Simulation of Streamflow and Estimation of Recharge to The

In cooperation with the San Antonio Water System

Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer in the Hondo Creek, Verde Creek, and San Geronimo Creek Watersheds, South-Central Texas, 1951–2003

Scientific Investigations Report 2005–5252

U.S. Department of the Interior U.S. Geological Survey Blank Page Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer in the Hondo Creek, Verde Creek, and San Geronimo Creek Watersheds, South-Central Texas, 1951–2003

By Darwin J. Ockerman

In cooperation with the San Antonio Water System

Scientific Investigations Report 2005–5252

U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior Gale A. Norton, Secretary

U.S. Geological Survey P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2005 For sale by U.S. Geological Survey, Information Services Box 25286, Denver Federal Center Denver, CO 80225

For more information about the USGS and its products: Telephone: 1-888-ASK-USGS World Wide Web: http://www.usgs.gov/ Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.

Suggested citation:

Ockerman, D.J., 2005, Simulation of streamflow and estimation of recharge to the Edwards aquifer in the Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas, 1951–2003: U.S. Geological Survey Scientific Investigations Report 2005–5252, 37 p. iii

Contents

Abstract ...... 1 Introduction ...... 1 Purpose and Scope ...... 2 Description of Study-Area Watersheds ...... 2 Simulation Approach ...... 5 Simulation of Streamflow and Estimation of Recharge ...... 7 Data Collection and Compilation ...... 7 Model Construction ...... 10 Model Calibration and Testing ...... 12 Streamflow ...... 12 Hondo Creek Model ...... 14 Verde Creek Model ...... 14 San Geronimo Creek Model ...... 14 Evapotranspiration ...... 17 Summary of HSPF Process-Related Parameters ...... 17 Streamflow, 1951–2003 ...... 17 Recharge, 1951–2003 ...... 19 Model Limitations and Assessment ...... 20 Summary...... 20 References ...... 21

Figures

1–2. Maps showing: 1. Location of Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas ...... 3 2. Land cover in Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas, early to mid-1990s ...... 4 3. Diagram showing Hydrological Simulation Program—FORTRAN (HSPF) flowchart for (a) impervious and (b) pervious land segments ...... 6 4–6. Maps showing: 4. Location of data-collection sites used for Hydrological Simulation Program— FORTRAN (HSPF) model calibration, testing, and simulations, Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas ...... 8 5. Subwatersheds in Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas, for Hydrological Simulation Program—FORTRAN (HSPF) model development ...... 11 6. Primary rain gages and associated rainfall areas for Hydrological Simulation Program—FORTRAN (HSPF) model calibration, testing, and simulations, Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas ...... 12 iv

7–9. Graphs showing: 7. Measured and simulated daily mean streamflow at U.S. Geological Survey streamflow-gaging station 08200000 Hondo Creek near Tarpley, Texas, 1992–2003 ...... 15 8. Measured and simulated daily mean streamflow at U.S. Geological Survey streamflow-gaging station 08200700 Hondo Creek at King Waterhole near Hondo, Texas, 1992–2003 ...... 16 9. Measured and simulated daily mean streamflow at U.S. Geological Survey streamflow-gaging station 08180590 San Geronimo Creek Reservoir inflow near Rio Medina, Texas, 1992–93 ...... 18

Tables

1. Process-related model parameters for the Hydrological Simulation Program—FORTRAN ...... 7 2. Basin-related model parameters for the Hydrological Simulation Program—FORTRAN ...... 7 3. Data-collection stations used for Hydrological Simulation Program—FORTRAN model input, calibration, and simulations for Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas ...... 9 4. Sources of rainfall data used to provide missing record and time-step disaggregation for primary rainfall time series used in the Hydrologic Simulation Program—FORTRAN (HSPF) model, Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas ...... 10 5. Subwatershed drainage areas, Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas ...... 13 6. Streamflow calibration and testing results, Hydrological Simulation Program—FORTRAN model, Hondo Creek watershed, south-central Texas, 1992–2003 ...... 23 7. Streamflow testing and recalibration results, Hydrological Simulation Program—FORTRAN model, San Geronimo Creek watershed, south-central Texas, 1992–93 ...... 25 8. Rainfall and simulated evapotranspiration, Hydrological Simulation Program—FORTRAN model, Hondo Creek watershed, south-central Texas, 1992–2003 ...... 26 9. Process-related annual parameters for pervious and impervious land segments, Hydrological Simulation Program—FORTRAN models, Hondo Creek and Verde Creek watersheds, south-central Texas ...... 27 10. Process-related annual parameters for pervious and impervious land segments, Hydrological Simulation Program—FORTRAN model, San Geronimo Creek watershed, south-central Texas ...... 31 11. Monthly values of lower-zone evapotranspiration (LZETP) parameter, Hydrological Simulation Program—FORTRAN models, Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas ...... 35 12. Simulated average annual streamflow volumes for Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas, 1951–2003 ...... 35 13. Estimated annual rainfall and Edwards aquifer recharge, Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas, 1951–2003 ...... 36 14. Comparison of Edwards aquifer recharge estimates from Hydrological Simulation Program—FORTRAN (HSPF) with historical (Puente method) U.S. Geological Survey recharge estimates for Hondo Creek and Verde Creek watersheds, south-central Texas, 1992–2003 ...... 37 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer in the Hondo Creek, Verde Creek, and San Geronimo Creek Watersheds, South-Central Texas, 1951–2003

By Darwin J. Ockerman

Abstract feet (5.04 inches), 26,000 acre-feet (3.36 inches), and 5,940 acre-feet (1.97 inches), respectively. Most of the recharge (about 77 percent for the three watersheds together) occurs as The U.S. Geological Survey, in cooperation with the San streamflow channel infiltration. Diffuse recharge (direct infil- Antonio Water System, constructed three watershed models tration of rainfall to the aquifer) accounts for the remaining 23 using the Hydrological Simulation Program—FORTRAN percent of recharge. For the Hondo Creek watershed, the HSPF (HSPF) to simulate streamflow and estimate recharge to the recharge estimates for 1992–2003 averaged about 22 percent Edwards aquifer in the Hondo Creek, Verde Creek, and San less than those estimated by the Puente method, a method the Geronimo Creek watersheds in south-central Texas. The three U.S. Geological Survey has used to compute annual recharge to models were calibrated and tested with available data collected the Edwards aquifer since 1978. HSPF recharge estimates for during 1992–2003. Simulations of streamflow and recharge the Verde Creek watershed average about 40 percent less than were done for 1951–2003. The approach to construct the mod- those estimated by the Puente method. els was to first calibrate the Hondo Creek model (with an hourly time step) using 1992–99 data and test the model using 2000–2003 data. The Hondo Creek model parameters then were Introduction applied to the Verde Creek and San Geronimo Creek watersheds to construct the Verde Creek and San Geronimo Creek models. The simulated streamflows for Hondo Creek are con- The Edwards aquifer is one of the most productive aquifers sidered acceptable. Annual, monthly, and daily simulated in the United States and is the major source of public water sup- streamflows adequately match measured values, but simulated ply for San Antonio and numerous smaller municipalities in hourly streamflows do not. The accuracy of streamflow simula- south-central Texas. In addition, the Edwards aquifer supplies tions for Verde Creek is uncertain. For San Geronimo Creek, large quantities of water for agriculture, industry, military the match of measured and simulated annual and monthly installations, and recreational activities. The aquifer also is a streamflows is acceptable (or nearly so); but for daily and source of water to major springs in the region. These springs hourly streamflows, the calibration is relatively poor. Simulated provide habitat for several threatened and endangered species average annual total streamflow for 1951–2003 to Hondo and supply water to downstream users. Creek, Verde Creek, and San Geronimo Creek is 45,400; Parts of each of the study-area watersheds are within the 32,400; and 11,100 acre-feet, respectively. Simulated average recharge zone of the Edwards aquifer. Recharge to the Edwards annual streamflow at the respective watershed outlets is 13,000; aquifer is an important issue for water-resource managers in the 16,200; and 6,920 acre-feet. The difference between total San Antonio area. As part of a continuing program in coopera- streamflow and streamflow at the watershed outlet is stream- tion with the Edwards Aquifer Authority (EAA), the U.S. Geo- flow lost to channel infiltration. Estimated average annual logical Survey (USGS) computes annual estimates of Edwards Edwards aquifer recharge for Hondo Creek, Verde Creek, and aquifer recharge. These estimates are based on a method for San Geronimo Creek watersheds for 1951–2003 is 37,900 acre- computing recharge developed by Puente (1978) using data 2 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003

collected from a network of USGS streamflow-gaging stations the upper part of Hondo Creek, above the confluence with and a network of rain gages, some operated by the National Verde Creek and including 161 square miles (mi2) of drainage Weather Service (NWS) and some by the EAA. The USGS has area, is included in the model simulations. Verde Creek begins compiled annual estimates of Edwards aquifer recharge from in three branches in south-central Bandera County and runs 1934 to present (2004). south for about 20 mi to its confluence with Hondo Creek, about The method to estimate Edwards aquifer recharge (herein- 4 mi east of the town of Hondo. The drainage area of the water- after referred to as the Puente method) uses monthly time peri- shed is 219 mi2. San Geronimo Creek enters the Medina River, ods. A watershed model, which uses smaller time steps (hourly) which is part of the San Antonio-Guadalupe River system. San that more realistically simulate the relatively rapid rainfall- Geronimo Creek, beginning in northwestern Bexar County, runoff-recharge processes in the study-area watersheds, might runs southwest for about 20 mi through Bexar, Bandera, and provide a more accurate estimation of aquifer recharge for Medina Counties to its mouth on the Medina River. The drain- these watersheds. Also, a calibrated watershed model enables age area of the watershed is 68.5 mi2. predictive scenarios to estimate changes in recharge resulting The study area is described as having a subtropical, subhu- from land-use changes including implementation of best- mid climate, characterized by hot summers and mild, dry win- management practices (BMPs). Removal of juniper (Juniperus ters (Larkin and Bomar, 1983). Heaviest rainfall tends to occur ashei) trees (Owens and others, 2001), construction of flood- in spring, early summer, and fall, but heavy rainfall can occur control or recharge-enhancement structures (HDR Engineering, throughout the year. Average annual rainfall (1961–90) in the 1998), and rainfall enhancement (Edwards Aquifer Authority, study area is about 30 inches per year (in/yr). The San Antonio 2004) are examples of BMPs that have been proposed or imple- average monthly low temperatures range from 37.9 degrees mented in the three or similar watersheds in the region. Fahrenheit (°F) in January to 75.0 °F in July. Average monthly To address the need for accurate recharge estimates for high temperatures range from 60.8 °F in January to 95.3 °F in the Edwards aquifer, the USGS, in cooperation with the San August (Bomar, 1995). Antonio Water System conducted a study to apply a watershed The surface of the area upstream from the Edwards aquifer model to simulate streamflow and recharge to the Edwards recharge zone (outcrop), the upper part of each watershed, is aquifer in the Hondo Creek, Verde Creek, and San Geronimo composed of Glen Rose Limestone (the uppermost unit of the Creek watersheds in south-central Texas (fig. 1). Trinity aquifer, which is juxtaposed against the northern boundary of the Edwards aquifer). Glen Rose Limestone generally has low permeability compared to rocks of the Edwards Group, Purpose and Scope which compose almost all of the Edwards aquifer (Clark, 2003). Uneroded remnants of Edwards Group rocks (hilltops) that The purpose of this report is to document application of the overlie Glen Rose Limestone in places are moderately perme- HydrologicalSimulation Simulation Program—FORTRAN of (HSPF) water- able but are not connected to the Edwards aquifer. The central shed model (Bicknell and others, 2001) to simulate streamflow part of each watershed is in the Edwards aquifer recharge zone. and estimateStreamflow recharge to the Edwards and aquifer in the Hondo In the Edwards aquifer recharge zone, outcropping Edwards Creek, Verde Creek, and San Geronimo Creek watersheds. Cal- Group rocks are characterized by high permeability because of ibrationEstimation and testing of a model forof each watershed are based on faults, sinkholes, and other karst features. Relatively imperme- data collected during 1992–2003. Model simulations of stream- able confining units overlie the Edwards aquifer south of the flow andRecharge estimates of recharge to are providedthe for 1951–2003. recharge zone, which is coincident with the lower part of each Model estimates of Edwards aquifer recharge also are com- watershed. Relatively thin alluvial deposits of sand and silt also pared toEdwards previous recharge estimatesAquifer, computed by the USGS are at the surface, mainly along stream channels (Small and using the Puente method. Clark, 2000). South-Central Streams in the study-area watersheds originate in the topographically rugged Texas Hill Country north of the recharge DescriptionTexas, of Study-Area 1951–2003 Watersheds zone and generally flow south, crossing the recharge zone and continuing onto units that overlie and confine the Edwards aqui- The study-area watersheds are in south-central Texas west fer south of the recharge zone. Most, if not all of the streamflow of San Antonio, along the southern part of the Edwards Plateau is lost to faults, fractures, caves, and sinkholes as the streams (often referred to as the Texas Hill Country) and near the tran- cross the recharge zone. Although major streams on the Glen sition to the Gulf Coastal Plain. The study-area watersheds Rose Limestone outcrop (upstream from the recharge zone) are encompass parts of the Edwards aquifer outcrop, which is not classified as perennial, flow is much more frequent than on essentially coincident with the recharge zone (fig. 1). the Edwards Group outcrop (recharge zone). Hondo Creek at Hondo and Verde Creeks are part of the Frio-Nueces River USGS streamflow-gaging station 08200000 Hondo Creek near system. Hondo Creek originates in south-central Bandera Tarpley (upstream from the recharge zone) flows more than 90 County and flows southeast for about 65 miles (mi) through percent of the time (1952–2004) and in some years is continu- Bandera, Medina, and Frio Counties to its confluence with the ous. In contrast, Hondo Creek at USGS streamflow-gaging Frio River about 5 mi northwest of Pearsall. For this study, only station 08200700 Hondo Creek at King Waterhole near Hondo Introduction 3

99˚

30˚ Guadalupe Rive r KENDALL COUNTY 100˚ BANDERA COUNTY Study area

TEAU COMALGUADALUPE COUNTY COUNTY EDWARDS PLA (Texas Hill Country) REAL COUNTY Verde

UVALDE COUNTY 08200000 Creek

m ONE Creek i RGE Z San n ECHA o IFER R er San Antonio AQU G ARDS 98˚ EDW 08200700 Hondo Medina River Seco Cr IN ASTAL PLA BEXAR COUN GULF CO ATA eek Nueces River SCOSA COUN San Antoni TY WILSON COUNTY MEDINA COUNTY TY Frio River ZAVALA COUNTY o River 29˚

Hondo Creek

Pearsall

FRIO COUNTY

Base modified from U.S. Geological Survey Recharge zone from 1:250,000 quadrangles Ashworth and Hopkins, 1995 Universal Transverse Mercator Projection TEXAS Zone 14 Area enlarged 015 3045 60 MILES

EXPLANATION Hondo Creek watershed Verde Creek watershed LOCATION MAP San Geronimo Creek watershed Edwards aquifer recharge zone boundary Boundary between Edwards Plateau and Gulf Coastal Plain 08200700 U.S. Geological Survey gaging station and number

Figure 1. Location of Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas.

