources among the various Hachured areas are where a shallow aquifer overlies a deeper aquifer. AMAs, however, and recent analysis of trends in groundwater levels indicates Two large aquifer systems in the California has had the highest depletion that depletion of many of the aquifers has Pacific Northwest region of the United intensity since 2000 (Konikow 2015), again increased (Scanlon et al. 2015). States, the Columbia Plateau aquifer with the depletion varying across the (Washington and Oregon) and the Snake aquifer and the most severe depletion oc- River Plain aquifer (Idaho), have had curring in the southern part of the valley. Causes of Groundwater a net accretion of groundwater levels A complete assessment of the rates of Depletion as compared to predevelopment condi- depletion, and depletion intensity, for all tions. In other words, on the whole, more major aquifers across the United States Aquifer Depletion and the groundwater existed in these two aquifer can be found in Konikow (2015). Water Budget Myth systems in 2008 than before major ir- Although groundwater depletion con- When water is pumped from a well, rigated agriculture development occurred, tinues to worsen in many aquifer systems, it causes water levels in the well and primarily from sustained application of some show signs of stabilization. During adjacent aquifer to decline. This draw- diverted surface water to irrigate fields, the 1960s and 1970s, groundwater was down in turn creates a hydraulic gradi- which increased recharge to the under- used extensively in Arizona, largely to ent that induces water to flow from the lying aquifers above that which would supply crop irrigation. This pumping led aquifer toward and into the well—to occur naturally. In these areas, however, to groundwater level declines of more replace water pumped from the well. This the trends in recent decades have been than 100 m (300 ft) in some areas, as well drawdown and water removal also disturb toward depletion—as groundwater as earth fissures and land subsidence of 6 the natural equilibrium in the aquifer and withdrawals have increased, surface m (20 ft) in some areas (Galloway, Jones, the balance between natural recharge and water diversions have remained more and Ingebritsen 1999; Tillman and Leake natural discharge to springs and streams. or less constant, with an increase in the 2010). In 1980, the Arizona legislature Lowering the water level in the aquifer efficiency of delivering these diversions passed the Groundwater Management Act also means that some water has been to irrigated fields, which has resulted in (GMA) to protect shared groundwater removed from “storage” within the pore a decrease in recharge to groundwater resources and to control severe depletion spaces near the well, although much of resources. of groundwater resources; the GMA cre- this storage depletion may be recover- The High Plains aquifer underlies a ated Active Management Areas (AMAs), able when the pump is turned off. If there very large area—about 450,000 km2 (110 where groundwater resources were to are many wells pumping large quantities million acres), and, in absolute terms, has be actively monitored and managed, of groundwater for long periods of time had the highest rate of depletion since including the use of surface water basins from a single aquifer, however, substan- 2000 (Konikow 2015). If we assess the to increase the recharge to groundwater tial recovery may not be possible within a intensity of aquifer depletion by account- systems, often referred to as artificial re- reasonable time period. ing for the size of the aquifer, however, charge basins (Tillman and Leake 2010). Before major groundwater develop- then it turns out that the Central Valley of An analysis of the trends of water- ment (typically, prior to the 1940s), most

6 COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY (a) (b)

Figure 5. Rural family and their well— circa 1930s. (Photo courtesy Large capacity well at a paper mill, of U.S. Department of Agri- St. Marys, Georgia. culture–Natural Resources (Photo by Alan M. Cressler, USGS) Conservation Service.) (c) wells were shallow hand-dug wells with primitive pumps and limited capacity Los Angeles County production well. to produce water (Figure 5). Wells were (Photo by Loren Metzger, USGS) few and far between, with little chance of pumpage from one well interfering with or affecting another well. With few ex- Figure 6. Examples of modern ceptions, the yield of groundwater from a high-capacity wells for well was low enough that any depletion (a) municipal, (b) industrial, and of the groundwater was not of significant (c) agricultural uses. Irrigation well used for flood concern. Sources: (a) Galloway et al. (2003); (b) irrigation of a rice field in the Since the 1930s, well drilling and Kenney et al. (2009); (c) Barlow and Mississippi Delta region. pumping technology have substantially Leake (2012). (Photo by David E. Burt, Jr., USGS) improved, and millions of wells have been drilled throughout the United the natural predevelopment recharge to States and globally—allowing extensive the aquifer does not define the available groundwater development to help meet amount of water that can be sustainably expanding municipal, industrial, and pumped from an aquifer, which used to agricultural use with higher water well be a common belief that is sometimes extractions (Figure 6). The largest use of referred to as “the water budget myth” these groundwater resources is for crop (Bredehoeft 1997; Bredehoeft, Papadopu- irrigation; with 71% of all groundwater los, and Cooper 1982) and has been used withdrawals in the United States in 2010 as the basis for establishing groundwa- being used for irrigation (see Maupin et ter rights in some states (e.g., Nevada). al. 2014). Rather, the manner in which groundwater In a predevelopment groundwater flow can be sustainably pumped from an aqui- Figure 7. Generalized schematic system, the inflows (recharge) are typical- fer must address all three factors, with the illustrating how the sources ly in dynamic balance with the outflows latter two factors together often referred of water to a well may shift (discharge) and the groundwater flow to as “capture” because the groundwater over time (modified from system is in a long-term equilibrium (ex- system is capturing water that would oth- Heath [1983]). cept for fluctuations caused by seasonal erwise not recharge the aquifer or would or annual climatic cycles and short-term otherwise flow out of the aquifer (Leake the aquifer (see Bredehoeft and Durbin precipitation variability). In a developed 2011; Lohman 1972). 2009). As long as no new pumpage or cli- groundwater system, all water pumped When a well is first turned on, all matic changes affect the aquifer system, from a well must be balanced by some extraction is supplied by storage deple- existing withdrawals should be sustain- combination of three factors: (1) removal tion. Over time, more and more of the able. If recharge processes and/or surface of water from storage in the aquifer water usually is derived from capture water resources at the aquifer boundaries (often called depletion); (2) an increase of surface water and less is derived are limited, however, sufficient water in recharge to the aquifer from some from storage depletion (Figure 7). After may not be available for capture to fully other source of water; or (3) a decrease in enough time has elapsed, the fraction of balance the well discharge. In such situ- discharge from the aquifer (e.g., Barlow water extracted from storage will reach ations, the aquifer system cannot reach a and Leake 2012; Theis 1940) to another zero and at that time a new water bal- new equilibrium with the established rate water body or to the atmosphere. Thus, ance equilibrium has been reached for of pumpage, which means storage will

COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY 7 continue to be depleted and water levels result in the overuse of the resource for the scenario in which groundwater use in the aquifer will continue to decline; and a decline in the overall value of the is “optimal” versus one in which ground- such a situation is not sustainable in the resource over time unless measures are water is used with existing controls long run. The actual timing for these taken to restrict use of the resource, such linked to ownership of overlying land. effects depends on several factors, which as regulations to limit or privatize use of The work of Gisser and Sanchez (1980) include the hydraulic properties of the the resource. assumed that wells constructed to pump aquifer, the rate of groundwater pumping, Groundwater is not a completely water from the aquifer never went dry, the location and rate at which aquifers are open resource if its use is restricted to however, which is not always the case recharged from other water bodies, and owners of land that overlie the aquifer. when aquifers are severely depleted. As the distance of the pumping wells to these Because groundwater movement within discussed in MacEwan et al. (2017), the water bodies. an aquifer can cross land ownership capital costs associated with drilling new, Drought can exacerbate the problem in boundaries, it does have the charac- or deeper, wells are significant and indi- several ways. During periods of drought, teristics of a CPR. Thus when private cates that avoiding these costs can play less surface water is available for distri- ownership of a groundwater resource an important role in limiting the overdraft bution to farmers, as well as decreased within an aquifer is not clearly defined, in depleting aquifers. Thus, MacEwan et precipitation falling directly on their or regulations have not been developed to al. (2017) suggest that when the depletion fields, which leads them to increase their promote sustainable use of groundwater, of aquifers leads to dry wells, the optimal groundwater use for irrigation, if pos- there is a tendency to use groundwater allocations of groundwater can provide sible. If streamflow adjacent to aquifer in a nonsustainable manner and thereby substantial economic benefits (or avoided boundaries is decreased or other surface deplete the resource. Over time, overuse costs), contrary to the findings of Gisser water is depleted during drought, the po- of groundwater can be a somewhat self- and Sanchez (1980). tential for recharge to help balance well correcting problem, because the costs of The High Plains aquifer of West Texas withdrawals will be lessened and more groundwater extraction tend to increase was analyzed (Nieswiadomy 1985) using pumpage will be balanced by storage as water levels drop. Thus the costs of the approach developed by Gisser and depletion, which accelerates water-level extracting groundwater can exceed the Sanchez (1980) and concluded that “the declines in the aquifer. Thus groundwater benefits for some lower-value water uses, benefits from groundwater management pumping directly affects the extent to which reduces overall pumpage from the most likely are small for the Texas High which depletion occurs, with the rate and aquifer. This behavior is seen in por- Plains, especially relative to any reason- extent of groundwater depletion signifi- tions of the High Plains aquifer in Texas, able cost of regulating pumping.” This cantly impacted by changes to land use or Oklahoma, and Kansas; the Pecos aquifer result was similar to that for the Pecos surface conditions that impact groundwa- in Texas; and the Central Valley aquifer aquifer analysis, referred to as the Gisser- ter recharge and discharge processes. in California (Maupin et al. 2014). Sanchez effect (GSE), which has been Left unregulated, the sustainable use observed in several aquifer management Impacts of Managing of groundwater could be achieved in an studies. These studies were reviewed by aquifer if some increase in groundwater Koundouri (2004), who found that the Groundwater as a Common recharge occurs and the cost of ground- most important cause of GSE is the sig- Pool on Depletion water extraction becomes high enough nificant benefit to agriculture compared to The fact that the flow of groundwa- that groundwater pumpage is decreased the costs of developing groundwater for ter occurs across ownership boundaries to the point of a dynamic equilibrium of irrigation. In semi-arid regions, water is a (whether boundaries separate individual the groundwater table. This laissez-faire major factor limiting agricultural produc- ownership, water management districts, approach to managing groundwater, tion, with irrigation greatly increasing states, or even sovereign nations) indi- however, does not prevent groundwater agricultural yields and profits (Ward and cates that groundwater is a nonexclusive depletion, nor has it been advocated as Michelsen 2002). This is especially true or common pool resource (CPR). A CPR a viable policy by any state. Thus an for water applied during critical crop is an economic term used to identify important policy question remains—Is growth periods. finite resources whose characteristics there an optimal method to allocate water The GSE effect often means that a make it difficult to exclude persons from to users that will improve the economic strict regulatory approach to addressing obtaining benefits from its use, even if benefit of groundwater use of an aquifer? the CPR issue for groundwater may not they don’t provide support to manage or Studying the Pecos aquifer in eastern give the best economic outcome for areas sustain the use of the resource. Examples New Mexico, Gisser and Sanchez (1980) that rely heavily on groundwater, espe- of CPRs include open ocean fisheries and found that the economic benefits of opti- cially for high-value uses. An alternative open pasture used by multiple livestock mally managing groundwater were insig- approach of addressing the CPR problem owners. The difficulty in sustaining the nificant. Their economic analysis showed is through privatization of groundwater use of a CPR is that individuals may seek a near-identical height of the water pumping rights. This can be done through to maximize their benefit from using the table (and by implication the amount of an allotment or allocation process in resource without regard to long-term groundwater depletion) at a steady state which private entities have fixed alloca- maintenance of the resource. This can and near-identical future income streams tions of groundwater over a specified

8 COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY period of time. This approach is used connected to surface water bodies, how- tems, and they often provide substantial in Nebraska through the use of natural ever, the use of groundwater during one recharge to underlying aquifers. As pres- resource districts (NRDs) that typically year increases infiltration of surface water surized irrigation technologies such as allocate water on a five-year basis and to recharge the depleted groundwater in sprinkler or drip irrigation have evolved, occasionally allow some “banking” or future years. Thus the long-term impact along with a better understanding of crop carryover of water across allocation peri- of groundwater pumping on future avail- water needs, the conversion to more ef- ods. In the High Plains aquifer region of ability of surface water will decrease both ficient irrigation methods has decreased Texas, groundwater conservation districts surface water and groundwater resources recharge to underlying groundwater. (GCDs) establish pumping limits as part over time. Recognizing the impact of Increasing water delivery system efficien- of their management plans to achieve groundwater pumping, and depletion, cy to agricultural fields and increasing adopted desired future conditions; these on the future availability of water in irrigation efficiency can reduce recharge limits are to be revisited periodically both surface and groundwater systems to aquifers that underlie irrigated agricul- to assess their effectiveness, and they is needed to effectively administer water tural regions (e.g., see Maurer and Berger may be modified as needed (NPGCD rights in states that follow the prior ap- 2006). In some cases, this reduction in 2016). Assessment of the effectiveness of propriations doctrine. recharge due to changes in how surface groundwater management plans generally What is less recognized is the role water is managed can exceed historical includes groundwater availability model- of surface water management on deple- pumpage rates in the aquifers, resulting in ing and annual water level monitoring tion of groundwater for aquifers that a decline in the groundwater table and in (TWDB 2017). The Kansas Groundwater are hydrologically connected to surface essence depleting groundwater resources. Management Districts are now doing water systems. Many areas that use this after a change in state law. Diamond surface water to irrigate agriculture crops Agricultural Financial Policy Valley, Nevada, also has a “water rights” recharge to nearby aquifers as a direct bundling experiment (see Young n.d.), result of either conveying water (espe- Impacts on Groundwater and Idaho has a program to recharge the cially via earthen-lined channels) to the Depletion Snake River Plain aquifer. fields or applying it to the fields at rates Decisions about crop, soil, and irriga- that exceed crop evapotranspiration. tion management are complex, affected Relationship of Surface Water Open-channel water delivery systems by many important factors, including typically have some amount of water producer experience, preferences, avail- Use to Groundwater that seeps into the ground, with seepage able equipment and labor capabilities, Depletion—The Conjunctive rates varying significantly depending on and a variety of financial considerations Management Problem characteristics, setting, and operation of (crop insurance options, credit con- The conjunctive management of the delivery system. Some systems have straints, landowner/tenant relationships, surface water and groundwater resources seepage as high as 75% of the amount of availability and requirements of loan has different administrative definitions the diverted water. This seepage is often and cost-share programs). Of special depending on how each state defines its a large source of recharge to groundwater note are crop insurance requirements, water rights. Some states manage surface resources in areas such as the Eastern because in most cases commodity crops water and groundwater as a single inte- Snake River Plain and Treasure Val- can be insured as either “irrigated” or grated resource (e.g., Idaho and Kansas). ley aquifers in Idaho and the Deschutes “not irrigated”—with pilot programs Other states use groundwater more as a Basin aquifer in Oregon. When these addressing “limited irrigation” not being supplemental source when surface water delivery systems are made more efficient widely available. Insurance of “irrigated” supplies are lacking (e.g., Nevada), while from the water delivery perspective (i.e., crops requires adequate facilities and a others administer groundwater as sepa- seepage is lessened because of canal reasonable expectation of receiving water rate privately or state-owned resources lining or replaced by closed conduits), it adequate for “good irrigation practice,” (e.g., Texas, California, and Oklahoma). decreases the recharge to the aquifer and as well as other requirements and condi- Managing surface water and groundwater water tables can drop. tions (USDA–FCIC 2017). Often this is as a single resource, such as the water Another source of groundwater interpreted to mean that during drought, rights framework in Idaho, can help recharge is through the irrigation of irrigation must continue through the crop ensure that water resources are managed cropland, where applied water not used season to meet “good irrigation practice” in a sustainable and predictable way, be- by crops can seep below the crop’s roots, and therefore ensure that drought-related cause junior water users are required to downward to an underlying aquifer. His- crop loss is due to an unexpected shortfall stop diverting when there is not enough torical irrigation practices tended to have of normally expected rainfall rather than water for everyone’s needs, regardless of a higher fraction of applied water seeping failure to use the irrigator’s full allocation the source of water. below a crop’s root zone, which the ir- of water. Using groundwater resources when rigator sees as a less efficient use of water During extreme drought, however, surface water is no longer available (e.g., see Urban 2004). These historical irrigation sometimes may be discontinued can help ensure water availability. If a irrigation practices include surface flood or diverted to salvage crops, with prior groundwater system is hydrologically as well as furrow and subirrigation sys- approval from the crop insurance

COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY 9 company (USDA–RMA 2011). The ing the dry summer months. In addition, to a “well-drilling boom” in the Central requirements to follow good irrigation flow from aquifers is typically more Valley, increasing associated problems practices can provide incentives for farm- moderate in temperature than water flow- including tensions between neighboring ers to use their full allocations of ground- ing on the surface, and it is also typically landowners and local land subsidence. water, even for situations in which the cleaner because it is filtered as it flows Even municipal water supply custom- full allocation still results in crop failure, through aquifer soils. In summer, when ers anticipating restrictions on water use because this is perceived as a requirement surface water supplies are typically at due to the enaction of the Sustainable for a farmer to receive crop insurance their lowest, this cool and clean supply Groundwater Management Act (SGMA payments. Although this is not a primary can help maintain habitat for fish and 2014) have resorted to drilling additional driver of groundwater depletion, it can re- other aquatic species, in addition to sup- wells. This increase in well-drilling activ- sult in the nonproductive use of ground- porting human recreation. In the winter, ity has raised concern over groundwater water, thereby increasing groundwater the addition of flow may be warmer than depletion (Galbraith 2012) and results in resources depletion. that on the surface and can again improve an increased cost for developing ground- the aquatic habitat. water resources for use as water supplies onsequences of Spring flows also are an important for all groundwater users. C source of base flow. In southwestern Ida- Depleting Aquifers ho, spring flow from Thousand Springs Subsidence of Land and Although a large direct consequence supplies moderate temperature and clean of depleting groundwater resources is water to fish hatcheries. Groundwater Ground Failures the loss of water supply, many other pumping and decreased recharge have As groundwater is extracted from an consequences of depletion also must be led to declines in aquifer levels, with aquifer, the materials that comprise the considered. These include impacts of some springs flowing at a decreased rate, aquifer and aquitard (confining clay/silt groundwater depletion on hydrologically resulting in hatcheries no longer having beds) can compact, which can cause the connected surface water bodies, how de- enough water for aquaculture production. land surface to move in both vertical and clining groundwater levels can decrease They cannot use nearby water from the horizontal directions. In addition, ground- well productivity, how falling water Snake River because of its lower quality water pumpage can decrease the volume tables can cause subsidence of lands that and temperature variation for aquaculture of water in an aquifer, which can lead overlie an aquifer, and impacts of declin- production. to land settlement or subsidence. When ing groundwater levels on aquifer water groundwater is pumped from the aquifer, quality through leakage of poor-quality pore pressure decreases and the effec- water from adjacent aquitards or from Loss of Productivity of tive stresses between sediment particles sea-water intrusion. Furthermore, declin- Groundwater Wells increase (total stress minus pore pressure ing groundwater levels will increase the For unconfined aquifers, a decline as defined by Terzaghi [1943], assuming energy requirements and pumping costs, in the water table results in a decline total stress is constant). This increase in and drilling deeper wells to access the re- in the saturated thickness of an aquifer. effective stress causes the volume of the maining groundwater may be very costly. As the saturated thickness of an aquifer aquifer to decrease. decreases, there is less area for ground- Aquifers that contain a high propor- water to flow from the aquifer into a well, tion of clays, especially as clay lenses, Reduced Flow to Surface which reduces well productivity. At some are highly susceptible to compaction, be- Water Systems and point the saturated thickness decreases to cause pore water slowly drains from these Ecosystems reduce well productivity below the water lenses from the surrounding deformable Groundwater can be a source for needs of a water user, which leads to sediments. This is accompanied by a surface water if the water in the aquifer the need to redrill (deepen) the existing reduction of the voids between soil par- has an outlet point on the surface. This well or construct additional newer wells ticles and a gradual transfer of effective can include base flow to streams, spring in more favorable aquifer locations to stress from the pore water to the stress flows, and inflows to lakes and wetlands. supplement or replace the original well. between soil particles for the porous When groundwater levels in an aquifer In West Texas and the Texas South- sediments (Li and Sheng 2011). Once decline to below the level of surface ern High Plains, declining groundwater the compaction of clayey material has water bodies, or are no longer in contact resources have often decreased well ca- occurred, it cannot simply be reversed by with surface outlet points, groundwater pacities and led to increased well drilling. recharging the aquifer with the amount of can no longer replenish surface water In this area, it is common for producers water that was extracted. This is because bodies but rather drain surface water to to combine water from multiple wells to the deformation of the fine-grained (clay recharge groundwater. provide sufficient water flow for a single or silt) material is mostly inelastic; that The base flow to streams is often a center pivot or subsurface drip irrigation is, once the soil particles are compressed critical resource in the western United system. California irrigators also have and the stress between the soil particles States, especially for ecosystems, because begun drilling more wells to replace lost has reached a new equilibrium, to expand this may be the only stream habitat dur- surface water (Richtel 2015), leading the soil particles would require a much

