Sustainable Yield Groundwater in Arid and Semi-Arid Lands in Texas Presented at Geological Society of America South-Central Section by Ronald T. Green, F. Paul Bertetti, S. Beth Fratesi, and Nathaniel J. Toll Southwest Research Institute® March 13, 2017

Sustainable Groundwater Groundwater sustainability is the development and use of groundwater resources to meet current and future beneficial uses without causing unacceptable environmental or socioeconomic consequences. (USGS Circular 1186)

Common Perception There are those that believe that Texas does not have a water shortage problem, rather Texas has a water transmission problem. Further, this perspective believes that local water shortages can be met if water resources are governed by free market forces.

Texas Projected Water Usage 2010-2060 (Texas Water Development Board Website, 2010)

Basic Facts 1/4-1/3 of Texas is arid and semi-arid (< 20 inches/year) Urban (municipal) and rural (irrigation) interests compete for finite resources Recharge and available water are dependent on precipitation and are highly variable Future threats to water availability from (i) drought (short-term trends) (ii) climate change (long-term trends)

Managing Water Resources in the Edwards Plateau Mostly carbonate aquifer with limited storage Watersheds act independently Mostly arid and semi-arid

Discharge measured at river gauging stations is used to estimate recharge

Recharge rate is calculated as baseflow component of river discharge Storm flow Baseflow Storm Surge Storm flow Baseflow

Distributed Recharge Rates Calculated from Stream Discharge Measurement

Recharge rates in study area corrected for actual groundwater catchment areas 10

Recharge is Minimal when Precipitation is Less than 20 inch/year Distributed Recharge Focused Blue data – Texas Hill Country

Recharge is Minimal when Precipitation is Less than 20 inch/year Additional Supporting Data R = 0.15 (P – 16.5) Blue data – Texas Hill Country Red data – Pretoria South Africa

Average Precipitation

Distributed Recharge Calculated from Average Precipitation R = 0.15 (P – 16.5) No Distributed Recharge Distributed Recharge

Short-Term Variability in Annual Precipitation Del Rio (inch/year) (1920 to 2000) Long-term Average 10% 20% 30% 40% occurrence 30% occurrence 20% occurrence

Short-Term Recharge as a Function of Precipitation (acre-feet/year)   Crockett Edwards Kimble Menard Real Schleicher Sutton Val Verde Recharge under average conditions 56,140 144,160 95,076 61,268 81,200 57,684 75,556 90,496 Predicted recharge at 90% precipitation 13,474 101,760 69,055 43,248 63,840 34,610 48,821 38,784 80% precipitation 59,360 43,034 25,228 46,480 11,537 22,086 70% precipitation 1,837 2,127 300 5,193 40% 30% 20%

Watershed Independence is of Critical Importance if Water Resource Management Assumes a Regional Aquifer (i.e., Edwards-Trinity) is a Single Reservoir

Each Watershed Has a Separate Water Budget Colorado River-Concho Lower Pecos River Colorado River-Llano Rio Grande Amistad Devils River Nueces River Rio Grande Falcon Preliminary - Subject to Revision Edwards-Trinity Aquifer Water Budget Preliminary - Subject to Revision

Effect of Long-Term Drought or Climate Change on Recharge in West Texas

Average Year Precipitation threshold below which 17 Precipitation threshold below which distributed recharge is negligible Area that experiences distributed recharge Area that experiences negligible 17

Dry Year (30% decrease) Precipitation threshold 17 Precipitation threshold below which distributed recharge is negligible migrates east during periods of drought 30% decrease in precipitation occurs 20% of the time 17

Texas Can be Sub-Divided by Area into Three Categories of Recharge Area that rarely experiences distributed recharge Area that almost always experiences distributed recharge Area that may or may not experience distributed recharge

Intermediate Zone Does Not Provide Sustainable Sources for Water Supply

Area that almost always Climate Change that Causes Less Precipitation Will Shift these Zones to the East Area that may or may not experience distributed recharge Area that rarely experiences distributed recharge Area that almost always experiences distributed recharge

Locations Targeted for Groundwater Export

Managing Water Resources in the Edwards Plateau Mostly arid and semi-arid Mostly carbonate aquifer with limited storage Watersheds act independently

Unless significant quantities of water can be stored during times of plenty, carbonate aquifers in arid and semi-arid environments cannot provide sustainable sources of excess water

Continental Variability in Precipitation and Recharge

Approximate Threshold of distributed recharge

Sustainable Yield Groundwater in Arid and Semi-Arid Lands in Texas Summary Recharge is limited, uncertain, and varies from year to year Recharge becomes negligible when precipitation decreases below a threshold of 15-17 in/yr Arid and semi-arid regions are vulnerable to limited recharge during short-term periods of drought Long-term climate change could exacerbate water shortage by increasing magnitude of variability and reducing precipitation/recharge Water-resource planning needs to account for these potential threats

Contact Information Ronald T. Green, Ph.D., P.G. Institute Scientist Earth Science Section Space Science Division Southwest Research Institute 6220 Culebra San Antonio, Texas 78238 1.210.522.5305 (office) 1.210.522.5184 (fax) 1.210.316.9242 (cell) rgreen@swri.edu