(downstream from the recharge zone) flows less than 10 percent Land cover in the study-area watersheds (fig. 2), especially of the time (1961–2004) and in some years does not flow in the upper and middle parts of each watershed, is oak-juniper (Aragon Long and others, 2005). forest and rangeland (shrubland and grassland). Much of the Streamflow losses in the Edwards aquifer recharge zone lower part of each watershed (south of the recharge zone) is are believed to contribute directly to Edwards aquifer recharge pasture and cultivated land. Development in the study-area (Puente, 1978; Land and others, 1983). Most recharge to the watersheds is sparse, limited to farms, ranches, and a few small Edwards aquifer occurs as streamflow losses directly out of towns. channels and other water courses. Infiltration of rainfall, or Land-surface elevation in the study-area watersheds diffuse recharge, is relatively small compared to recharge ranges from about 730 to 2,000 feet above sea level. Land occurring through streamflow losses (Maclay, 1995). slopes generally are steeper in the upper parts of the watershed 4 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003

99˚30' 99˚15' 99˚00' 98˚45'

29˚45'

Hondo Creek watershed San Geronimo Creek watershed

29˚30'

Verde Creek watershed

Base modified from U.S. Geological Survey 1:24,000 quadrangles Universal Transverse Mercator projection 04 812 16 MILES Zone 14 EXPLANATION Edwards aquifer recharge zone (from Ashworth and Hopkins, 1995) Land-cover category Evergreen forest Deciduous/mixed forest Shrubland Grassland Bare/transitional Cultivated/pasture (agricultural) Water Quarry Developed Watershed boundary

Figure 2. Land cover in Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas, early to mid-1990s (U.S. Geological Survey, 2003).

than in the lower parts. Soils in most of the watersheds, espe- Several small (less than about 1,000 acre-feet [acre-ft]) cially upstream from and on the Edwards aquifer recharge zone reservoirs and numerous smaller stock ponds are in the three are thin, rocky, and clayey. In the lower parts of the watersheds, watersheds. Most of these reservoirs (many constructed in the in the transition to Gulf Coastal Plains, the soils are deeper clay 1960s and 1970s) are in the upper parts of the watersheds, above and sand loams. the Edwards aquifer recharge zone, and probably have little Introduction 5 influence on recharge to the Edwards aquifer. However, acteristics. These land segments do not need to be contiguous. two structures in the recharge zone in the Verde Creek and Stream or reservoir segments (open or closed channels, or San Geronimo Creek watersheds were constructed to enhance completely mixed lakes) are simulated using hydraulic routing recharge to the Edwards aquifer by retaining and allowing methods (Donigian and others, 1995). for infiltration of streamflow that might otherwise exit the A schematic diagram depicting how HSPF simulates water recharge zone (Edwards Aquifer Authority, 2004). The EAA movement through and across pervious and impervious land has estimated that these two structures have increased recharge to the atmosphere, ground water, or surface runoff is shown in by an average of 1,699 acre-feet per year (acre-ft/yr) since their figure 3. The flow of water between the storage zones, stream, construction in the late 1970s (Edwards Aquifer Authority, and atmosphere is affected by the process-related parameters 2004). listed in table 1. Domestic and agricultural water supply in the watersheds The process-related model parameters for each land is obtained from Edwards aquifer wells in the lower parts and segment are adjusted to calibrate the model. Some process- Trinity aquifer wells in the upper parts. No wastewater treat- related parameters can be adjusted monthly to account for ment discharge occurs in the study-area watersheds. seasonal variations: interception storage capacity (CEPSC), interflow index (INTFW), interflow recession coefficient Simulation Approach (IRC), lower-zone evapotranspiration (LZETP), Manning’s roughness coefficient (n) for assumed overland flow plane (NSUR), and upper-zone nominal storage (UZSN). For this Because numerous watershed characteristics and hydro- study, monthly variation was implemented only for the param- logic processes affect streamflow and recharge in the study-area eter LZETP. watersheds, a comprehensive watershed model was needed to The six basin-related model parameters listed in table 2 account for the complex interactions between watershed char- define the areal extent of each land segment and other charac- acteristics and hydrologic processes and to simulate streamflow teristics of each stream or reservoir segment, including length and recharge. Also, a watershed model is necessary to evaluate and tables of surface area, volume, and discharge, as a function changes in streamflow and recharge that might occur because of of depth. Collectively, these parameters represent the physical future changes in watershed characteristics (for example, land- characteristics of each land or stream segment in a watershed use changes and possible implementation of BMPs). and generally remain unchanged during calibration of the The HSPF model was selected to model the study water- model. sheds because it is one of the most comprehensive watershed The HSPEXP computerized expert system (Lumb and models available, can simulate a wide variety of stream and others, 1994) was used to assist with process-related parameter watershed conditions with reasonable accuracy, and enables adjustment for model calibration. The HSPEXP procedures flexibility in adjusting model inputs to simulate alternative con- consist of a set of hierarchical rules designed to guide the ditions, or scenarios (Donigian and others, 1995). HSPF is a calibration of the model through a systematic evaluation of watershed model used to simulate hydrologic processes in com- the model parameters. Simulation errors are evaluated on plex agricultural, rural, and urban watersheds. HSPF uses such the basis of seven criteria: total streamflow volume, low- information as the time history of rainfall, temperature, evapo- flow recession, highest 10 percent of daily flow volumes, ration, and parameters related to land cover, land-use practices, lowest 50 percent of daily flow volumes, storm volumes, and soil characteristics to simulate the hydrologic processes that seasonal volumes, and summer storm volumes. Statistics occur in a watershed. The result of an HSPF simulation is a time computed by HSPEXP provide the analyst with an evaluation history of the quantity of water transported over the land surface of the agreement between simulated and observed runoff and through the various soil zones down to an aquifer (Donigian values. Specifically for this study, simulation errors were and others, 1995). evaluated by comparing total streamflow volume, 50-percent HSPF is an empirical model; conceptually, the process- lowest flows, 10-percent highest flows, and selected storm related parameters of the model have physical meaning but are peaks. not physically measurable (or are difficult to measure) and must The Hondo Creek watershed includes two long-term be specified by calibration. The HSPF model is divided into gaging stations. Few streamflow data are available from the three components to simulate the hydrology of a watershed: Verde Creek and San Geronimo Creek watersheds. The pervious areas (segments), impervious areas (segments), and approach used in calibrating the study-area watershed models stream (and reservoir) segments. Pervious land conceptually is was to first calibrate and test the Hondo Creek model using represented within HSPF by a series of interconnected water- available data for 1992–2003. Then the resulting calibrated storage zones: an upper zone, a lower zone, and a ground-water parameters were transferred to the Verde Creek and San zone. Impervious land is represented by much simpler surface Geronimo Creek watersheds to create the Verde and San storage, evaporation, and runoff (overland flow) processes. Geronimo models. The few streamflow data available from the Each user-defined, pervious or impervious land segment repre- San Geronimo Creek watershed were used for testing transfer- sents its own unique hydrologic response on the basis of soil ability of Hondo Creek model parameters to the other two type, land cover, watershed slope, or other important basin char- watersheds. 6 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003 Refer To stream flow flow flow Shallow Overland subsurface Ground-water Model input Storage Outflow Decision point Evapotranspiration Designates controlling HSPF parameter— to table 1 LSUR* IRC* NSUR* SLSUR* KVARY* ein and Schiffer, 2002, fig. 3). 2002,ein Schiffer, fig. and AGWRC* * ET EXPLANATION storage storage Surface Interflow detention To flow stream Overland LSUR* NSUR* SLSUR* storage Active ground-water INTFW* UZSN* storage Upper-zone BASETP* AGWETP* Potential runoff RETSC* Direct infiltration Delayed Infiltration CEPSC* DEEPFR* Rainfall storage storage potential ET storage Rainfall Surface retention Surface detention Interception potential ET INFILT* Inactive ground water INFILD* INFEXP* LZSN* zone Lower- storage LZETP* ET ET ET ET ET Actual Hydrological Simulation Program—FORTRAN (HSPF) flowchart for (a) impervious and (b) pervious land segments (modified from Wickl from (modified pervious land segments impervious (b)(a) and Simulation Hydrological Program—FORTRAN(HSPF) flowchart for evaporation ET Actual (a) Impervious land segments (b) Pervious land segments Figure 3. Simulation of Streamflow and Estimation of Recharge 7

Table 1. Process-related model parameters for the Hydrological Simulation Program—FORTRAN (modified from Wicklein and Schiffer, 2002, table 1).

[PERLND, pervious segment; IMPLND, impervious segment]

Parameter Description1 Land area AGWETP Fraction of available potential evapotranspiration demand that can be met from stored ground water PERLND AGWRC Ground-water recession parameter. Index of the rate at which ground water drains from land PERLND BASETP Fraction of available potential evapotranspiration demand that can be met from ground-water outflow; PERLND simulates evapotranspiration from riparian vegetation CEPSC Interception storage capacity PERLND DEEPFR Fraction of ground water that does not discharge to surface within boundaries of modeled area PERLND INFEXP Infiltration equation exponent; controls rate of infiltration decrease as a function of increasing soil moisture PERLND INFILD Ratio of maximum and mean infiltration capacities PERLND INFILT Index to infiltration capacity of soil; also affects percolation to ground-water zone PERLND INTFW Interflow index; controls amount of infiltrated water that flows as shallow subsurface flow PERLND IRC Interflow recession coefficient; index for rate of shallow subsurface flow PERLND KVARY Ground-water outflow modifier; index of how much effect recent recharge has on ground-water outflow PERLND LSUR Length of assumed overland flow plane PERLND or IMPLND LZETP Lower-zone evapotranspiration; index value (ranging from 0 to 0.99) representing density of deep-rooted PERLND vegetation LZSN Lower-zone nominal storage; index to soil moisture holding capacity of unsaturated zone PERLND NSUR Manning’s n for assumed overland flow plane PERLND or IMPLND RETSC Impervious retention storage capacity IMPLND SLSUR Slope of assumed overland flow plane PERLND or IMPLND UZSN Upper-zone nominal storage; index to amount of surface storage in depressions and upper few inches of soil PERLND

1 Users manual for Hydrological Simulation Program—FORTRAN (Bicknell and others, 2001) provides a more complete description of each parameter.

Simulation of Streamflow and Estimation of resulting land-cover map (fig. 2) comprises nine land-cover categories. Recharge Geology is based on that of Maclay (1995) and Small and Clark (2000). Watershed topography was obtained from USGS The major steps for simulating streamflow and estimating 7.5-minute digital elevation model (DEM) files (U.S. Geologi- Edwards aquifer recharge for the study area included data col- cal Survey, 2001). The DEMs also were used to delineate sub- lection and compilation, model development, model calibration basins as part of the model construction. Channel characteristics and testing, and simulation of streamflow and recharge during 1951–2003. Table 2. Basin-related model parameters for the Hydrologi- Data Collection and Compilation cal Simulation Program—FORTRAN. [PERLND, pervious segment; IMPLND, impervious segment, FTABLE, table of depth, surface area, volume, and discharge for each stream reach] Input data for the HSPF models include spatial data (land cover, geology-soils, topography, and stream characteristics Parameter Description Units such as stream-segment length and cross-section dimensions) AREA Drainage area of each PERLND Acres and meteorologic time-series data (rainfall and pan evapora- or IMPLND tion). Spatial data were used to develop model hydrologic LEN Stream reach length Miles response units (HRUs) (PERLND [PERvious LaND] and DEPTH FTABLE depth Feet IMPLND [IMPervious LaND], in HSPF terminology). Time series of rainfall, streamflow, and evapotranspiration were used SAREA FTABLE surface area Acres as input for model calibration, testing, and simulations. VOL FTABLE volume Acre-feet Land-cover spatial data were derived from early to DISCH FTABLE discharge Cubic feet per mid-1990s satellite data (U.S. Geological Survey, 2003). The second 8 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003

99˚30' 99˚15' 99˚00' 98˚45'

29˚45'

24 17 8 15 16 10 11 18 19 29˚30' 20 21 12

13 7 22

14 6 9 23

Base modified from U.S. Geological Survey 1:24,000 quadrangles Universal Transverse Mercator projection 0 4 8 12 16 MILES Zone 14 EXPLANATION Hondo Creek watershed Verde Creek watershed San Geronimo Creek watershed Edwards aquifer recharge zone (from Ashworth and Hopkins, 1995) Site description—Number refers to site number in table 3 5 National Weather Service daily rain gage 7 National Weather Service pan evaporation station 1 National Weather Service hourly rain gage 9 U.S. Geological Survey streamflow-gaging station and hourly rain gage 24 Natural Resources Conservation Service evapotranspiration station 17 U.S. Geological Survey partial-record station 10 U.S. Geological Survey discontinued streamflow-gaging station

Figure 4. Location of data-collection sites used for Hydrological Simulation Program—FORTRAN (HSPF) model calibration, testing, and simulations, Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas. for each segment (surface area, volume, and discharge as a collection sites are shown in figure 4. Station information for function of depth) also were determined from DEM files using each data-collection site identified in figure 4 is listed in table 3. geographic information system tools available through the Five rain gages were used as the primary source of rainfall data Watershed Modeling System software (U.S. Army Corps of to the models (sites 1, 4, 5, 6, and 8 in fig. 4 and table 3). Engineers, Coastal and Hydraulic Laboratory, 2004). Because some of the gages were not in operation during the Meteorologic data compiled and used as input to the water- entire study period (1992–2003), data from three additional shed models included rainfall and pan evaporation data. Data- sites (2, 3, and 6a in table 3) also were used. Because hourly Simulation of Streamflow and Estimation of Recharge 9

Table 3. Data-collection stations used for Hydrological Simulation Program—FORTRAN model input, calibration, and simulations for Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas.