10 COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY greater soil water pressure to separate fornia, Arizona, Nevada, New Mexico, subsidence (Figure 9) (Faunt et al. 2016; the soil particles to reach the original Texas, and other areas. In Las Vegas, Galloway, Jones, and Ingebritsen 1999). porosity of the material. The nonrecover- Nevada, land surfaces have subsided Ground failure, including surface able and cumulative vertical deformation about 1.9 m (6 ft) in some areas (Bell and faulting and earth fissures, also can occur of the different types of sediments in an Price 1993; Bell et al. 2002), with some from groundwater pumping and land aquifer manifests itself as land subsid- areas of Houston, Texas, seeing around subsidence (Li and Sheng 2011; Sheng, ence or settlement of the land surface. 3 m (10 ft) of land subsidence (Figure Helm, and Li 2003). Surface faults have Note that coarse-grained material, such 8) (Kasmarek 2013). Parts of Phoenix, a dominating shear component, whereas as sand and gravel, is often assumed to Arizona, have seen more than 5.8 m (18 earth fissures are primarily soil tensile deform elastically. ft) of subsidence (Galloway and Burbey failure. An earth fissure typically starts Land subsidence from groundwater 2011; Miller and Shirzaei 2015), and in as a small (mm-scale) tensile crack in depletion has occurred in many areas of California’s San Joaquin Valley, some the subsurface sediments, then grows the U.S. arid southwest, including Cali- areas have seen 9.5 m (29 ft) of land upward because of mechanical piping and additional pumping stress, and eventually reaches the ground surface after breaking the sedimentary cover. Land subsidence and ground failures are closely related to

Figure 9. Land subsidence in San Joaquin Valley, California (Galloway, Jones, and Ingebritsen 1999). The person Figure 8. Predicted (1891–2009) and observed (1906–2000) subsidence in Houston, standing by the post in the Texas (Kasmarek 2013). photo is Dr. J. F. Poland.

COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY 11 aquifer movement caused by groundwater height of the groundwater table above sea igate or reverse groundwater depletion. pumping (Helm 1994). It should be noted level (Ghyben-Herzberg principle [Freeze This ranges from more direct approaches that ground failure is not necessarily the and Cherry 1979]). Such an interface is that attempt to address the hydrologic direct result of land subsidence (Li and typically in an equilibrium under non- imbalance in aquifers that lead to ground- Sheng 2011; Sheng, Helm, and Li 2003). disturbed conditions (no groundwater water depletion (e.g., decreasing pump- Ground failures caused by groundwa- pumpage). age or increasing recharge), to changes ter pumping have been observed in the When fresh groundwater in the coastal in how pumped groundwater is managed, Fremont Valley, San Jacinto Valley, and aquifer is pumped, the reduction in the to the better alignment of water manage- other areas in California (Holzer 1984; groundwater level near the well causes ment institutions and policies at local Shlemon and Davis 1992; Shlemon and the salt/freshwater interface to move and state levels to encourage sustainable Hakakian 1992); Las Vegas Valley, Ne- upward. With continued groundwater groundwater use. vada (Bell and Price 1993, Sheng, Helm, extraction, the salt/freshwater interface and Li 2003); southern Arizona (Carpen- will migrate upward and inland and can ter 1993; Jachens and Holzer 1982); and degrade groundwater quality as the saline Physical Approaches to Escalante Valley, Utah (Lund et al. 2005). water mixes into the original freshwater Mitigating Depletion Both land subsidence and ground fail- zone. An example of seawater intru- The most direct approach to decreas- ures have damaged buildings, roads, and sion is in the Central and West Coast ing the depletion of groundwater is to utilities, resulting in costly repairs needed Basins in southern California, where simply extract less groundwater from to maintain and rebuild infrastructure. groundwater pumping has brought large aquifers. With agriculture being the larg- The lowering of land levels can also re- groundwater-level declines (Hendley and est user of groundwater in the United sult in the changing of land slopes, which Stauffer 2002; Johnson and Whitaker States, this means that any reduction can have negative impacts on a variety of 2003; Reichard et al. 2003). Groundwater in groundwater pumpage for irrigating water infrastructure. Land subsidence can development in the Los Angeles area ini- cropland could decrease groundwater affect the drainage of water from flood tially supported irrigated agriculture and depletion. Irrigators have become highly events, which can result in flood drain- is now predominantly used for municipal savvy in water-limited regions in regard age infrastructure becoming ineffective, water supplies. To mitigate problems of to their water use, applying new tech- thereby increasing the impacts of flood- seawater intrusion, several management nologies to manage their water allotments ing events. In addition, land subsidence measures have been employed, includ- to maximize crop yields and, ultimately, can result in lowering of the slope of ing freshwater well injection, surface profits. One widely adopted technique in open water canals, which decreases their spreading, and pumping restrictions to areas that rely heavily on groundwater for operating capacity for delivering irriga- halt saltwater intrusion into freshwater irrigation combines advanced application tion water to croplands. aquifers. Surface water and increased technology with an understanding of the use of reclaimed water have been used spatial and temporal crop water needs. to replace the decreased availability of Weather, crop type, and soil conditions Degradation of Groundwater groundwater supplies (Johnson 2007; can help optimize water use by a crop to Quality Reichard et al. 2003; WRD 2018). Inland produce a reasonable yield. Groundwater in coastal aquifers, or aquifers, such as the Hueco Bolson, that Networks of automated weather sta- that overlies brackish groundwater, may are underlain by brackish aquifers lack tions—such as Agrimet, ASMET, CoAg- also face degradation of groundwater as sharp a salt/freshwater interface as Met, AgWeatherNet, and CIMIS—collect quality from intensive groundwater coastal aquifers. Intrusion of the underly- weather data for agricultural regions and pumping (Sheng and Devere 2005). ing brackish water, however, also can use it to compute reference crop evapo- Wherever a freshwater aquifer is hydrau- occur as fresh groundwater in the aquifer transpiration (ET), which is used with lically connected to large bodies of brack- is depleted (Sheng and Devere 2005) and research-based crop-specific coefficients ish or saline water, such as the ocean, brackish groundwater begins to mix with to estimate crop water requirements. pumping fresh groundwater can induce the freshwater. Depleting fresh ground- Irrigators can use ET-based information saltwater to flow into the aquifer to re- water will accelerate the degradation of to match irrigation applications (rates place the extracted freshwater. Saltwater groundwater quality and cause further and timing) to crop water needs, thus is denser than freshwater, which drives loss of fresh groundwater supplies. decreasing over-irrigation without undue seawater to move inland under freshwa- risk of crop yield loss. This practice can ter in an aquifer. Thus coastal aquifers have both positive and negative effects will typically contain freshwater floating Mitigating the on groundwater, depending on where nearer the ground surface and saltwater Consequences of the water is coming from. If the irrigator deeper in the aquifer, with an interface previously irrigated with a less-efficient between salt- and freshwater usually be- Groundwater Depletion practice and used surface water, some low sea level. As a general rule of thumb, There is a growing recognition of the of the excess delivery becomes aquifer this depth to the salt/freshwater interface consequences of groundwater depletion. recharge; eliminating this recharge will is typically forty times greater than the This has led to several approaches to mit- decrease recharge to the underlying aqui-