[dd, degrees; mm, minutes; ss, seconds; NWS, National Weather Service; --, unknown; USGS, U.S. Geological Survey; CR, County Road; FM, Farm Road; SH, State Highway, NRCS, Natural Resources Conservation Service]

Site Station name, number Latitude Longitude Period of number Type of data (short name) (ddmmss) (ddmmss) record used (fig. 4) 1 NWS near Tarpley, Tex., 418845 (Tarpley NWS) 29°40'--" 99°17'--" Hourly rainfall 1951–2003 12 NWS near Leaky, Tex., 414254 (Leaky NWS) 29°44'--" 99°46'--" Hourly rainfall 1951–2003 3 NWS near Hondo, Tex., 414254 (Hondo NWS) 29°20'--" 99°08'--" Daily rainfall 1952–75 4 NWS at Hondo Airport, Tex., 414256 (Hondo Airport NWS) 29°22'--" 99°10'--" Daily rainfall 1975–2003 5 NWS at Boerne, Tex., 410902 (Boerne NWS) 29°48'--" 98°44'--" Daily rainfall 1951–2003 6 NWS at Rio Medina, Tex., 417628 (Rio Medina NWS) 29°26'--" 98°53'--" Daily rainfall 1951–2003 16a NWS at San Antonio International Airport, San Antonio, Tex., 29°32'--" 98°28'--" Hourly rainfall 1951–2003 417945 (San Antonio Airport NWS) 7 NWS at Sea World, San Antonio, Tex., 418169 (Sea World NWS) 29°27'--" 98°42'--" Pan evaporation 1988–2003 17a NWS at Canyon Dam, Comal County, Tex., 411429 (Canyon Dam 29°52'--" 98°12'--" Pan evaporation 1961–88 NWS) 8 USGS Hondo Creek near Tarpley, Tex., 08200000 (Tarpley USGS) 29°34'10" 99°14'47" Streamflow and hourly 1992–2003 rainfall 9 USGS Hondo Creek at King Waterhole near Hondo, Tex., 29°23' 26" 99°09' 04" Streamflow and hourly 1992–2003 08200700 (King Waterhole) rainfall 10 USGS San Geronimo Creek Reservoir inflow near Rio Medina, 29°32'23" 98°49'06" Streamflow 1992–93 Tex., 08180590 (discontinued) (San Geronimo Creek Reservoir inflow) 11 USGS Hondo Creek at CR 232 near Hondo, Tex., 08200020 29°31'37" 99°13'22" Partial-record streamflow 1981 measurement 12 USGS Hondo Creek at FM 462 near Hondo, Tex., 08200100 29°28'23" 99°12'16" Partial-record streamflow 1981 measurement 13 USGS Hondo Creek near Hondo, Tex., 08200500 (discontinued)2 29°27'05" 99°11'07" Partial-record streamflow 1981 measurement 14 USGS Hondo Creek near FM 462 near Hondo, Tex., 08200650 29°24'17" 99°10'22" Partial-record streamflow 1981 measurement 15 USGS Middle Verde Creek upstream SH 173 near Bandera, Tex., 29°34'08" 99°05'55" Partial-record streamflow 1980–81 08200976 measurement 16 USGS Middle Verde Creek above East Verde Creek near Bandera, 29°32'17" 99°04'47" Partial-record streamflow 1980–81 Tex., 08200978 measurement 17 USGS East Verde Creek near Bandera, Tex., 08200979 29°34'41" 99°04'48" Partial-record streamflow 1980–81 measurement 18 USGS Middle Verde Creek below East Verde Creek near Bandera, 29°31'17" 99°05'04" Partial-record streamflow 1980–81 Tex., 0820097960 measurement 19 USGS Middle Verde Creek below reservoir near Bandera, Tex., 29°30'11" 99°06'02" Partial-record streamflow 1980–81 08200992 measurement 20 USGS Middle Verde Creek Reservoir outflow near Hondo, Tex., 29°30'05" 99°06'31" Partial-record streamflow 1980–81 08201000 measurement 21 USGS Middle Verde Creek above Martin Creek near Hondo, Tex., 29°29'01" 99°07'18" Partial-record streamflow 1980–81 08201005 measurement 22 USGS Verde Creek at Seifert Ranch near Hondo, Tex., 08201030 29°27'00" 99°07'30" Partial-record streamflow 1980–81 measurement 23 USGS Verde Creek near Hondo, Tex., 08201050 29°24'16" 99°06'59" Partial-record streamflow 1980–81 measurement 24 NRCS Seco Creek study area, Uvalde County, Tex. 29°35'--" 99°27'--" Evapotranspiration 1992–95

1 Not shown in figure 4. 2 Streamflow 1951–64. 10 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003

Table 4. Sources of rainfall data used to provide missing record and time-step disaggregation for primary rainfall time series used in the Hydrological Simulation Program—FORTRAN (HSPF) model, Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south- central Texas.

[NWS, National Weather Service; USGS, U.S. Geological Survey; --, not used] Primary rain Period of Data time Gages used to fill Gages used for gage record step missing data disaggregation Tarpley NWS 1951–2003 Hourly Leakey NWS (1951–91); Tarpley USGS -- (1992–2003) Tarpley USGS 1992–2003 Hourly Tarpley NWS -- Hondo Airport NWS 1975–2003 Daily Tarpley NWS Tarpley NWS, San Antonio Airport NWS Boerne NWS 1951–2003 Daily Rio Medina NWS Tarpley NWS, San Antonio Airport NWS Rio Medina NWS 1951–2003 Daily Hondo Airport NWS, Hondo NWS Tarpley NWS, San Antonio Airport NWS

rainfall data are preferred for the model simulations, data from unique HRUs on the basis of combinations of land cover, geol- NWS daily stations were disaggregated1 to hourly data by using ogy, and slope; and (3) and determining initial (uncalibrated) the nearest NWS or USGS stations with available hourly data. values of associated model parameters. Initial parameter esti- Information on which gages were used to fill missing record and mates were obtained primarily from previous studies (Brown gages used for dissaggregation is listed in table 4. and Raines, 2002; Ockerman, 2002) and secondarily from Two NWS stations were used as the source of potential hydrologic judgment. evaporation data for the models. The Sea World NWS station at The HSPF models were configured by dividing each San Antonio (site 7a in table 3) provided data for 1961–88. watershed into subwatersheds (fig. 5) to produce discrete HSPF Average 1988–2003 monthly values of pan evaporation from stream and reservoir segments. Guidelines for developing the Sea World were used to estimate evaporation for 1951–60 stream-segment configuration include defining segments with (the Canyon Dam station began operation in 1961). The (1) similar flow travel times that approximate the model simu- NWS daily (monthly, before 1961) pan evaporation data lation time step, (2) homogeneous channel properties, such as were disaggregated1 to hourly rates for each day for model slope and conveyance, and (3) outlets of subwatersheds at simulations. important points, such as streamflow-gaging stations, reser- Streamflow data for calibrating HSPF streamflow were voirs, outcrop boundaries, and inflows or outflows. The area of available for 1992–2003 from two active USGS streamflow- each subwatershed is listed in table 5. gaging stations (fig. 4, table 3). Station 08200000 Hondo Creek Five rain-gage sites provided the primary rainfall inputs to near Tarpley monitors streamflow from the upper 95.6 mi2 of the models. Rainfall data were applied to the model subwater- the watershed; station 08200700 Hondo Creek at King Water- sheds (fig. 6) on the basis of a modified Thiessen distribution hole near Hondo monitors the upper 149 mi2 of the watershed. (Wanielista, 1990). A single pan evaporation time-series data Station 08200500 Hondo Creek near Hondo, operated during set (Sea World NWS, fig. 4 and table 3) was applied to the 1953–64, measured streamflow from the upper 132 mi2 of the model subwatersheds. watershed. Verde Creek is ungaged. Supplementary streamflow In each subwatershed, pervious and impervious HRUs measurements were made at selected sites on Hondo and Verde were classified according to combinations of three factors: Creeks after several storms in 1980 and 1981 (Land and others, geology, land cover, and land slope. Five geology categories are 1983) (sites 11–23, fig. 4 and table 3). These measurements (1) Glen Rose Limestone (Trinity aquifer outcrop), (2) Edwards were made to help quantify streamflow losses in the Edwards Group outcrop (Edwards aquifer recharge zone), (3) Edwards aquifer recharge zone. Data from a discontinued streamflow- Group remnants (hilltops) overlying Glen Rose Limestone, gaging station on San Geronimo Creek (08180590 San Geron- (4) Edwards upper confining units, and (5) alluvial deposits. imo Creek Reservoir inflow near Rio Medina) were available Land cover was one or more of the eight categories (excluding for 1992–93. water) shown in figure 2. Land slopes were classified into three categories: Low slope, 0–5 percent; medium slope, 5–10 percent; and steep slope, greater than 10 percent. Model Construction Combinations of geology, land cover, and land slope result in 120 possible unique HRUs, not including impervious land HSPF models of the three watersheds were constructed by cover. The actual number of HRUs in the models is 111 because (1) defining subwatersheds for the watersheds; (2) classifying some combinations do not exist (for example, alluvial deposits

1 The rainfall and potential evaporation disaggregation programs are part of the BASINS watershed modeling software package (U.S. Environmental Protection Agency, 2003). Daily rainfall is disaggregated according to the temporal rainfall distribution at one or more nearby hourly stations. Daily potential evaporation is distributed on the basis of latitude and time of year. Simulation of Streamflow and Estimation of Recharge 11

99˚30' 99˚15' 99˚00' 98˚45'

29˚45'

22 36 10 24 26 32 12 2288 34 10 1616 3030 42 10 16 14 3838 12 12 18 20 18 20 14 46 24 18 14 54 16 20 40 48 50 24 22 58 52 68 80 22 26 6262 82 28 60 28 32 70 8844 8866 60 3430 56 72 30 62 66 36 74 8888 92 25 9494 64 70 29˚30' 36 66 90 32 27 68 98 38 52 54 74 42 48 76 44 72 38 40 11000 46 90 106 1020202 56 78 88 10411004 1088 86 58 94

Figure 5. Subwatersheds in Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas, for Hydrological Simulation Program—FORTRAN (HSPF) model development.

on steep slopes). In HSPF teminology, HRUs correspond to is based on gain-loss measurements made by Land and others PERLND and IMPLND segments. (1983) at selected sites along Hondo and Verde Creeks. Addi- In each watershed, substantial amounts of runoff are lost to tional streamflow measurements were made on Hondo Creek in streamflow channel infiltration (recharge) as the creeks flow June 2004 (Aragon Long and others, 2005). across the Edwards aquifer recharge zone. HSPF does not Model construction was completed by assigning initial include process parameters or specific features to account for values to process-related HSPF parameters. These values natural streamflow losses. Instead, these losses are simulated as represented a starting point for calibration of the models water withdrawals from each stream segment. The withdrawals by trial-and-error parameter adjustment. Initial values of are specified in the stream segment (FTABLE) configuration as parameters related to streamflow and recharge were based on a function of streamflow. The relation between streamflow previous HSPF studies in the region (Brown and Raines, 2002; losses and streamflow for a specific channel segment/watershed Ockerman, 2002). 12 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003

99˚30' 99˚15' 99˚00' 98˚45'

Boerne National Weather Service 29˚45'

Tarpley National Weather Service

Tarpley U.S. Geological Survey

29˚30'

Rio Medina National Weather Service

Hondo Airport National Weather Service

Base modified from U.S. Geological Survey 1:24,000 quadrangles Universal Transverse Mercator projection 04 812 16 MILES Zone 14

EXPLANATION Hondo Creek watershed Boundary of subwatershed by rainfall area for HSPF model Verde Creek watershed Hondo Airport National Weather Service San Geronimo Creek watershed Rio Medina National Weather Service Edwards aquifer recharge zone (from Tarpley National Weather Service Ashworth and Hopkins, 1995) Tarpley U.S. Geological Survey Boerne National Weather Service Rain gage and name—Color corresponds to associated rainfall area Boerne National Weather Service

Figure 6. Primary rain gages and associated rainfall areas for Hydrological Simulation Program—FORTRAN (HSPF) model calibration, testing, and simulations, Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas.

Model Calibration and Testing are in the Hondo Creek watershed; no long-term stations are in the Verde Creek and San Geronimo watersheds. The model was calibrated according to the guidelines of Donigian and others Streamflow (1984) and Lumb and others (1994). Calibration generally involved adjusting the process-related parameter values to A primary goal of watershed-model calibration is to match improve the model fit—that is, the match between simulated simulated streamflow to actual streamflow measured at a gag- and measured streamflow. Criteria such as error in total stream- ing station. Two long-term USGS streamflow-gaging stations flow volume for the calibration period, low-flow and high-flow Simulation of Streamflow and Estimation of Recharge 13

Table 5. Subwatershed drainage areas, Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-central Texas.

[ID, identification number; --, not applicable]

Hondo Creek watershed Verde Creek watershed San Geronimo Creek watershed Subwatershed ID Drainage area Subwatershed ID Drainage area Subwatershed ID Drainage area (fig. 5) (acres) (fig. 5) (acres) (fig. 5) (acres) 10 3,160 10 1,761 10 5,016 12 2,335 12 1,940 12 2,871 14 4,262 14 6,328 14 4,905 16 1,143 16 2,476 16 3,241 18 2,600 18 410 18 2,555 20 3,490 20 8,135 20 1,200 22 2,071 22 5,652 22 1,637 24 3,365 24 10,089 24 3,339 26 1,523 25 3,154 26 2,659 28 869 27 2,030 28 2,596 30 1,001 28 7,190 30 753 32 2,809 30 4,451 32 1,921 34 1,829 32 1,028 34 1,621 36 681 36 2,476 36 1,808 38 1,788 38 3,194 38 7,769 40 700 40 170 -- -- 42 4,277 42 1,262 -- -- 46 2,413 44 2,244 -- -- 48 2,275 46 3,400 -- -- 50 6,687 48 5,057 -- -- 52 1,981 50 3,258 -- -- 54 5,231 52 2,340 -- -- 56 531 54 2,751 -- -- 58 2,003 56 3,777 -- -- 60 1,640 58 469 -- -- 62 653 60 2,465 -- -- 66 91 62 4,617 -- -- 68 2,156 64 1,553 -- -- 70 4,230 66 3,605 -- -- 72 1,498 68 2,668 -- -- 74 2,548 70 2,536 -- -- 80 3,028 72 5,965 -- -- 82 2,647 74 9,216 -- -- 84 1,065 76 3,662 -- -- 86 1,579 78 6,983 -- -- 88 1,434 86 3,158 -- -- 90 197 88 1,632 -- -- 92 3,811 90 6,647 -- -- 94 2,088 94 314 -- -- 98 4,910 ------100 1,738 ------102 1,327 ------104 1,008 ------106 3,259 ------108 2,774 ------

Total 102,705 140,063 43,891 14 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003

distribution, and error in peak flows were used to evaluate how Simulation errors for other criteria generally are within well simulated streamflow matched measured streamflow. acceptable limits (compared to default criteria guidelines of Because most process-related parameters cannot be measured HSPEXP [Lumb and others, 1994]). During the testing period directly, they are specified through iterative calibration. (2000–2003), simulated lowest 50 percent of daily flow vol- Model basin-related parameter values were computed umes at the Tarpley gage was 11.1 percent greater than mea- from available spatial data, to the extent possible. For example, sured flows. For the entire calibration and testing period the average slopes of PERLNDS were computed from digital ele- lowest 50 percent of simulated daily flow volumes was 2.6 per- vation data. cent greater than measured flows. Both Hondo Creek gages were rendered inoperative during a July 2002 flood, and daily Hondo Creek Model mean streamflows at both sites were estimated (A.M. Miller, U.S. Geological Survey, oral commun., 2004). The July 2002 The Hondo Creek model was calibrated for the period Jan- flood accounted for about 90 percent of the total streamflow uary 1, 1992, to December 31, 1999, using an hourly time step. volume at the King Waterhole gage during the 2000–2003 The response of the calibrated model was further tested using period. So, potential error in the estimates of gaged streamflow data for the period January 1, 2000, to December 31, 2003. Data might affect the comparison statistics listed in table 6 (at end of from USGS streamflow-gaging station 08200000 Hondo Creek report). near Tarpley were used to calibrate model streamflow for the Simulated flows at both USGS gages were evaluated by Hondo Creek watershed upstream from the gage (subwater- graphical comparison of measured and simulated daily time sheds 10–66). Data from USGS streamflow-gaging station series, exceedance-probability curves, and scatter plots (figs. 7, 08200500 Hondo Creek at King Waterhole near Hondo were 8). General agreement between the measured and simulated used to calibrate subwatersheds 68–102. Calibrated parameters exceedance-probability curves indicates adequate calibration for PERLND and IMPLND HRUs in subwatersheds 68–102 over the range of flow conditions. were transferred to the lower, ungaged subwatersheds Donigian and others (1984) present general guidelines for (104–108). characterizing HSPF calibrations. For annual and monthly run- Streamflow calibration and testing results for Hondo off volumes, model calibration is considered very good when Creek near Tarpley and Hondo Creek at King Waterhole near the error is less than 10 percent, good when the error is 10 to 15 Hondo are listed in table 6. The measured and simulated stream- percent, and fair when the error is 15 to 25 percent. By these flow and percentage differences are reported for total flow vol- guidelines, calibration results for annual flow volumes at both ume, highest 10 percent of daily flow volumes, lowest 50 per- gages are very good. Coefficients of determination and model- cent of daily flow volumes, and selected storm peaks. Storm fit efficiency near or above 0.9 indicate an acceptable match peaks included in the analyses are all measured peaks greater between measured and simulated streamflows. By this criterion, than 1,000 cubic feet per second (ft3/s). There were 24 such annual, monthly, and daily simulated streamflows adequately streamflow peaks at the Tarpley station and 16 peaks at the matched measured values, but simulated hourly streamflows King Waterhole station. did not. Two model-fit statistics were used to examine the quality of the model fit on an annual, monthly, daily, and hourly basis: Verde Creek Model (1) the coefficient of determination of the linear regression between measured and simulated streamflow, which provides a Process-related parameters from the calibrated Hondo measure of the variance between measured and simulated Creek model were applied to the Verde Creek watershed to pro- streamflows explained by the regression—in other words, a duce the Verde Creek model. Discrete discharge measurements rough measure of the goodness of fit of the regression; and made along Verde Creek in 1981 (Land and others, 1983) were (2) the coefficient of model-fit efficiency (Nash and Sutcliffe, used to help develop streamflow channel infiltration capacity 1970), which indicates how well the graph of simulated for the FTABLEs of all models. No other streamflow data from streamflow as a function of measured streamflow matches the Verde Creek were used to calibrate or test the Verde Creek line of equal value (Knapp and others, 2004). Thus model-fit model. Because the accuracy of the Verde Creek model is efficiency is a measure of the match between measured and sim- uncertain, results are not provided in this report. ulated streamflows. Error in simulated total flow volume at the Tarpley gage San Geronimo Creek Model for the calibration period (1992–99) was 0. Simulated annual flow volume for the testing period (2000–2003) exceeded mea- Process-related parameters from the calibrated Hondo sured annual flow volume by 2.7 percent. For the entire Creek model were applied to the San Geronimo Creek water- 1992–2003 period, simulated total flow volume exceeded the shed to produce the San Geronimo Creek model. To assess the measured amount by 1.1 percent. Error in simulated total flow validity of using parameters from the Hondo Creek model in the volume at the King Waterhole gage for the calibration period San Geronimo Creek model, the simulated streamflow was (1992–99) was 1.0 percent; for the testing period, 3.9 percent; compared to measured streamflow from a discontinued USGS and for the entire period, 1.1 percent. streamflow-gaging station (San Geronimo Creek Reservoir Simulation of Streamflow and Estimation of Recharge 15

100,000 D N

10,000 Measured Simulated

1,000

100 CUBIC FEET PER SECO N , I

W 10

1 STREAMFLO

0.1 1992 1994 1996 1998 2000 2002 YEAR 10,000 D N

Measured 1,000 Simulated

100 CUBIC FEET PER SECO N , I W

10 STREAMFLO

1 0.2 0.51 2 58010 20 3050 70 9095 98 99 99.5 99.8 EXCEEDANCE PROBABILITY, IN PERCENT D

N 10,000

Line of equal value

1,000 CUBIC FEET PER SECO

N 100 , I W

SIMULATED STREAMFLO SIMULATED 1 10 100 1,000 10,000 MEASURED STREAMFLOW, IN CUBIC FEET PER SECOND

Figure 7. Measured and simulated daily mean streamflow at U.S. Geological Survey streamflow-gaging station 08200000 Hondo Creek near Tarpley, Texas, 1992–2003. 16 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003

100,000 D N Measured 10,000 Simulated

1,000 CUBIC FEET PER SECO

N 100 , I W

10 STREAMFLO

1 1992 1994 1996 1998 2000 2002 YEAR 10,000 D N

Measured 1,000 Simulated

100 CUBIC FEET PER SECO N , I W

10 STREAMFLO

1 0.2 0.51 2 58010 20 3050 70 9095 98 99 99.5 99.8 EXCEEDANCE PROBABILITY, IN PERCENT D

N 10,000

Line of equal value

1,000 CUBIC FEET PER SECO

N 100 , I W

SIMULATED STREAMFLO SIMULATED 1 10 100 1,000 10,000 MEASURED STREAMFLOW, IN CUBIC FEET PER SECOND

Figure 8. Measured and simulated daily mean streamflow at U.S. Geological Survey streamflow-gaging station 08200700 Hondo Creek at King Waterhole near Hondo, Texas, 1992–2003. Simulation of Streamflow and Estimation of Recharge 17 inflow, site 10 in fig. 4, table 3) during 1992–93. The compari- recharge likely accounts for about 90 percent of rainfall. Maclay son is summarized in table 7 (at end of report). Generally, sim- (1995, p. 31) reports that evapotranspiration in the area is about ulated volumes and peak flows were higher than measured 85 to 90 percent of rainfall. Uncertainty in ET simulations flows. Simulated flow volume (43,600 acre-ft) during 1992–93 can result in considerable potential error in diffuse recharge was 31.7 percent greater than measured flow volume (33,100 estimates. acre-ft). Simulated storm peaks (three measured peaks that HSPF process parameters related to ET simulation were exceeded 1,000 ft3/s) exceeded measured peaks by 77.9 not calibrated with actual data because very few measured ET percent. data are available for the region, and no ET data have been col- Because a substantial part of recharge to the Edwards aqui- lected in the study area. ET-related parameter values from other fer occurs as infiltration of flow in the stream channel, simu- HSPF models (Brown and Raines, 2002; Ockerman, 2002) lated streamflow that exceeds measured streamflow in San were used to guide selection of those parameters for the Hondo Geronimo Creek likely results in overestimation of recharge. Creek model. Also, parameters that affect ET also affect Therefore, on the basis of 1992–93 streamflow data from the streamflow and are adjusted during the streamflow calibration. San Geronimo Creek watershed, the model was recalibrated by To test the validity of the ET simulations, only general compar- adjusting selected process-related parameters, primarily isons with few available data were possible. The simulated INFILT and LZSN (table 1). Channel-loss characteristics in the average annual ET values from several PERLND categories are FTABLEs also were adjusted. The simulation results after reca- listed in table 8 (at end of report). libration are shown in figure 9 and table 7. For the three land categories listed in table 8, average The resulting simulated total flow volume and the highest 1992–2003 ET ranged from 77 to 89 percent of rainfall. Annual 10 percent (1.9 percent error) and lowest 50 percent (0 percent ET showed greater variability, ranging from 54 to 115 percent error) of daily flow volumes are close to measured data. Coef- of annual rainfall. ficients of determination and model-fit efficiency indicate an Probably the best-available ET data for comparison were acceptable match between measured and simulated annual and collected during 1991–95 by the Natural Resources Conserva- monthly streamflows. However, coefficients of determination tion Service in the Seco Creek watershed (site 24, fig. 4 and and model-fit efficiency are low (compared to those of the table 3) adjacent to the Hondo Creek watershed in northeastern Hondo Creek simulations) for daily and hourly simulations. Uvalde County (Dugas and others, 1998). Average ET rates in Therefore, the model calibration for daily and hourly simula- the Seco Creek watershed during March through October were tions is relatively poor. One factor that affects the calibration is 0.075 inch per day (in/d). HSPF simulations for the Hondo few available streamflow data from San Geronimo Creek. Creek watershed during the same time period and with similar Because streamflow is intermittent at the measurement site, few land cover (Glen Rose Limestone, medium slope, evergreen discharge measurements were made during the 1992–93 data- forest) yielded an average daily ET rate of 0.079 in/d. Com- collection period to develop and confirm the stage-discharge pared to the Seco Creek measured ET, the HSPF simulated ET rating (the relation between measured water stage and com- is considered reasonable. puted discharge). Also, gain-loss measurements, similar to the measurements made along Hondo and Verde Creeks to estimate streamflow infiltration, have not been made along San Geron- Summary of HSPF Process-Related Parameters imo Creek. Through calibration and testing, sets of process-related parameters were developed for the Hondo Creek, Verde Creek, Evapotranspiration and San Geronimo Creek watershed models. The process- related annual parameters for the Hondo Creek and Verde In addition to streamflow calibration, another goal of Creek models are listed in table 9 (at end of report). The pro- watershed-model calibration is to accurately simulate the over- cess-related annual parameters for the San Geronimo Creek all water budget in the watershed, which includes recharge and model are listed in table 10 (at end of report). Values of the evapotranspiration (ET). Model simulations of recharge cannot monthly varying lower-zone evapotranspiration parameter be compared to measured values because diffuse recharge (LZETP) for all three models are listed in table 11 (at end of (direct infiltration of rainfall through the soil layers to the water report). table), especially within the faulted karst environments of the study-area watersheds, cannot be measured directly. Estimates of recharge therefore depend on accurate model representations Streamflow, 1951–2003 of the remaining water-budget components not associated with recharge (streamflow, ET, and changes in soil-water storage The results of streamflow simulations (average annual over the simulation period). Annual streamflow in Hondo Creek streamflow volumes) for each watershed model are listed in (King Waterhole streamflow-gaging station) usually is less than table 12 (at end of report). Total streamflow for a watershed 10 percent of rainfall. Changes in soil-water storage (over long includes all surface runoff, interflow, and base flow that origi- simulation periods) are relatively small. Therefore, ET plus nates in the watershed and reaches the stream channel. Because 18 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003

100,000 D N Measured 10,000 Simulated

1,000 CUBIC FEET PER SECO

N 100 , I W

10 STREAMFLO 1 1992 1993 YEAR 10,000 D N Measured 1,000 Simulated

100 CUBIC FEET PER SECO N , I W 10 STREAMFLO 1 0.2 0.51 2 58010 20 3050 70 9095 98 99 99.5 99.8 EXCEEDANCE PROBABILITY, IN PERCENT 10,000 D N

Line of equal value

1,000

100 CUBIC FEET PER SECO N , I W

1 1 10 100 1,000 10,000

SIMULATED STREAMFLO SIMULATED MEASURED STREAMFLOW, IN CUBIC FEET PER SECOND

Figure 9. Measured and simulated daily mean streamflow at U.S. Geological Survey streamflow-gaging station 08180590 San Geronimo Creek Reservoir inflow near Rio Medina, Texas, 1992–93. Simulation of Streamflow and Estimation of Recharge 19 of streamflow channel infiltration, much of the total streamflow Hondo Creek, about 0.5 percent in Verde Creek, and about 9 is lost to recharge before it reaches the watershed outlet. Simu- percent San Geronimo Creek). The recharge estimates in table lated average annual total streamflow for 1951–2003 in the 13 do not include channel infiltration downstream from the Hondo Creek, Verde Creek, and San Geronimo Creek water- recharge zone, and they do not include estimates of subsurface sheds is 45,400; 32,400; and 11,100 acre-ft; respectively. Sim- inflow to the Edwards aquifer from other aquifers. ulated average annual streamflow at the respective watershed For the Hondo Creek watershed, estimated average annual outlets is 13,000; 16,200; and 6,920 acre-ft; the difference recharge for 1951–2003 is 37,900 acre-ft/yr, or 5.04 inches (in.) between total streamflow and streamflow at the watershed out- (about 17 percent of average annual rainfall). Most of the let is streamflow lost to channel infiltration. recharge (82 percent2) occurred as streamflow channel infiltra- Model simulations also were done to assess the effects on tion. About 18 percent occurred as diffuse recharge, or direct streamflow of recharge-enhancement structures completed for infiltration of rainfall to the aquifer. Annual Hondo Creek Verde Creek in April 1978 and for San Geronimo Creek in recharge is highly variable (depending on annual rainfall), rang- November 1979. The recharge-enhancement structures divert ing from 1,920 acre-ft (0.26 in.) in 1956 to 117,000 acre-ft (15.6 streamflow to recharge. Three simulations were done for each in.) in 1992. of the two watersheds: (1) for 1951–2003 for actual conditions, For the Verde Creek watershed, estimated average annual which means with the structures simulated in the Verde Creek recharge for 1951–2003 is 26,000 acre-ft/yr, or 3.36 in. (about watershed beginning in April 1978 and in the San Geronimo 11 percent of average annual rainfall). About 62 percent2 of the watershed beginning in November 1979; (2) for 1951–2003 recharge occurred as streamflow channel infiltration. About 38 without the structures simulated in either watershed for the percent occurred as diffuse recharge. Annual Verde Creek entire period; and (3) for 1951–2003 with the structures simu- recharge varied from 1,760 acre-ft (0.23 in.) in 1956 to 73,800 lated in both watersheds for the entire period. acre-ft (9.54 in.) in 1992. For Verde Creek simulation (1) streamflow at the water- For the San Geronimo Creek watershed, estimated average shed outlet was 16,200 acre-ft. Simulations (2) and (3) resulted annual recharge for 1951–2003 is 5,940 acre-ft/yr, or 1.97 in. in average annual streamflow of 16,800 and 15,700 acre-ft, (about 6 percent of average annual rainfall). About 64 percent2 respectively. So, simulation with the recharge-enhancement of the recharge occurred as streamflow channel infiltration. structure in place during 1951–2003 for the entire period About 36 percent occurred as diffuse recharge. Annual San resulted in a reduction of streamflow exiting the watershed of Geronimo Creek recharge varied from 100 acre-ft (0.03 in.) in about 6.5 percent. 1954 to 26,700 acre-ft (8.87 in.) in 1992. For San Geronimo Creek simulation (1), streamflow at the Average annual recharge estimates from the San Geron- watershed outlet was 6,920 acre-ft. Simulations (2) and (3) imo Creek model before and after recalibration with measured resulted in average annual runoff of 7,680 and 6,160 acre-ft, streamflow data were compared to assess the sensitivity of the respectively. So, simulation with the recharge-enhancement recharge estimation to streamflow calibration. During 1992–93 structure in place during 1951–2003 for the entire period (period when streamflow calibration data were available) the resulted in a reduction of streamflow exiting the watershed by simulated runoff volume exiting the San Geronimo Creek about 20 percent. The drainage area above the San Geronimo watershed before recalibration, using process-related parame- Creek recharge-enhancement structure is about 78 percent of ters from the calibrated Hondo Creek model (43,600 acre-ft, the watershed (compared to about 29 percent of the Verde table 7), was about 32 percent greater than after recalibration Creek watershed upstream from the Verde Creek structure). So, (33,100 acre-ft, table 7). Before recalibration the simulated as might be expected, the effect of the Verde Creek structure on average annual (1951–2003) recharge was 6,220 acre-ft/yr, or streamflow at the watershed outlet is not as great as the effect of 2.06 in. This estimate is about 4.7 percent greater than the esti- the San Geronimo Creek structure. mate after recalibration (5,940 acre-ft, table 13). During the period when streamflow calibration data were available (1992–93), the recharge estimate before recalibration was 5.0 Recharge, 1951–2003 percent greater than the estimate after recalibration. So, for the San Geronimo Creek watershed, the recharge estimate was not The HSPF models for the Hondo Creek, Verde Creek, particularly sensitive to the recalibration. and San Geronimo Creek watersheds were used to estimate HSPF 1992–2003 recharge estimates for Hondo Creek Edwards aquifer recharge during 1951–2003. Annual recharge and Verde Creek were compared to USGS estimates of recharge estimates (simulated recharge) by watershed are listed in made using the Puente method (Puente, 1978; R.N. Slattery, table 13 (at end of report). Recharge comprises direct infiltra- U.S. Geological Survey, written commun., 2003). Because tion of rainfall (diffuse recharge) and infiltration of streamflow the USGS recharge estimates are annual values, only annual within the stream channels. Most of the simulated streamflow comparisons with HSPF simulations are possible. Estimates channel infiltration occurs in the recharge zone, but some of recharge for the San Geronimo Creek watershed by the occurs downstream from the recharge zone (about 5 percent in Puente method are not available because, in the Puente