12 COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY fer. If the irrigator is pumping groundwa- ter, this technique could reduce pumping from the aquifer, decreasing groundwater depletion and the farmer’s costs. Another direct approach to arrest- ing groundwater depletion is to enhance groundwater replenishment using alternative water sources, by either directly injecting water into an aquifer or by increasing recharge using other water sources. Managed aquifer recharge (MAR) has been used to mitigate the ef- fects of groundwater depletion in several areas (Dillon 2005; Pyne 2005; Sheng 2005; Sheng and Zhao 2015). Aquifer recharge typically falls into two catego- ries—passive recharge and groundwa- Figure 10. Flow trends in the Snake River below Thousand Springs, Idaho. (Data ter injection—with the cost and legal sources: U.S. Geological Survey National Water Information System.) restrictions being very different for each category. Passive recharge occurs when this time, water was primarily delivered annual equilibrium (Urban 2004). This water is placed on the ground surface or by unlined canals and applied to cropland is largely from the irrigation delivery in a shallow infiltration gallery and is al- using flood irrigation, resulting in signifi- system contributing to aquifer annual lowed to seep into the aquifer by gravity cant aquifer recharge. recharge. When the canal system was flow. Because the infiltration is similar to Around the middle of the 20th century, constructed and implemented in the late natural processes (i.e., rain falling on the however, the annual low monthly flows in 1800s, it was noted that groundwater lev- ground and seeping into the ground), the the Snake River below Thousand Springs els increased by as much as 30.5 m (100 permitting process is typically more le- began to fall, along with the discharge of ft) in some parts of the valley (Petrich nient than injecting water into an aquifer, groundwater from the East Snake Plain 2004). As of this writing, groundwater where water quality is a higher concern. aquifer to the Snake River. This decline pumping has not exceeded the recharge Groundwater injection typically requires in low flows and groundwater discharge resulting from delivery and application of a well and typically needs more energy to the Snake River coincided with im- surface water to fields, and recharge has when water is forced into a confined proved groundwater extraction technolo- not been decreased by lining or piping aquifer. In many cases, water must be gies, resulting in more irrigators sourcing substantial portions of the delivery sys- treated prior to injection to ensure that the their water from groundwater and declin- tem. This system presents an opportunity interaction of the injected water and the ing groundwater levels in the Eastern for thoughtful planning when creating aquifer soils does not clog the injection Snake Plain aquifer. In addition, surface efficiencies to maintain the balance that well and to meet water quality standards water irrigators improved their delivery currently exists while improving the for aquifers that can potentially be used and application efficiency through the delivery and application of water. for drinking water. use of lined canals and sprinklers, which The passive form of MAR has oc- further decreased aquifer recharge and Agricultural Management curred for many years in the western caused groundwater levels to continue to United States with the delivery and ap- decline. Because many water rights were Approaches to Mitigating plication of irrigation water. An interest- issued when groundwater levels were Depletion ing example is in the Eastern Snake Plain higher, there is now a deficit of about Another method to decrease ground- aquifer in Idaho. Figure 10 depicts the 0.75 km3 (600,000 acre-ft) of water annu- water depletion is through changes to annual-monthly low flow at the King Hill ally (Idaho Water Resource Board 2009). crop selection and agricultural prac- gage, below Thousand Springs (from In recent years, to counteract declining tices. Changes in cropping in the Texas 1910 to present), along with the differ- aquifer levels, the State Water Board in High Plains reflect responses to limited ence in flow in the Snake River between Idaho began a managed recharge program and declining (quantity and quality) the Buhl gage (mostly upstream of Thou- to use unlined canals and infiltration groundwater. In the Northern Texas High sand Springs) and the King Hill gage ponds during the nonirrigation season to Plains, grain corn is the predominant (1950 to present). Figure 10 shows that increase recharge by as much as 0.31 km3 irrigated crop, whereas in the Southern the trend in annual low monthly flow was (250,000 acre-ft) per year. High Plains where aquifer storage and increasing from the early 1900s through The Treasure Valley aquifer in Idaho is well capacities are more limited, more the 1950s, which corresponded with an example of how managed recharge oc- drought-tolerant crops, including cotton, groundwater wells showing increased curs as part of an irrigation and delivery grain sorghum, and winter wheat, are water levels over the same period. During system, maintaining groundwater in an prevalent (Colaizzi et al. 2009). Applied

COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY 13 research programs in the region evalu- and strategies and more drought-tolerant while not often explicitly stated, this ate—and regional water planning efforts crops and varieties has been encouraged implies that groundwater must be used advocate—water conservation strategies, by water-limited conditions (limited well in a sustainable manner under the prior including conversion to higher efficiency capacities), availability of low-interest appropriations doctrine. irrigation technologies, data-based irriga- loan programs and cost-share programs, The governance structure for ground- tion scheduling, changes in crop types to and suitability of the technologies to the water under the prior appropriation less water-demanding or more drought- local production systems. Where water doctrine is in a state water agency (e.g., tolerant crops and varieties, conservation saved by improved irrigation efficiency the Idaho Department of Water Re- tillage methods, and conversion from is simply used to irrigate more acreage, sources), where disputes of access and “full” irrigated production to limited ir- however, overall consumptive use and use of groundwater are resolved through rigation or dryland (rainfed) production aquifer depletion can increase (Grafton et administrative or legal proceedings. The (Amosson et al. 2016). Adoption of these al. 2018). management of groundwater in these strategies in the High Plains region of states is typically more localized, where Texas has led to significant reductions in Policy and Institutional decisions on the timing of water use are groundwa­ter pumpage, where improved at the discretion of individual water users irrigation scheduling for corn has reduced Approaches to Mitigating or groundwater districts. On the surface, water use by 5.1–7.6 hectare-cm per hect- Depletion the underlying principle of requiring that are (2–3 acre-inches per acre); adoption Since each state has primacy over its water be used in a “sustainable” manner of more efficient irrigation application water resources (in essence allowing each would appear to prevent the depletion methods (e.g., conversion from furrow to state to set its own rules on access and of groundwater resources. In practice, low-pressure center-pivot irrigation) has use of its water resources), a wide range however, governing access and use of decreased water use by 3.3–8.9 hectare- of policy and institutional approaches has groundwater under the prior appropria- cm per hectare (1.3–3.5 acre-inches per developed to address groundwater deple- tion doctrine has not addressed ground- acre); and changes in cropping systems tion across the United States. These ap- water depletion for three primary reasons. from corn to cotton, wheat, and grain sor- proaches tend to fall into three categories: First, the prior appropriations doctrine ghum production have decreased water • Centralized regulatory control by typically includes a rule for the consistent use by 19.8–21.8 hectare-cm per hectare including groundwater under prior ap- beneficial use of water. If the water right (7.8–8.6 acre-inches per acre) (Amosson propriations doctrine holder does not use water in a beneficial et al. 2016). • Decentralized regulatory control by al- manner, they can lose their future water In some cases, producers seek to lowing quasi-independent groundwa- use right. Often referred to as the “use it concentrate limited irrigation supplies on ter management agencies with limited or lose it” rule, this provides incentive to smaller acreages of higher value crops, regulatory authority use water by a groundwater rights holder, such as wine grapes (Latzke 2017). • Financial incentive programs even in wet years when groundwater is Considerations of whether or not to adopt Each approach has advantages and not needed, or profitable, to produce a cover crops include weighing the relative disadvantages, and no one approach can crop. This increases long-term groundwa- value of improved soil conditions (soil be applied to all groundwater depletion ter extraction and exacerbates depletion health) afforded by cover crops against situations. All states in the continental of groundwater. the water demand for establishing the western United States (west of the 100th The second prior appropriations doc- cover crops. Agricultural research pro- meridian) use the doctrine of prior ap- trine issue is that its governance structure grams are advancing—and producers are propriations for allocating surface water does not address groundwater resources adopting—drought-tolerant, short-season, rights. All of these states—except Cali- in nonreplenishable conditions, such as and salt-tolerant varieties and irrigation fornia, Nebraska, Oklahoma, and Texas— the southern High Plains aquifer. Strict management strategies to lower water use also use the doctrine of prior appropria- interpretation of the prior appropriations and increase water use efficiency while tion to allocate rights to use groundwater. doctrine would severely curtail pump- maintaining yields and quality (Xue et al. The underlying rules of the doctrine of age throughout the southern High Plains 2017). prior appropriation are that (1) no junior aquifer because recharge rates are a small By 2012, an estimated 87% of ir- water user may negatively impact a fraction of current water use and the cur- rigated crop acreage (or about1.6 million senior water user, and (2) water must be rent groundwater use is not sustainable. hectares [3.9 million acres]) in Texas was used in a beneficial manner. If the first Because rights to pump groundwater irrigated with low-pressure center-pivot rule is to be met, there is an underlying were granted long ago (many more than irrigation (Wagner 2012); an estimated assumption that there is a sustainable 50 years old) and significant investments 101,171 hectares (250,000 acres) of amount of groundwater use associated were made to develop an agricultural subsurface drip irrigation had been with an aquifer, because any use beyond economy in the region, accommodations installed by 2004 (Colaizzi et al. 2009), this sustainable amount would negatively have been made within the application of increasing to more than 141,640 hectares impact the ability of senior water users the prior appropriations doctrine to allow (350,000 acres) by 2015. Adoption of to receive future water allocations if an for continued use of groundwater within more efficient irrigation technologies aquifer is being depleted over time. Thus, the High Plains aquifer, even though