2 Does not include streamflow channel infiltration downstream from the recharge zone. 20 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003

method, recharge in that watershed is included in the recharge occurs as infiltration of streamflow and because recharge events computation for a larger watershed. Recharge estimates for the and processes tend to occur on time scales substantially greater Hondo Creek and Verde Creek watersheds for 1992–2003 com- than hourly. puted from HSPF simulation and the Puente method are listed The accuracy of streamflow and recharge estimates from in table 14 (at end of report). Overall, for Hondo Creek, the the Verde Creek and San Geronimo Creek models is difficult to HSPF recharge estimates average 40,100 acre-ft/yr compared to assess because of the lack of streamflow data available for 51,700 acre-ft/yr estimated by the Puente method (about 22 per- model calibration. Transfer of calibrated process-related param- cent less). HSPF recharge estimates for Verde Creek average eters from the Hondo Creek model to the San Geronimo Creek 28,700 acre-ft/yr compared to 47,700 acre-ft/yr estimated by model resulted in simulations that did not compare favorably the Puente method (about 40 percent less). For years of greater- with the few measured data. Therefore, using Hondo Creek than-average rainfall in both watersheds, the HSPF recharge process-related parameters in the Verde Creek model also estimates are substantially less than the Puente recharge esti- might not produce simulations that reasonably represent actual mates. For years of average or less-than-average rainfall, the conditions. relation between estimates from the two methods is mixed in Because most recharge to the Edwards aquifer occurs as terms of which is greater. streamflow infiltration and most of that streamflow originates in the Glen Rose Limestone outcrop upstream from the recharge Model Limitations and Assessment zone, gaged streamflow upstream from and downstream from the Edwards aquifer recharge zone (similar to that measured in Hondo Creek) might be useful for improving the calibration of Errors in model calibration can be classified either as sys- the Verde Creek and San Geronimo Creek HSPF models. Addi- tematic or measurement errors (Raines, 1996). Systematic tional gain-loss measurements, especially along San Geronimo errors are associated with limitations of the model to represent Creek, also might help improve model calibration. Because ET the hydrologic processes of the study watershed. There are pos- is a large component of the water budget in these study water- sible limits on how well the model parameters and equations sheds, additional ET data, especially in the Edwards aquifer describe the physical characteristics of runoff and recharge. recharge zone, might improve model calibration and estimates Measurement errors are introduced as a result of inaccurate or of recharge. missing data. Limitations in the available rainfall data are potentially the most serious source of measurement error for the study. Runoff and recharge in the watershed are highly depen- Summary dent on the occurrence of relatively extreme storms. The available network of rain gages does not always adequately represent the spatial and temporal variability of rainfall in the watershed. The U.S. Geological Survey (USGS), in cooperation with HSPF model calibration for the three study-area water- the San Antonio Water System, constructed HSPF watershed sheds primarily is based on calibration of HSPF process-related models to simulate streamflow (runoff) and estimate recharge parameters for the Hondo Creek watershed. Process-related to the Edwards aquifer in the Hondo Creek, Verde Creek, and parameters obtained from the Hondo Creek calibration were San Geronimo Creek watersheds in south-central Texas. The applied directly to the other study watersheds. The Hondo models were calibrated and tested with available data collected Creek process parameters were applied to the Verde Creek during 1992–2003. Simulations of runoff and recharge to the model without recalibration because of a lack of streamflow Edwards aquifer were done for 1951–2003. data for calibration for the Verde Creek watershed. Therefore, Outcropping rocks in the upper part of each watershed are the accuracy of streamflow and recharge estimations for the Glen Rose Limestone; outcropping rocks in the central part of Verde Creek watershed is uncertain. Using Hondo Creek each watershed—the Edwards aquifer recharge zone—are parameter values, the San Geronimo Creek model simulations Edwards Group. Glen Rose Limestone generally has low per- for 1992–93 did not satisfy calibration criteria. Therefore, meability compared to rocks of the Edwards Group. Edwards the San Geronimo Creek model was recalibrated on the basis Group rocks are characterized by high permeability because of of the 1992–93 streamflow data from the San Geronimo faults, sinkholes, and other karst features. Relatively imperme- watershed. able confining units overlie the Edwards aquifer south of the The Hondo Creek model-fit statistics indicate that recharge zone, the lower part of each watershed. Streams in the streamflow simulations compare well with measured data on an study-area watersheds originate in the topographically rugged annual, monthly, and daily basis. However, the coefficient of Texas Hill Country north of the recharge zone and generally model-fit efficiency for hourly comparisons is low. The model, flow south, crossing the recharge zone and continuing onto therefore, is not as effective in predicting hourly streamflow Edwards aquifer confining units. values—for example, peak discharges. However, daily or Two USGS streamflow-gaging stations on Hondo Creek monthly streamflow volume is predicted with reasonable accu- provided adequate data to calibrate the HSPF model of the racy and either daily or monthly is likely more important than Hondo Creek watershed. However, suitable calibration data hourly for estimating recharge because much of the recharge for the Verde Creek and San Geronimo Creek watersheds is References 21 lacking. Verde Creek is ungaged. Data from the San Geronimo as the Puente method. For the Hondo Creek watershed, the Creek watershed is limited to streamflow data from a discontin- HSPF recharge estimates for 1992–2003 average 40,100 acre- ued USGS streamflow-gaging station operated during 1992–93. ft/yr compared to 51,700 acre-ft/yr estimated by the Puente The approach to develop the models was to first calibrate method (about 22 percent less). HSPF recharge estimates for the the Hondo Creek model (with an hourly time step) using Verde Creek watershed average 28,700 acre-ft/yr compared to 1992–99 data and test that model using 2000–2003 data. Then 47,700 acre-ft/yr estimated by the Puente method (about 40 per- the calibrated Hondo Creek model parameters were applied to cent less). During years of greater-than-average rainfall in both the Verde Creek and San Geronimo Creek watersheds. For the watersheds, the HSPF recharge estimates are substantially less entire calibration and testing period (1993–2003), the simulated than the Puente recharge estimates. During years of average or total flow volumes for Hondo Creek exceeded measured flow less-than-average rainfall, the relation between estimates from volumes at both USGS gages by about 1.1 percent, within limits the two methods is mixed in terms of which is greater. considered acceptable for calibration. Two other measures of simulation quality, coefficients of determination and model-fit efficiency, indicate (by values of 0.9 or greater) that annual, References monthly, and daily simulated streamflows for Hondo Creek adequately matched measured values, but simulated hourly Aragon Long, S.C., Reece, B.D., and Eames, D.R., 2005, Water streamflows did not. resources data, Texas, water year 2004—Volume 5. Guada- The Verde Creek model, using parameters from the Hondo lupe River Basin, Nueces River Basin, Rio Grande Basin, and Creek model, remains untested. The accuracy of streamflow intervening coastal basins: U.S. Geological Survey Water- simulations for Verde Creek thus is uncertain. Data Report TX–04–5, 414 p., accessed July 20, 2005, at After applying the Hondo Creek model parameters to the http://water.usgs.gov/wdr/2004/WDR-TX-04-5/. San Geronimo Creek model, the simulation was tested using Ashworth, J.B., and Hopkins, Janie, 1995, Aquifers of Texas: 1992–93 streamflow data from the discontinued USGS gage. Texas Water Development Board Report 345, 69 p. Because the simulated streamflow volumes compared poorly to Bicknell, B.R., Imhoff, J.C., Kittle, J.L. Jr., Donigian, A.S., and the measured volumes, the San Geronimo Creek model was Johanson, R.C., 2001, Hydrological Simulation Program— recalibrated using the 1992–93 data. The streamflow calibration FORTRAN, user’s manual for version 12: Research Triangle was improved but not to the level of the Hondo Creek model Park, N.C., U.S. Environmental Protection Agency, National calibration. The resulting simulated total flow volume and the Exposure Research Laboratory, Office of Research and highest 10 percent and lowest 50 percent of daily flow volumes Development, 725 p. are within 1.9 percent of measured data. Coefficients of deter- Bomar, G.W., 1995, Texas weather (2d ed.): Austin, University mination and model-fit efficiency indicate an acceptable (or of Texas Press, 275 p. nearly so) match between simulated and measured annual and Brown, D.S., and Raines, T.H., 2002, Simulation of flow and monthly streamflows. However, those coefficients are low effects of best-management practices in the upper Seco (compared to those of the Hondo Creek simulations) for daily Creek Basin, south-central Texas, 1991–98: U.S. Geological and hourly simulations. Therefore, the model calibration for Survey Water-Resources Investigations Report 02–4249, daily and hourly streamflows is relatively poor compared to the 22 p. Hondo Creek model calibration. Clark, A.K., 2003, Geologic framework and hydrologic Simulated average annual total streamflow for 1951–2003 features of the Glen Rose Limestone, Camp Bullis Training in the Hondo Creek, Verde Creek, and San Geronimo Creek Site, Bexar County, Texas: U.S. Geological Survey Water- watersheds is 45,400, 32,400, and 11,100 acre-ft, respectively. Resources Investigations Report 03–4081, 9 p. Simulated average annual streamflow at the respective water- Donigian, A.S., Jr., Bicknell, B.R., and Imhoff, J.C., 1995, shed outlets is 13,000, 16,200, and 6,920 acre-ft; the difference Hydrological Simulation Program—FORTRAN (HSPF), in between total streamflow and streamflow at the watershed out- Singh, V.P., ed., Computer models of watershed hydrology: let is streamflow lost to channel infiltration. Highlands Ranch, Colo., Water Resources Publications, Estimated average annual Edwards aquifer recharge p. 395–442. (direct infiltration of rainfall [diffuse recharge] and channel Donigian, A.S., Jr., Imhoff, J.C., Bicknell, B.R., and Kittle, J.L., infiltration) for Hondo Creek, Verde Creek, and San Geronimo Jr., 1984, Application guide for Hydrological Simulation Creek for 1951–2003 is 37,900 acre-ft (5.04 in.), 26,000 acre-ft Program—FORTRAN (HSPF): U.S. Environmental (3.36 in.), and 5,940 acre-ft (1.97 in.), respectively. Most of the Protection Agency, Environmental Research Laboratory, recharge (about 77 percent for the three study watersheds EPA–600/3–84–065, 177 p. together) occurs as streamflow channel infiltration. Diffuse Dugas, W.A., Hicks, R.A., and Wright, P., 1998, Effect of recharge accounts for the remaining 23 percent of recharge. removal of Juniperus ashei on evapotranspiration and runoff HSPF 1992–2003 annual recharge estimates for the Hondo in the Seco Creek watershed: Water Resources Research, Creek and Verde Creek watersheds were compared to USGS v. 34, no. 6, p. 1,499–1,506. estimates of historical annual recharge for those watersheds Edwards Aquifer Authority, 2004, Hydrologic data report for computed using a method documented in 1978 and referred to 2003: San Antonio, Report 04–02, 188 p. 22 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003

HDR Engineering, 1998, Trans-Texas water program, west munities on the Edwards aquifer recharge area: Uvalde, Tex., central study area—Phase II Guadalupe-San Antonio River Texas Agricultural Experiment Station and Texas Agricul- Basin recharge enhancement study feasibility assessment: tural Extension Service, 9 p. Austin, Tex., HDR Engineering [variously paged]. Puente, Celso, 1978, Method of estimating natural recharge to Knapp, H.V., Singh, Jaswinder, and Andrew, Karka, 2004, the Edwards aquifer in the San Antonio area, Texas: U.S. Hydrologic modeling of climate scenarios for two Illinois Geological Survey Water-Resources Investigations Report watersheds: Watershed Science Section, Illinois State Water 78–10, 34 p. Survey, Contract report 2004–07, accessed July 20, 2005, at Raines, T.H., 1996, Simulation of storm peaks and storm vol- http://www.sws.uiuc.edu/pubdoc/CR/ISWSCR2004-07.pdf umes for selected subbasins in the West Fork Trinity River Land, L.F., Boning, C.W., Harmsen, L., and Reeves, R.D., Basin, Texas, water years 1993–94: U.S. Geological Survey 1983, Streamflow losses along the Balcones fault zone, Water-Resources Investigations Report 96–4110, 41 p. Nueces River Basin, Texas: U.S. Geological Survey Water- Small, T.A., and Clark, A.K., 2000, Geologic framework and Resources Investigations Report 83–4368, 72 p. hydrogeologic characteristics of the Edwards aquifer out- Larkin, T.J., and Bomar, G.W., 1983, Climatic atlas of Texas: crop, Medina County, Texas: U.S. Geological Survey Water- Texas Department of Water Resources, Limited Printing Resources Investigations Report 00–4195, 10 p., 1 pl. Report LP–192, 151 p. U.S. Army Corps of Engineers, Coastal and Hydraulics Labora- Lumb, A.M., McCammon, R.B., and Kittle, J.L., Jr., 1994, tory, 2004, WMS—Watershed modeling system: accessed Users manual for an expert system (HSPEXP) for calibration April 20, 2004, at http://chl.erdc.usace.army.mil/ of the Hydrological Simulation Program—FORTRAN: U.S. CHL.aspx?p=s&a=Software;20 Geological Survey Water-Resources Investigations Report U.S. Environmental Protection Agency, 2003, BASINS— 94–4168, 102 p. Better assessment science integrating point & nonpoint Maclay, R.W., 1995, Geology and hydrology of the Edwards sources: accessed April 30, 2004, at http://www.epa.gov/ aquifer in the San Antonio area, Texas: U.S. Geological OST/BASINS/. Survey Water-Resources Investigations Report 95–4186, U.S. Geological Survey, 2001, Elevation program: accessed 64 p., 12 pls. May 12, 2004, at http://rockyweb.cr.usgs.gov/elevation/. Nash, J.E., and Sutcliffe, J.V., 1970, River flow forecasting U.S. Geological Survey, 2003, National land cover character- through conceptual models—Part 1. A discussion of princi- ization: accessed July 20, 2004, at http://landcover.usgs.gov/ ples: Journal of Hydrology, v. 10, no. 3, p. 282–290. natllandcover.asp Ockerman, D.J., 2002, Simulation of runoff and recharge and Wanielista, M.P., 1990, Hydrology and water quantity control: estimation of constituent loads in runoff, Edwards aquifer New York, Wiley, 565 p. recharge zone (outcrop) and catchment area, Bexar County, Wicklein, S.M., and Schiffer, D.M., 2002, Simulation of runoff Texas, 1997–2000: U.S. Geological Survey Water- and water quality for 1990 and 2008 land-use conditions Resources Investigations Report 02–4241, 31 p. in the Reedy Creek watershed, east-central Florida: U.S. Owens, M. K., Lyons, Robert, and Kneuper, Charles, 2001, Geological Survey Water-Resources Investigations Report Evaporation and interception water loss from juniper com- 02–4018, 62 p. Table 6 23

Table 6. Streamflow calibration and testing results, Hydrological Simulation Program—FORTRAN model, Hondo Creek watershed,Table south-central 6 Texas, 1992–2003. [acre-ft, acre-feet; ft3/s, cubic feet per second; --, not applicable] Hondo Creek near Tarpley (08200000)

Comparison of streamflow Measured Simulated Error1 Criteria2 volumes and peaks streamflow streamflow (percent) (percent)

Calibration period 1992–99

Total flow volume, acre-ft 282,000 282,000 0 10

Total of highest 10 percent of daily flow volumes, acre-ft 192,000 183,000 -4.7 15

Total of lowest 50 percent of daily flow volumes, acre-ft 15,000 14,700 -2.0 10

Average of selected storm peaks, ft3/s (20 storms) 9,030 7,640 -15.4 --

Testing period 2000–2003

Total flow volume, acre-ft 186,000 191,000 2.7 10

Total of highest 10 percent of daily flow volumes, acre-ft 125,000 130,000 4.0 15

Total of lowest 50 percent of daily flow volumes, acre-ft 7,730 8,590 11.1 10

Average of selected storm peaks, ft3/s (4 storms) 20,100 16,900 -15.9 --

Entire (calibration and testing) period 1992–2003

Total flow volume, acre-ft 468,000 473,000 1.1 10

Total of highest 10 percent of daily flow volumes, acre-ft 317,000 313,000 1.3 15

Total of lowest 50 percent of daily flow volumes, acre-ft 22,700 23,300 2.6 10

Average of selected storm peaks, ft3/s (24 storms) 14,000 11,900 -15.0 --

Model-fit statistics 1992–2003 Annual Monthly Daily mean Hourly3

Number of years, months, days, or hours 12 144 4,383 104,448

Coefficient of determination .98 .92 .94 .72

Coefficient of model-fit efficiency4 .96 .90 .91 .68

Percentage of time simulated within 10 percent of measured 41.7 18.0 18.4 18.7

Percentage of time simulated within 25 percent of measured 58.3 36.8 33.6 33.8 24 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003

Table 6. Streamflow calibration and testing results, Hydrological Simulation Program—FORTRAN model, Hondo Creek watershed, south-central Texas, 1992–2003—Continued.

Hondo Creek at King Waterhole near Hondo (08200700)

Comparison of streamflow Measured Simulated Error1 Criteria2 volumes and peaks streamflow streamflow (percent) (percent)

Calibration period 1992–99

Total flow volume, acre-ft 104,000 103,000 1.0 10

Total of highest 10 percent of daily flow volumes, acre-ft 104,000 103,000 1.0 15

Total of lowest 50 percent of daily flow volumes, acre-ft 0 0 0 10

Average of selected storm peaks, ft3/s (13 storms) 10,800 8,800 -18.5 --

Testing period 2000–2003

Total flow volume, acre-ft 81,100 84,300 3.9 10

Total of highest 10 percent of daily flow volumes, acre-ft 81,100 84,300 3.9 15

Total of lowest 50 percent of daily flow volumes, acre-ft 0 0 0 10

Average of selected storm peaks, ft3/s (3 storms) 19,900 33,000 65.8 --

Entire (calibration and testing) period 1992–2003

Total flow volume, acre-ft 185,000 187,000 1.1 10

Total of highest 10 percent of daily flow volumes, acre-ft 185,000 187,000 .8 15

Total of lowest 50 percent of flow volumes, acre-ft 0 0 0 10

Average of selected storm peaks, ft3/s (16 storms) 12,500 13,300 6.4 --

Model-fit statistics 1992–2003 Annual Monthly Daily mean Hourly3

Number of years, months, days, or hours 12 144 4,383 105,192

Coefficient of determination .94 .94 .92 .67

Coefficient of model-fit efficiency4 .93 .93 .90 .66

Percentage of time simulated within 10 percent of measured 33.3 84.7 94.2 94.6

Percentage of time simulated within 25 percent of measured 41.7 85.4 94.7 95.0

1 Error = ([simulated-measured]/measured) X 100. 2 Default error criteria from Donigian and others (1984). 3 Hourly statistics for 0820000 and 08200700 do not include July 2002. 4 From Nash and Sutcliffe (1970). Table 7 25

Table 7. Streamflow testing and recalibration results, Hydrological Simulation Program—FORTRAN model, San Geronimo Creek watershed,Table south-central 7 Texas, 1992–93. [acre-ft, acre-feet; ft3/s, cubic feet per second; -- not applicable]

Model testing results using process-related parameters from calibrated Hondo Creek HSPF model

Comparison of streamflow Measured Simulated Error1 Criteria2 volumes and peaks streamflow streamflow (percent) (percent) Testing period 1992–93 Total flow volume, acre-ft 33,100 43,600 31.7 10 Total of highest 10 percent of daily flow volumes, acre-ft 31,900 37,000 16.0 15 Total of lowest 50 percent of daily flow volumes, acre-ft 0 0 0 10 Average of selected storm peaks, ft3/s (3 storms) 7,700 13,700 77.9 --

Model-fit statistics 1992–93 Annual Monthly Daily Hourly Number of years, months, days, or hours 2 24 731 17,544 Coefficient of determination 1.00 .87 .60 .23 Coefficient of model-fit efficiency3 .84 .83 .55 .23 Percentage of time simulated within 10 percent of measured 0 45.8 66.1 67.0 Percentage of time simulated within 25 percent of measured 0 58.3 69.5 70.8

Model results after recalibration with measured San Geronimo Creek streamflow4 for 1992–93

Comparison of streamflow Measured Simulated Error1 Criteria2 volumes and peaks streamflow streamflow (percent) (percent) Calibration period 1992–93 Total flow volume, acre-ft 33,100 33,100 0 10 Total of highest 10 percent of daily flow volumes, acre-ft 31,900 32,500 1.9 15 Total of lowest 50 percent of daily flow volumes, acre-ft 0 0 0 10 Average of selected storm peaks, ft3/s (3 storms) 7,700 6,650 -13.6 --

Model-fit statistics 1992–93 Annual Monthly Daily Hourly Number of years, months, days, or hours 2 24 731 17,544 Coefficient of determination 1.00 .88 .65 .20 Coefficient of model-fit efficiency3 .99 .88 .64 .13 Percentage of time simulated within 10 percent of measured 50.0 70.8 72.9 75.1 Percentage of time simulated within 25 percent of measured 50.0 70.8 75.5 75.4

1 Error = ([simulated-measured]/measured) X 100. 2 Default error criteria from HSPEXP (Lumb and others, 1994). 3 From Nash and Sutcliffe, 1970. 4 Measured at San Geronimo Creek Reservoir inflow near Rio Medina (08180590). 26 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003

Table 8. Rainfall and simulated evapotranspiration, Hydrological Simulation Program—FORTRAN model, Hondo Creek watershed, south-centralTable Texas, 8 1992–2003. [In inches; ET, evapotranspiration]

Land category

Confining units of Edwards Glen Rose Limestone, Edwards Group, evergreen Year aquifer, evergreen forest, evergreen forest, low slope forest, low slope low slope

Rainfall ET Rainfall ET Rainfall ET 1992 49.74 32.40 49.74 32.48 44.56 35.49 1993 24.47 24.78 22.55 23.13 41.83 37.89 1994 37.10 28.93 23.31 21.30 16.60 19.15 1995 27.94 27.54 25.11 22.49 33.18 27.22 1996 19.33 18.51 13.50 13.49 19.12 21.65 1997 43.62 30.16 35.03 25.19 20.13 19.86 1998 39.17 28.14 32.65 23.98 59.08 42.45 1999 16.06 16.46 15.27 16.15 31.32 30.43 2000 30.94 21.46 23.06 19.07 17.94 19.67 2001 33.25 26.87 23.50 20.18 32.49 25.11 2002 51.01 27.75 35.56 21.97 42.37 34.56 2003 26.99 25.07 27.54 23.46 19.11 21.69