14 COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY depletion of the aquifer is continuing. to address the broad range of groundwa- can be complex to describe and under- The third issue with the prior appro- ter depletion issues. It appears to only stand. A brief case study helps illustrate priations doctrine is that its governance address groundwater depletion issues this topic. structure does not address the wide range in a reactive fashion, however, because of consequences caused by groundwater groundwater regulatory entities have The Causes and Conse- depletion—only directly related conse- tended to be established in response quences—impairing other water right to groundwater depletion after it has quences of Groundwater holders. Thus, the use of the prior ap- become a concern, and the approach does Depletion in the Pumpkin propriations doctrine does not address the not provide a mechanism to prevent the Creek Watershed issues of water quality degradation and consequences of groundwater depletion The Pumpkin Creek watershed is a land subsidence caused by groundwater before they occur. small area of the High Plains aquifer in depletion. A final approach to help decrease the Nebraska Panhandle, primarily in California, Nebraska, Oklahoma, and groundwater depletion is through fi- Banner and Morrill Counties (Figure 11). Texas have taken a different approach nancial incentives that encourage water Pumpkin Creek is a small tributary to for developing frameworks to govern conservation. These include low-interest the North Platte River. The creek begins access to and use of groundwater. These loan programs (TWDB 2016, 2018) and about 8 miles east of the Wyoming- states treat access to groundwater as cost-share programs (USDA–NRCS Nebraska state line and flows east for correlative rights, in which a land owner 2016, 2018) to financially assist agricul- about 30 miles to its confluence with has the right to capture groundwater tural irrigators in converting from lower- the North Platte River near Bridgeport, beneath their land for a beneficial use, efficiency to higher-efficiency irrigation Nebraska. Flows to the North Platte River which is often referred to as the rule methods. These can include replacing ranged from 67,500 to 100,000 m3/day of capture. This approach has led to a furrow irrigation with low-pressure preci- (100,000 to 150,000 ft3/hour) from before variety of issues caused by the depletion sion irrigation systems, such as center- 1930 to the mid-1960s (Sievers 2005). of groundwater resources, because in its pivot and subsurface drip irrigation, The aquifer in this watershed is limited initial form groundwater use was almost in addition to the use of soil moisture and groundwater is contained primarily completely unregulated. The issues that sensing and ET-based irrigation schedul- in Quaternary Period (2.6 million to 11.7 have occurred across Texas include land ing. Payment programs that incentivize thousand years ago) alluvium deposited subsidence along the Gulf Coast Region, temporary or permanent conversion of along Pumpkin Creek and its tributaries. to decreases in spring flows for critical irrigated land to dryland production or The alluvium is sand and gravel deposited habit in Central Texas, to drying up of taking marginal lands out of production in a channel approximately 0.8 km (0.5 wells in the Texas Panhandle. (USDA–FSA 2017) are also being used mile) wide with a maximum thickness of To address “unregulated use,” these to decrease the extraction of groundwater 30.5 m (100 ft). The sands and gravels states have developed frameworks to from the aquifer. These approaches have have excellent hydraulic conductivity, require decentralized groundwater regula- the largest positive impact on decreas- and irrigation well yields can exceed tory authorities and management. Within ing groundwater depletion for agricul- 3,780 liters per minute (1,000 gallons Texas these are called GCDs (TCEQ tural areas irrigated using groundwater per minute) in Morrill County where the 2017), typically formed along county resources because they provide financial alluvium is thicker. The underlying Brule political boundaries; within California incentives to extract less groundwater Formation (Oligocene Epoch, 33 to 23 these are being developed as groundwa- with little to no impact on surface water million years ago) is a thick sequence, ter sustainability agencies (California supplies. Conversely, these incentives can 120–210 m (400–700 ft) of siltstone that Department of Water Resources 2018), have a negative impact on groundwater is considered bedrock except where it is with administrative boundaries that align depletion if used to increase irrigation fractured or has isolated lenses of sand more to hydrographic than general politi- efficiency for agricultural areas using and gravel. The fracture zones of the cal jurisdictions; and within Nebraska surface water resources because this can Brule Formation have excellent hydrau- they are referred to as NRDs to permit lead to decreased groundwater recharge, lic conductivity but low storage (Sibray the development and use of groundwater as is noted earlier (Grafton et al. 2018). and Zhang 1994). The fracture zones are wells within an aquifer or region. These found closely proximate to the alluvium localized, or regionalized, groundwater of Pumpkin Creek. regulatory entities are responsible for de- Case Study on Causes, Irrigation wells developed in the veloping metrics of groundwater condi- fractured Brule Formation can yield tions that address impacts of groundwater Consequences, and large quantities of water where there is a depletion and establish limits on ground- Mitigation of good hydraulic connection to saturated water extraction for groundwater users alluvium with good storage character- within their jurisdiction, subject to review Groundwater Depletion istics. Fractured Brule Formation wells and approval by state water agencies. The causes of groundwater depletion, with little or no hydraulic connection to This approach allows groundwater its consequences, and the development of saturated alluvium will quickly lose yield regulatory and management structures effective measures to mitigate depletion during the growing season (Smith and

COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY 15 because of unfavorable geology and were (a) no longer able to irrigate. During the 1990s, precipitation was above aver- age—380–430 mm/yr (15–17 in/yr)—but flows in Pumpkin Creek continued to decline. By 1999, the Nebraska Depart- ment of Water Resources recorded zero flow at the Banner/Morrill County line. In 1939, there was as much as 0.4 m3/second (14 ft3/second) of flow at this measuring station (T. Hayden, Per- sonal communication). Beginning in the year 2000, drought greatly exacerbated the situation in Pumpkin Creek and groundwater level declines became noticeable in areas where the surface water was depleted; the (b) North Platte Natural Resource District placed a moratorium on new irrigation wells in the Pumpkin Creek area. At that time, there were 587 registered irriga- tion wells in the watershed (Figure 11b). Because of highly variable precipitation, allocations are now given on more than a five-year water year basis. Pumping capacity of individual wells is highly variable because of the thin and variable thickness of the aquifer. As a result, in many irrigated fields, pumping capacity is less than allocation and the regulatory burden is on the better producing wells. Because of drought and groundwater Figure 11. Groundwater pumping wells in Pumpkin Creek Watershed (a) circa 1965 pumping, Spear T Ranch, just east of the and (b) current (2001). Banner/Morrill County line, could no longer provide water for cattle. In 2003, the Spear T Ranch sued 23 upstream Souders 1975). The age of groundwater creased through the summer. Beginning groundwater irrigators in a case that went in the fractured Brule Formation had a in the 1960s, drought and the adoption of to the Nebraska Supreme Court. In 2005, median value in the 1970s, whereas the center-pivot irrigation technology greatly that court decided that surface water water from the alluvial aquifer varied expanded irrigated acreage using ground- users could sue groundwater irrigators from 1980 to modern (Steele et al. 2005). water. In the 1970s, surface water irriga- for damages, provided they could prove These age dates indicate the groundwater tors were experiencing insufficient flows. unreasonable interference. In 2004, resource is relatively renewable. Because The Nebraska Department of Water due to the drought and potential legal of the high hydraulic conductivity of Resources, now Department of Natural conflict between surface and groundwater both the alluvium and the fractured Brule Resources (DNR), responded by clos- users, the Nebraska Legislature passed Formation and the close proximity of the ing the Pumpkin Creek drainage to any LB962, which gives the DNR authority to wells to surface water, however, ground- additional surface water permits in 1979. jointly regulate hydrologically connected water pumping can impact Pumpkin Since the local NRD (North Platte NRD) groundwater and surface water within Creek quickly. As noted by Bredehoeft lacked authority to regulate groundwater local NRDs. The Pumpkin Creek area is (1997), pumping does not have to exceed in response to impacts on surface water, now in an “over appropriated” groundwa- recharge for streams to be depleted. groundwater-irrigated acres continued to ter management area in which groundwa- During the 1890s, the irrigation po- grow. In 1965, there were 123 registered ter depletions of surface water must be tential of Pumpkin Creek was recognized irrigation wells (Figure 11a). Faced with decreased to levels that existed in 1997. and farmers began to obtain water rights diminishing flows, most surface water Beginning in 2006, the irrigated area in from the state of Nebraska under the prior irrigators drilled wells and relinquished the Pumpkin Creek basin was decreased appropriations doctrine. Flows in Pump- their surface water rights. A few surface by 1,123.6 hectares (2,776.4 acres) to kin Creek were highest in spring and de- water irrigators were unable to drill wells 15,935.4 hectares (39,377.3 acres) with

16 COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY conservation easements administered by land, the complete absence of any other • Several longer-term consequences of the U.S. Department of Agriculture (B. restrictions would result in the “survival groundwater depletion must also be Cross, Personal communication). These of the deepest” well. This may not be an considered, including actions give some relief to the Spear T economically optimal outcome for agri- úúreduction of groundwater inflows to Ranch, but it is uncertain if sufficient cultural water users within the Pumpkin streams, springs, and wetlands that water flow in Pumpkin Creek will be Creek watershed and it could deprive degrade the ability of surface water restored to allow diversions for surface downstream surface water irrigators of users to receive their allocations and water irrigation. their water rights during drought. threaten the sustainability of riparian ecosystems; Lessons from Pumpkin Creek ummary and úúshifting or subsidence of land sur- The most important lesson from the S faces that overlie depleting ground- Pumpkin Creek conflicts is that in an area Key Points water; and where groundwater and surface water As highlighted in the above discus- úúdegradation of groundwater quality. are scarce and hydrologically connected, sion, there are a number of consequences • The consequences of groundwater groundwater pumping will degrade and that arise when groundwater resources depletion can be mitigated through a possibly eliminate surface water flows are depleted, as well as a wide range mixture of water management policies and severely impair the ability of senior of factors that cause the depletion of that directly addresses the hydrologic water users to receive water, unless there groundwater resources. Thus, it should be imbalance in an aquifer, either by are legal protections for surface water no surprise that the development of plans úúincreasing the recharge to an aquifer users. to effectively mitigate or reverse the through the use of MAR or altering Another lesson learned in the Pump- impacts of groundwater depletion must land use practices to enhance the kin Creek case is contrary to the com- account for all of these factors, and the infiltration of rainfall below the soil mon misperception that groundwater key issues that must be understood to ad- surface; or development has a unique sustainable dress the complex issues associated with úúdecreasing groundwater demand pumping rate directly related to recharge groundwater depletion are the following: through the use of more efficient irri- rates because the impacts of groundwa- • The continued population growth in gation methods and encouraging the ter pumping on streamflow happened the United States and the world will transition from irrigation to dryland quickly. The rapid impact was due to the increase competition for food and agricultural production systems at aquifer’s high hydraulic conductivity water supplies, which will increase the regional scale. and the close proximity of the wells to stress on water resources and amplify Use of a groundwater resource re- Pumpkin Creek. In addition, there were the importance of sustainable water quires that the groundwater table must no noticeable groundwater level declines and food. be drawn down to some degree before until the surface water was completely • This heightened stress will increase it can be used in a beneficial manner. depleted because the surface water reliance on groundwater systems for This means that lowering of an aquifer’s in Pumpkin Creek replenished water direct supply and buffering the vari- groundwater table in small amounts is extracted from the alluvial aquifer. In ability of surface water supplies. unavoidable and not in and of itself a contrast to a simple concept of a single • Reliance on groundwater will con- negative condition. Long-term and exces- “sustainable” pumping rate policy, there tinue to put groundwater at risk of sive declines in an aquifer’s water table, is a range of groundwater pumping policy depletion, which is already a growing however, can result in many undesirable options depending on the desired flow in problem across the United States. impacts. Thus, when developing policies Pumpkin Creek. • The direct consequences of ground- that regulate groundwater and practices Because of the heterogeneous nature water depletion are declines in water that manage the use of groundwater and the shallow nature of the aquifer, tables, which decrease well yields and resources, the potential consequences of many marginal wells exist in the Pump- may cause shallower wells to go dry. groundwater depletion need to be fully kin Creek alluvium, and the current Drilling new, deeper wells is expen- assessed to determine the trade-offs that allocation of water puts the regulatory sive, and many agricultural producers exist between the undesired impacts of burden on the most productive ground- will choose (or be forced) to decrease groundwater depletion and whether these water wells. Policymakers who would irrigated acreage or take other steps impacts outweigh the benefits associated like to increase surface water flows are in response to reduced availability of with groundwater use. confronted with whether decreasing groundwater. water allocations or retiring marginal • For aquifers in arid regions primarily acres would be the most effective and recharged by seepage from inefficient economic means of restoring flows to water delivery and irrigation, ground- Pumpkin Creek. The heterogeneous and water depletion can be exacerbated renewable nature of the aquifer also leads by reductions in recharge caused by to the conclusion that as long as pump- increases in water delivery and irriga- ing is restricted to irrigating overlying tion system efficiencies.

COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY 17 water right has a priority date that cor- Dethloff, H. C. and G. L. Nall. 2018. Agriculture. Glossary responds to the date they first diverted Handbook of Texas Online. Texas State Histori- cal Association, http://www.tshaonline.org/ To effectively improve understand- the water and a volume or flow rate handbook/online/articles/ama01 (11 April 2018) ing of the causes and consequences of that limits the amount of water they Dillon, P. J. 2005. Future management of aquifer groundwater depletion, definitions must can divert. recharge. Hydrogeol J 13 (1): 313–316. Döll, P., H. M. Schmied, C. Schuh, F. T. Portmann, be provided for a variety of terms that Water user. An entity that diverts water and A. Eicker. 2014. Global-scale assessment of are used to describe the use of water, the from a river or pumps it from an groundwater depletion and related groundwater management and governance systems aquifer for agricultural, municipal, abstractions: Combining hydrological modeling with information from well observations and that have been created to determine who industrial, or other beneficial use. GRACE satellites. Water Resour Res 50 (7): has access to groundwater resources, and 5698–5720. what is meant by the term groundwater Faunt, C. C., M. Sneed, J. Traum, and J. T. Brandt. 2016. Water availability and land subsidence in depletion itself. Literature Cited the Central Valley, California, USA. Hydrogeol Correlative right. Limits the rights of a Amosson, S., S. Nair, B. Guerrero, and T. Marek. J 24 (3): 675–684. landowner to use a common resource, 2016. 2016 Panhandle regional water plan task Freeze, R. A. and J. A. Cherry. 1979. 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COUNCIL FOR AGRICULTURAL SCIENCE AND TECHNOLOGY 19 Texas Water Development Board (TWDB). 2016. FactSheets/2017/crep_kansas_upper_arkan- 2000. Treasure Valley Hydrologic Project Texas Water Development Board approves $1 sas_river_jan2017.pdf (11 April 2018) Research Report, Idaho Department of Water million in financial assistance to the Panhandle U.S. Department of Agriculture–Federal Crop Insur- Resources. 57 pp., https://idwr.idaho.gov/files/ Groundwater Conservation District for an agri- ance Corporation (USDA–FCIC). 2017. 2018 projects/treasure-valley/TVHP-Water-Bud- cultural conservation loan program. November Crop Insurance Handbook. FCIC, USDA Man- get-1996-2000.pdf (16 January 2019) 2, http://www.twdb.texas.gov/newsmedia/ agement Agency. 789 pp., https://www.rma. Wagner, K. 2012. Status and Trends of Irrigated press_releases/2016/11/panhandle.asp (11 April usda.gov/handbooks/18000/2018/18_18010-2. Agriculture in Texas—A Special Report by 2018) pdf (11 April 2018) the Texas Water Resources Institute. Publica- Texas Water Development Board (TWDB). 2017. U.S. Department of Agriculture–Natural Resources tion TWRI EM-115. Texas Water Resources Water for Texas—2017 State Water Plan. Conservation Service (USDA–NRCS). 2016. Institute, Texas A&M University, http://twri. TWDB. 150 pp., https://www.twdb.texas.gov/ Ogallala Aquifer Initiative. USDA–NRCS, tamu.edu/docs/education/2012/em115.pdf (11 waterplanning/swp/2017/doc/SWP17-Water- https://www.nrcs.usda.gov/wps/portal/nrcs/ April 2018) for-Texas.pdf?d=6718.700000026729 (15 detailfull/national/programs/initiatives/?cid=ste Ward, F. A. and A. Michelsen. 2002. The economic October 2018) lprdb1048809 (11 April 2018) value of water in agriculture: Concepts and pol- Texas Water Development Board (TWDB). 2018. U.S. Department of Agriculture–Natural Resources icy applications. Water Policy 4 (5): 423–446. Agricultural Water Conservation Grant and Conservation Service (USDA–NRCS). 2018. Water Replenish District of Southern California Loan Programs (AG). Financial Assistance Financial assistance. USDA–NRCS, https:// (WRD). 2018. Engineering Survey and Report, Programs, Texas Water Development Board, www.nrcs.usda.gov/wps/portal/nrcs/main/na- 2018, http://www.wrd.org/sites/pr/files/ https://www.twdb.texas.gov/publications/shells/ tional/programs/financial/ (11 April 2018) WRD%202018%20ESR%20-%20March%20 AWCLGP.pdf (11 April 2018) U.S. Department of Agriculture–Risk Management 7%20Adopted.pdf (22 June 2018) Theis, C. V. 1940. The source of water derived from Agency (USDA–RMA). 2011. Frequently Winter, T. C., J. W. Harvey, O. L. Franke, and W. M. wells. Civil Eng 10:277–280. asked questions: Crop insurance and drought- Alley. 1998. Ground Water and Surface Water: Tillman, F. D. and S. A. Leake. 2010. Trends in damaged crops. USDA–RMS, https://www. A Single Resource. U. S. Geological Survey groundwater levels in wells in the active rma.usda.gov/help/faq/drought.html (11 April Circular 1139. management areas of Arizona, USA. Hydrogeol 2018) Xue, Q., T. H. Marek, W. Xu, and J. Bell. 2017. J 18 (6): 1515–1524, doi:10.1007/s10040-010- University of Nebraska–Lincoln, Conservation Irrigated corn production and management 0603-3. and Survey Division (UNL, CSD). 2013. in the Texas High Plains. J Contemp Water U.S. Department of Agriculture–Farm Service Groundwater-level changes in Nebraska from Res Educ 162 (1): 31–41, doi:10.1111/j.1936- Agency (USDA–FSA). 2017. Conservation predevelopment to spring 2013. UNL, CSD, 704X.2017.03258.x. Reserve Enhancement Program—Kansas Upper Lincoln, Nebraska. Young, M. n.d. Water allocation in the West: Chal- Arkansas River. USDA–FSA, https://www.fsa. Urban, S. 2004. Water Budget for the Treasure lenges and opportunities, https://bit.ly/2TniyuT usda.gov/Assets/USDA-FSA-Public/usdafiles/ Valley Aquifer System for the Years 1996 and (4 December 2018)

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Citation: Council for Agricultural Science and Technology (CAST). 2019. Aquifer Depletion and Potential Impacts on Long-term Irrigated Agricultural Productivity. Issue Paper 63. CAST, Ames, Iowa.

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