Average 1992–2003 33.30 25.67 27.24 21.91 31.48 27.93 Table 9 27 models, Hondo Creek and Verde Verde and Creek Hondo models, Program—FORTRAN PERLND segment IMPLND segment IMPLND Process-related annual parameters for pervious and impervious land segments, Hydrological Simulation Simulation Hydrological segments, land impervious and pervious for parameters annual Process-related Land cover and slope and cover Land AGWETP AGWRC BASETP CEPSC DEEPFR INFILT INTFW IRC LSUR LZSN NSUR RETSC SLSUR UZSN Table 9 All imperviousAll ------400-- .10 .05 .06 -- Evergreen forest, low slopeDeciduous/mixed forest, low slopeShrubland, low slopeGrassland, low slope 0 0slope low Bare/transitional, Agricultural, low slope .96 0.96Developed, low slopeEvergreen 0.001 forest, medium slope 0 .001 0Deciduous/mixed forest, medium slope 0 0.27 .15Shrubland, medium slope .960.09 0 0 .96 0Grassland,medium slope .09 .960.24 .001slope medium Bare/transitional, 0 .001 .96 0.90 .24 .96Agricultural, .001 medium .96slope .12 0.75 .10Developed,medium slope .96 0 .001 .90 .10 .001 .001 400 0 .09Evergreen forest, steep slope 0 .09 .15 .75 .001 2.3 .09Deciduous/mixed forest, .10 steep slope .26 .96 .24 400 0 0.30 .96Shrubland, steepslope .24 .09 .10 .96 .09 .24 .09 2.3Grassland,steep .001 0 slope .90 0-- 0 .001 .90 .09 .23 .96 slope steep Bare/transitional, .001 .90 .24 .30 .23 .12 .75 0.04Agricultural, steep .75 slope .10 .96 .96 .90-- 400 .24 .10 .75 .001 .960.22 .90 .90 400 .09Developed,steep slope .09 400 2.3 0 .75 .001 .001 .90 .09 2.3 .10 .75 .04 .75 .001 0 2.3 .23 400 0 .30 400 .23 400 .15 .10 .30 .75 .09 .22 .23 .25 2.2 .96 .30 .90-- 2.3 400 2.2 0 .96-- .90 .09 .09 .96 .90 -- .30 .23 .09 2.3 .30 .75 .001 0 .30 .04 .75 .001 .04-- .22 400 .23 .96 .75 .001-- .90 .30-- .22 400 .04 .12 .22 400 2.2 .10 .22 .96 .90 .90 -- 2.2 .10 .08 .001 .75 .22 .90 .04 .09 2.2 .08 .30 .09 400 .75 .30 .001 .75 .09 .21 .10 .04 .75 .22 .30 .21 -- 2.2 .22 400 400-- .22 400 .10 .09 .22 -- .22 2.1 2.2 .30 .90 2.1 .08 .90 .08 .09 .90 .22-- .30 .30 .08 .75 .30 .21 .75 .21 -- .22-- 400 .75 .90 .21 -- .08 400 400 2.1 .90 2.1 .14 .75 .08 .21 2.1 .14 .30 400 .75 .30 .20 .21 .30 .20 2.1-- 400-- -- 2.1 .30 .14 .14-- .30 .14 .20 .20 -- .20 .14 .14 .20 .20 Creek watersheds, south-central Texas. Texas. south-central watersheds, Creek Table 9. [Parameter definitions in tablePERLND, 1; PERvious LaND;not --,applicable; IMPLND, IMPerviousLaND] outcrop) aquifer (Trinity Limestone Rose Glen 28 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003 models, Hondo Creek and Verde Verde and Creek Hondo models, Program—FORTRAN PERLND segment IMPLND segment IMPLND Process-related annual parameters for pervious and impervious land segments, Hydrological Simulation Simulation Hydrological segments, land impervious and pervious for parameters annual Process-related Land cover and slope and cover Land AGWETP AGWRC BASETP CEPSC DEEPFR INFILT INTFW IRC LSUR LZSN NSUR RETSC SLSUR UZSN All imperviousAll ------400-- .10 0.05 .04 -- Evergreen forest, low slopeDeciduous/mixed forest, low slopeShrubland, low slopeGrassland, low slope 0 0slope low Bare/transitional, Agricultural, low slope .70 0.70Developed, low slope 00 0 Evergreen forest, medium slope 0 0Deciduous/mixed forest, medium slope 0.27 .15 .70Shrubland, medium slope 0 0 0.90 .70 0 .70Grassland,medium slope .900 0 0.42slope medium Bare/transitional, 0 .70 .70 0 .42 .700.80 Agricultural, mediumslope .12 .70 0 0.600 .10Developed,medium slope0 .800 .10 0 400 .90 0Evergreen forest, steep slope .90 0 .60 2.2 .90 .15 .10Deciduous/mixed .70 forest, steep slope .26 400 .42 0 0.30 .70 .42Shrubland, steepslope .70 .90 .10 2.2 .90 .420 .90 0 .80 0-- Grassland,steep slope0 0 .800 .90 .70 .30 .42 .80 slope steep .42Bare/transitional, .60 0.04 .42 .12 .60-- .70 400 .70Agricultural, steep slope .100 0.30 .42 .60 .80 .70 .80 .10 400 .80 .90 2.2Developed,steep slope 400 0 .900 .04 2.2 0 .60 .80 .60 .900 2.2 .10 .60 0 .30 400 .42 0 400 .30 .30 400 .42 .60 .10 .30 .15 2.1 .90-- .42 2.2 .70 .25 2.1 400 .80-- 0 .70 .80 -- .70 .90 .30 .90 2.2 .30 .80 .04 .420 .30 .90 .60 0 .040 .60-- 400-- .04 .300 .70 .42 .60 .40-- .30 .80 400 .40 .30 .12 2.1 400 -- .70 2.1 .30 .08 .10 .040 .80 .80 .60 .08 .10 2.1 .80 .30 .90 400 .30 .30 .040 .90 .30 .60 .60 .30 .30 .90-- .10 2.1 .60 400 400 .40-- .30 400 .40-- .10 2.1 2.0 .30 .90 .40 .08 2.0 .80 .08 .80-- .08 .30 .90 .30 .80 .30 .40 .30 .60 .30 .60-- -- 400 .30 .08 .40 .60-- .80 400 2.0 400 .08 .14 2.0 .30 .80 .60 .14 2.0 .30 400 .30 .30 .30 .60 .30 .30-- 2.0 400-- -- 2.0 .30 .14 .14-- .14 .30 .30 .30 -- .30 .14 .14 .30 .30 Creek watersheds, south-central Texas—Continued. Texas—Continued. south-central watersheds, Creek zone) recharge aquifer (Edwards outcrop Group Edwards Table 9. Table 9 29 models, Hondo Creek and Verde Verde and Creek Hondo models, Program—FORTRAN PERLND segment IMPLND segment IMPLND Process-related annual parameters for pervious and impervious land segments, Hydrological Simulation Simulation Hydrological segments, land impervious and pervious for parameters annual Process-related Land cover and slope and cover Land AGWETP AGWRC BASETP CEPSC DEEPFR INFILT INTFW IRC LSUR LZSN NSUR RETSC SLSUR UZSN All imperviousAll ------400 -- .100.05 .04 -- Evergreen forest, low slope low forest, Evergreen Deciduous/mixed forest, low slopeShrubland, low slope slope low Grassland, .001 0.001slope low Bare/transitional, 0.94 .94 slope low Agricultural, 0.001 slope low Developed, .001 .001Evergreen forest, medium slope 0.27 .001 .001 .15Deciduous/mixed forest, medium slope 0.09 .94Shrubland, medium slope .94 .001 0.30 .09 .001 .94 .001 slopemedium Grassland, .0010.90 .001 .001slope .94 medium Bare/transitional, .30 .94 .001 .940.65 .12Agricultural, mediumslope 400 .10 .94 .90 .10 .001 .001 slopemedium Developed, .001 .09 .001 2.3 .09 .001 .65 .09 .001 .001Evergreen forest, steep slope .15 .10 0.30 .26 .94400 .30Deciduous/mixed forest, steep slope .94 .30 .10 .09 .09 -- .94 .30 .001 2.3 .09Shrubland, steepslope .90 .001 .90 .09 .001 .001 .001 0.04Grassland,steep slope .90 .30 .30 .30 .001 .94 .001 .30 .65 .12 slope steep Bare/transitional, 0.22 .65 -- .10400 .94 .30 .65 .94 .90 .90 .10400 .001 slope steep Agricultural, .94 .90 .09 400 2.3 .09 .90 2.3Developed, .65 .65steep .04 slope .09 .001 .001 2.3 .10 .65 .001 .001400 400 .30 .30400 .65.001 .30 .30 .22 .10 .30 .30 .15 .001 .09 2.3 2.2 -- 400 .25 .94 .90 2.2 -- -- .90 .09 .94 .09 2.3 .90 .001 .30 .30 .94 .30 .09 .04 .30 .65 .001 .04 .65 -- -- .30 .04400 .30 .65 .001 .001 .30 .94 -- .90 .001400 .22 .30400 .12 .22 2.2 -- .22 .90 .10 .04 2.2 .08 .94 .90 2.2 .65 .001 .10 .08 .90 .09 .30400 .04 .65 .09 .30 .22 .21 .65 .30 .001 .10 .09 .21 .65400 -- 2.2400 .30 -- .21 --400 .30 2.2 .09 .10 2.1 .30 .30 .08 .90 2.1 .08 .90 .30 .08 .09 -- .30 .30 .90 .21 .30 .65 .21 -- .65 .21 --400 .90 .30 .65 .08 --400 400 2.1 .08 2.1 .65 .14 .90 .21 .14 2.1400 .30 .21 .30 .20 .65 2.1 .30 .20 --400 -- -- .30 2.1 .14 .14 -- .30 .14 .20 .20 -- .14 .20 .20 .14 .20 Creek watersheds, south-central Texas—Continued. Texas—Continued. south-central watersheds, Creek Edwards Group remnants (hilltops) overlying Glen Rose Limestone Table 9. 30 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003 models, Hondo Creek and Verde Verde and Creek Hondo models, Program—FORTRAN PERLND segment IMPLND segment IMPLND Process-related annual parameters for pervious and impervious land segments, Hydrological Simulation Simulation Hydrological segments, land impervious and pervious for parameters annual Process-related Land cover and slope and cover Land AGWETP AGWRC BASETP CEPSC DEEPFR INFILT INTFW IRC LSUR LZSN NSUR RETSC SLSUR UZSN PERLND segment PERLND IMPLND segment IMPLND Land cover and slope and cover Land AGWETP AGWRC BASETP CEPSC DEEPFR INFILT INTFW IRC LSUR LZSN NSUR RETSC SLSUR UZSN All imperviousAll ------400-- .100.05 .04 -- Evergreen forest, low slopeDeciduous/mixed forest, low slopeShrubland, low slopeGrassland, low slopeslope .001 low Bare/transitional, 0.001Agricultural, low slopeDeveloped, low slope .75 0.75Evergreen forest, medium slopeDeciduous/mixed forest, medium slope 0.001 .001 .001 .001Shrubland, medium slope .001 0.27 .001Grassland,medium slope .15 .001 .75 .75slope medium Bare/transitional, .0010.20 .75 .75Agricultural, mediumslope .20 .0010.35 .75 .001Developed,medium .001 slope .75 .001 .001 0.40 .35 .001 .001 .75 .12 .10 .001 .001 .001 0.70 .10 .15 .40 .75 .75 400 .20 .001 .20 .001 .26 .10 .75 .20 .20 5.0 .70 .001 .10 .001 .001 .20 .35 .35 .75 .20 400 0.30 .001 .35 .35 .75 .20 5.0 .12 .10 .40 .40-- .35 .001 .35 .10 .40 .40 .30 slope .20 low .001 forest, .20Evergreen .70 .35 .70 0.04 .40 .10 .40 .20Deciduous/mixed forest, low slope 400 .70 400 .70-- 0.50 .10 slope low Shrubland, .35 .40 .35 .70 400 .20 400 5.0 .70 .001 slope low Grassland, .35 0.001 400 .04 4005.0 .20 5.0slope low Bare/transitional, .70 .40 .40 .30 .30 .35 5.00.85 slope low Agricultural, .85 5.0400 .40 .30 .50 .30-- slope low .70-- .70 .35 Developed, 5.0 .40 .30 0.001 .30 .70 400 400 ---- .001 .001 .001 .40 400-- .04 imperviousAll .04 .001 .30 5.0 5.0 .70-- 0.27 .04 .15 .08 .85 5.0 400 .001 .85 .70 --0.45 .50 .50 .30 .30 .08 .85 .04 400 5.0 .001 .50 .30 .50 .45 0.35-- .001 -- .85 .001 .04 5.0 .50 .50 .30 .001-- 1.50 .85 .35 .12 .08 .50 .08 .10 .30 .001-- 0.70 .10 .081.50 -- .001-- .45 .50 400 .50 .45 .10 .08 .45 .50 .70 3.0 .10 .08 .35-- .45 .35 400 .50 0.30 .351.50 .45 3.01.50 .50 -- .35 -- 1.50 .70 .30 .70 .351.50 0.04 .70 400-- 400 1.50 -- 400 0.60 .70 3.0 3.0 3.0 400-- .70 .04 .30 .30 3.0400 .30 -- -- .60 3.0 -- .30 -- .30 -- .04 -- .04 .04 -- .60 -- .60 .04 .60 .04 400 .60 -- .60 .10 0.05 .04 -- Creek watersheds, south-central Texas—Continued. Texas—Continued. south-central watersheds, Creek Edwardsaquifer confining units deposits Alluvial Table 9. Table 10 31 Geronimo Creek watershed, watershed, Creek Geronimo PERLND segment IMPLND segment IMPLND Process-related annual parameters for pervious and impervious land segments, Hydrological Simulation Program—FORTRAN model, San model, Program—FORTRAN Simulation Hydrological segments, land impervious and pervious for parameters annual Process-related Land cover and slope and cover Land AGWETP AGWRC BASETP CEPSC DEEPFR INFILT INTFW IRC LSUR LZSN NSUR RETSC SLSUR UZSN Table 10 All imperviousAll ------400-- .10 0.05 .06 -- Evergreen forest, low slopeDeciduous/mixed forest, low slopeShrubland, low slopeGrassland, low slope .001 0.001 slopelow Bare/transitional, 0.95 .95Agricultural, low slopeDeveloped, low slope 0.001 .001Evergreen forest, medium slope .001 0.27 .001Deciduous/mixed forest, medium slope .001 .15 0.22 .95Shrubland, medium slope .95 .001 .001 .22 .95 .001Grassland, 0.3 medium slope .001 .001 slope medium Bare/transitional, .001 .95 .95 .3 1.0 .001 .95Agricultural, .12 medium slope0.8 .10 .95 .001 1.0Developed, medium slope .10 .001 .001 .001 .22 400 .001Evergreen forest, .22 steep slope .001 .15 .8 .001 .22 .103.6 .26 .95Deciduous/mixed forest, steep slope .3 .95 400 .3 .22 .001 0.3 .10 .95Shrubland, steep slope .22 .3 .223.6 .001 1.0 .001Grassland, steep slope .001 1.0 .001-- .22 .001 .95 .3 .001 1.0 .3 slope steep .3Bare/transitional, .3 .12 .8 .8 .10 .95 .95 0.04 1.0 slope steep Agricultural, .3 .10 .95 .8 .001 1.0 400 1.0-- .22 400Developed, steep 0.24slope .223.6 400 .001 .8 .001 1.0 .22 .8 .10 .001 3.6 .001 .8 .04 .3 .001 3.6 400 .3 .3 .001 400 .15 .10 .8 400 .22 .3 .3 .25 .953.6 1.0 .24 .33.6 .953.6 1.0 400-- .001 .22 .22 .95 -- 1.0 .22 .3 .3 .001 .8 3.6 -- .3 .3 .001 .8 .001 .95 .04 .3 .3 .001 .8 1.0 400 .04-- .3 .3 .12-- 400-- .04 .10 .953.6 400 .24 1.0 1.0 .001 .10 .8 3.6 .24 1.0 -- .22 .08 3.6 .24 .04 .22 .08 .3 .8 400 .001 .8 .10 .22 .3 .8 .24 .3 .3 .04 3.6 .24 400 400 .24 -- .3 .22 .10 400 -- .33.6 1.0 3.6 -- .24 .33.6 1.0 .22 .08 .3 1.0 .08 .3 .3 .8 -- .3 .08 .8 .24 1.0 .3 .8 .24 400-- -- 400-- .24 .08 3.6 400 .81.03.6 .14 .08 3.6 .24 400 .14 .3 .8 .33.6 .22 .24 .3 .22 400 ---- .3 3.6 -- .14 .14-- .3 .14 .22 .22 -- .14 .22 .22 .14 .22 Table 10. [Parameter definitions in tablePERLND, 1; PERvious LaND;not --,applicable; IMPLND, IMPerviousLaND] outcrop) aquifer (Trinity Limestone Rose Glen south-central Texas. Texas. south-central 32 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003 Geronimo Creek watershed, watershed, Creek Geronimo PERLND segment IMPLND segment IMPLND Process-related annual parameters for pervious and impervious land segments, Hydrological Simulation Program—FORTRAN model, San model, Program—FORTRAN Simulation Hydrological segments, land impervious and pervious for parameters annual Process-related Land cover and slope and cover Land AGWETP AGWRC BASETP CEPSC DEEPFR INFILT INTFW IRC LSUR LZSN NSUR RETSC SLSUR UZSN All All impervious ------400 -- .10 0.05 .04 -- Evergreen forest, low slopeDeciduous/mixed forest, low slopeShrubland, low slopeslope low Grassland, 0 0slope low Bare/transitional, slope low Agricultural, .75 0.75slope low Developed, 0 0 slope 0 medium forest, Evergreen 0 0Deciduous/mixed forest, medium slope 0.27 .15 .75Shrubland, medium slope 0 0 .75 0 0.9 .75Grassland,medium slope .9 0 0slope medium Bare/transitional, 0 0.4 .75 .75 0 .75slope medium Agricultural, .4 .120.8 .75 0Developed,medium 0 slope 0 .10 0 .10 0 .80.6 .9 slope steep 0 forest, Evergreen 0 .9500 .9 slope steep .15 forest, .10Deciduous/mixed .75 .26 0 .6 2.8 .75 .4slope steep Shrubland, .75 .10500 .9 .9 .4 .9 0.3 .4 0 0 0Grassland,steep slope 0 2.8 0 .8 .75 .9 0 slope steep Bare/transitional, .8 -- .4 .4 .8 .4 .12 .3 .75 .75 slope steep Agricultural, .6 0 .10 .75 .4 0.04 .10 .6500 .8 .8Developed, .6steep .9 slope -- .8 0 0500 0 0.4 .9500 0 2.8 .9 .10 0 .8 2.8 0 .6 .6 .04 2.8 .6 .4 .3500 500 .15 .9 .10 .4 .75500 .6 .25 .3 .4 0 .3 .4 .75 2.8 2.8 .8500 2.8 .75 -- .9 .9 0 .8 .9 -- .4 0 -- 2.8 .8 .3 .3 0 .3 .75 .6 0 .04 .4 .4 .6 .04 .3500 .8 .12 .4 -- -- .04 .6 -- .75500 0 .4 .10 2.8500 .10 .4 .8 -- .9 .8 .4 2.8 .6 .8 .04 .08 .08 2.8 0 .9 .3500 .9 .10 .3 .6 .04 .6 .4 .4 .4 .6 .4 .3 2.8500 --500 .10 .9 .4500 -- .4 .4 2.8 .3 -- 2.8 .8 2.8 .9 .08 .8 .4 .08 .3 .8 -- .3 .08 .3 .6 .4 .4500 .6 .4 -- .8 -- .6 .08 .4 --500 2.8500 .8 .13 2.8 .6 .4 .08 .13 2.8 .3500 .3 .4 .6 .4 .3 .4 2.8 --500 -- .3 -- 2.8 .13 .13 -- .3 .13 .4 .4 -- .13 .4 .4 .13 .4 Table 10. south-central Texas—Continued. Texas—Continued. south-central zone) recharge aquifer (Edwards outcrop Group Edwards Table 10 33 Geronimo Creek watershed, watershed, Creek Geronimo PERLND segment IMPLND segment IMPLND Process-related annual parameters for pervious and impervious land segments, Hydrological Simulation Program—FORTRAN model, San model, Program—FORTRAN Simulation Hydrological segments, land impervious and pervious for parameters annual Process-related Land cover and slope AGWETP AGWRC BASETP CEPSC DEEPFR INFILT INTFW IRC LSUR LZSN NSUR RETSC SLSUR UZSN All imperviousAll ------400 -- .10 0.05 .04 -- Evergreen forest, low slopeDeciduous/mixed forest, low slopeShrubland, low slope slope low Grassland, .001 0.001slope low Bare/transitional, 0.94 .94 slope low Agricultural, Developed, low slope 0.001 .001 .001Evergreen forest, medium slope 0.27 .001 .001Deciduous/mixed forest, medium slope .15 0.2 .94Shrubland, medium slope .94 .001 .001 .2 .94 .001Grassland,medium 0.35 slope .001 .001 .001slope medium .94Bare/transitional, .940.9 .001 .35 .94 .12Agricultural, mediumslope .10 0.7 .94 .001 .10 .001 .9Developed,medium .001 slope .001 .2 400 .001 .2Evergreen forest, steep slope .001 .2 .15 .001 .10 3.6 .26 .7 .94Deciduous/mixed forest, steep slope .35 .94 0.3 400 .2 .001 .10 .2 .35 .94Shrubland, steepslope .35 .2 .001 3.6 .9-- .001 .001 .001Grassland,steep slope .2 .9 .94 .001 .001 .9 .35 .35 .12 .3 slope steep Bare/transitional, .35 0.04 .7 .10 .94 .94 .7 .10Agricultural, .001 steep slope .9 .35 .94 .9 400 .7 .2-- 0.4 .9 400 .2Developed, 400steep slope 3.6 .001 .001 .2 .10 .9 .001 3.6 .7 .001 .7 3.6 .04 .7 .35 .001 400 400 .3 .001 .15 .10 .35 .2 400 .3 .25 .7 .94 .35 .3 .4 3.6 3.6 .9 .94 400-- 3.6 .2 .2 .001 .9 .94-- .2 .9 .35 -- .001 .3 .3 3.6 .7 .3 .001 .001 .04 .94 .7 .001 .35 .35 400 .04 .9-- -- .12 .3 .7 .04 .35 400-- .10 .94 .4 3.6 400 .10 .001 .9 .9 .4 3.6 .2 -- .4 .7 .9 .04 .08 3.6 .2 .08 .3 .001 400 .2 .10 .3 .7 .7 .4 .4 .04 .3 .7 .35 3.6 .4 400 400-- .10 .35 .2 400-- .35 .4 3.6 3.6-- .3 .9 3.6 .2 .08 .9 .08 .9 .35 .3 .3-- .08 .7 .3 .4 .7 .35 .4 400 .9-- -- .7 400 .4 .08-- 400 3.6 .9 3.6 .7 .14 .08 3.6 .4 .14 .3 400 .3 .7 .4 .4 .3 3.6 .4 400-- -- 3.6-- .3 .14 .14 .3-- .14 .4 .4 -- .4 .14 .14 .4 .4 Table 10. south-central Texas—Continued. Texas—Continued. south-central Edwards Group remnants (hilltops) overlying Glen Rose Limestone 34 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003 Geronimo Creek watershed, watershed, Creek Geronimo IMPLND IMPLND PERLND segment Process-related annual parameters for pervious and impervious land segments, Hydrological Simulation Program—FORTRAN model, San model, Program—FORTRAN Simulation Hydrological segments, land impervious and pervious for parameters annual Process-related Land cover and slope AGWETP AGWRC BASETP CEPSC DEEPFR INFILT INTFW IRC LSUR LZSN NSUR RETSC SLSUR UZSN PERLND segment PERLND Land cover and slope and cover Land AGWETP AGWRC BASETP CEPSC DEEPFR INFILT INTFW IRC LSUR LZSN NSUR RETSC SLSUR UZSN Evergreen forest, low slopeDeciduous/mixed forest, low slopeShrubland, low slope slope low Grassland, slope low Bare/transitional, .001 0.001 slope low Agricultural, Developed, low slope .75 0.75Evergreen forest, medium slopeDeciduous/mixed forest, medium slope 0.001 .001 .001 .001Shrubland, medium slope .001 .001Grassland,medium slope 0.27 .15 .001slope medium .75Bare/transitional, .001 .75 0.2 .75Agricultural, mediumslope .75 .001 .2Developed,medium slope .75 .001 .75 .001 0.35 .001 .001 .001 .001 .750.4 .35 .12 .001 .10 .001 .001 .10 .15 .75 .75 .001 0.7 .001 .2 .4 .2 .26 .10 .75 400 .2 .001 .2 .001 .001 .10 .75 .2 .2 5.0 .7 .35 .35 .001 .75 400 .35 0.3 .35 .12 .10 .2 .001 .4 .4 .35 5.0 .35 .10 .4 .001-- .4 .2 .2 .10 .35 .3 .4 .2 .4 .7 .7 .10 0.04 .7 400 400 .7 .2 .35 .35 .4-- 400 .7 0.5 400 .7 5.0 .35 5.0 .2 400 400 5.0 .4 .4 5.0 .7 .35 .04 .3 .3 .4 5.0 5.0 400 .3 .35 .3 .7 .7 .4 .5 -- --5.0 .3 .3 400 400 .7 -- .4-- 400 .7 5.0 5.0 .3-- -- .04 .04 400 5.0 .04 .7 .08 .3 .3 -- .5 .5 400 5.0 .08 .04 .3 .5 .5 -- 5.0-- .04 .3 .5 .5 -- .3 .08 .08 .5 -- .08-- .5 .5 .08 .5 .08 .5 .5 All imperviousAll ------400 -- .10 0.05 .04 -- Evergreen forest, low slopeDeciduous/mixed forest, low slopeShrubland, low slope .001Grassland, low slope 0.001slope low Bare/transitional, Agricultural, low slope .85 0.85Developed, low slope 0.001 .001 .001 .001 imperviousAll .001 0.27 .15 .85 .001 .85 0.45 .85 .001 .45 .001 .85 .0010.35 .001 .85 .35 1.5 .12 .10 .001 .10 -- 1.50.7 .001 .45 .45 .10400 .45 .7 .10 -- 3.5 .35 .35 .45400 .35 0.3 .45 1.5 1.5 3.5 -- .35 1.5-- .35 .3 .7 .7 1.5 -- .7400 400 1.5 0.04-- 400 .7 3.5 3.5-- 0.6 400 3.5 .7 .04 .3 .3 400 3.5-- .3 3.5 .6 -- -- .3-- -- .3 .04 .04-- -- .04 -- .6 .6 400 .04 .6 .04 -- .6 .6 .100.05 .04. -- Table 10. south-central Texas—Continued. Texas—Continued. south-central Edwardsaquifer confining units deposits Alluvial Table 12 35

Table 11. Monthly values of lower-zone evapotranspiration (LZETP) parameter, Hydrological Simulation Program—FORTRAN models, HondoTable Creek, Verde11 Creek, and San Geronimo Creek watersheds, south-central Texas. [PERLND, PERvious LaND]

All geology-slope combinations

Land cover Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

PERLND segment

Evergreen forest 0.4 0.4 0.5 0.6 0.6 0.6 0.6 0.6 0.6 0.5 0.4 0.4 Deciduous/mixed forest .2 .2 .4 .6 .7 .7 .7 .7 .6 .5 .4 .3 Shrubland .2.2.4.6.7.7.7.7.6.5.4.3 Grassland .2 .2 .4 .6 .7 .7 .7 .7 .6 .5 .4 .3 Bare/transitional.2.2.4.6.7.7.7.7.6.5.4.3 Agriculture .2 .2 .4 .5 .5 .5 .5 .5 .4 .3 .2 .2 Developed .2.2.4.6.7.7.7.7.6.6.5.3

Table 12. Simulated average annual streamflow volumes for Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south- central Texas, 1951–2003.

[Total streamflow for a watershed includes all surface runoff, interflow, and base flow that originates in the watershed and reaches the stream channel. Streamflow at watershed outlet is difference between total streamflow and streamflow lost to channel infiltration.] Table 12 Hondo Verde San Geronimo Creek Creek Creek

Watershed area, acres 102,702 140,088 43,889 Approximate area of Edwards aquifer recharge zone and contributing 90,200 92,800 36,120 drainage area upstream from the recharge zone, acres Average annual rainfall, inches 30.40 29.72 32.09 Average annual total streamflow, acre-feet 45,400 32,400 11,100 Average annual total streamflow, inches 5.30 2.78 3.03 Average annual streamflow at watershed outlet, acre-feet 13,000 16,200 6,920 Average annual streamflow at watershed outlet, inches 1.52 1.39 1.89 36 Simulation of Streamflow and Estimation of Recharge to the Edwards Aquifer, South-Central Texas, 1951–2003

Table 13. Estimated annual rainfall and Edwards aquifer recharge, Hondo Creek, Verde Creek, and San Geronimo Creek watersheds, south-centralTable Texas, 13 1951–2003. Hondo Creek Verde Creek San Geronimo Creek1 Calendar year Rainfall Recharge Rainfall Recharge Rainfall Recharge (inches) (acre-feet) (inches) (acre-feet) (inches) (acre-feet) 1951 18.69 9,760 20.72 8,750 19.81 230 1952 25.17 14,200 26.56 10,500 34.11 3,930 1953 19.27 21,800 18.97 14,400 19.99 1,580 1954 14.59 5,840 13.60 3,960 11.44 100 1955 19.44 4,170 18.58 4,090 17.83 210 1956 10.95 1,920 11.90 1,760 12.14 150 1957 38.65 58,500 38.16 38,800 46.61 11,400 1958 43.17 73,200 42.55 48,500 41.44 9,750 1959 29.36 21,500 32.91 14,500 37.30 4,190 1960 34.87 36,700 33.86 24,800 32.76 2,870 1961 28.29 40,600 26.79 26,800 24.92 2,810 1962 20.57 6,360 21.61 4,850 25.16 1,600 1963 20.47 9,450 19.19 6,840 19.24 700 1964 21.90 20,800 24.66 14,200 27.23 2,780 1965 30.51 30,100 31.92 22,000 39.03 7,720 1966 19.37 9,040 25.11 9,170 30.20 5,230 1967 28.32 41,900 28.56 27,800 27.39 3,980 1968 45.45 85,800 35.21 53,800 29.02 4,420 1969 36.81 52,500 36.63 35,300 38.26 6,830 1970 27.02 33,300 23.60 22,500 22.11 2,910 1971 43.08 71,300 37.58 43,900 38.46 8,460 1972 36.44 55,900 32.66 33,700 33.05 3,530 1973 41.43 46,900 45.65 33,300 50.96 12,800 1974 40.85 57,600 37.69 38,800 37.08 5,260 1975 32.34 40,500 30.09 28,700 30.13 5,130 1976 40.60 68,700 42.30 48,200 44.46 10,600 1977 28.96 40,500 25.37 26,800 28.26 3,530 1978 34.08 37,500 32.71 25,100 35.09 5,990 1979 28.46 39,900 28.28 28,200 34.49 8,720 1980 30.00 34,900 26.20 24,100 26.33 1,890 1981 41.49 69,000 37.14 44,700 38.56 10,600 1982 22.68 15,400 22.77 11,700 25.82 2,380 1983 32.13 40,000 28.94 26,800 31.00 2,230 1984 28.84 20,500 24.64 16,200 22.47 1,660 1985 36.77 65,800 35.70 44,200 39.24 9,720 1986 30.84 36,300 34.26 28,800 40.16 12,800 1987 46.42 68,600 43.17 44,600 39.82 11,300 1988 20.64 22,500 17.00 13,800 16.56 180 1989 21.14 21,000 19.50 13,700 22.33 700 1990 28.01 36,700 25.73 24,900 34.19 7,750 1991 44.08 60,000 41.95 40,900 47.36 7,260 1992 48.98 117,000 46.67 73,800 55.65 26,700 1993 26.29 17,100 26.87 16,000 22.13 2,500 1994 28.95 26,000 28.57 20,400 37.54 6,640 1995 27.65 23,300 27.99 17,900 28.31 2,160 1996 17.12 3,800 18.17 3,780 22.12 730 1997 36.97 44,800 34.66 30,500 46.58 10,800 1998 39.64 62,200 43.10 46,900 40.69 11,600 1999 18.01 9,050 21.01 9,180 18.83 1,130 2000 26.09 30,800 27.52 21,200 36.85 10,100 2001 29.49 40,500 30.94 31,700 39.23 13,800 2002 43.97 73,600 44.34 51,900 53.60 18,400 2003 26.04 32,600 24.86 21,600 27.41 4,160 Average 30.40 37,900 29.72 26,000 32.09 5,940 Minimum 10.63 1,920 11.38 1,760 11.14 100 Maximum 51.71 117,000 52.71 73,800 58.61 26,700

1 Recalibrated values. Table 14 37

Table 14. Comparison of Edwards aquifer recharge estimates from Hydrological Simulation Program—FORTRAN (HSPF) with historical (Puente method) U.S. Geological Survey recharge estimates for Hondo Creek and Verde Creek watersheds, south-central Texas, 1992–2003.Table 14

Hondo Creek watershed Verde Creek watershed

Year HSPF-simulated recharge Puente method recharge HSPF-simulated recharge Puente method recharge (acre-feet) (acre-feet) (acre-feet) (acre-feet) 11992 117,000 209,000 73,800 133,000 1993 17,100 17,900 16,000 18,200 1994 26,000 15,500 20,400 16,300 1995 23,300 18,100 17,900 22,200 1996 3,800 1,530 3,780 5,570 11997 44,800 61,600 30,500 55,800 11998 62,200 64,500 46,900 62,600 1999 9,050 20,400 9,180 19,900 2000 30,800 14,600 21,200 21,000 2001 40,500 39,800 31,700 39,300 12002 73,600 135,000 51,900 156,000 2003 32,600 22,800 21,600 22,100

Average 40,100 51,700 28,700 47,700

1 Years of greater-than-average rainfall in both watersheds. Prepared by the USGS Texas Water Science Center: 8027 Exchange Drive Austin, TX 78754–4733

Information regarding water resources in Texas is available at http: //tx.usgs.gov/