SUSTAINABLE YIELD GROUNDWATER IN ARID AND SEMI-ARID LANDS IN TEXAS

There is debate whether there is water in excess of the sustainable yield of groundwater in south and west Texas to allow for groundwater export to regions in the state with inadequate water resources. Recent research has shown that semi-arid lands, such as south and west Texas, receive negligible recharge when precipitation is less than about 400-450 mm/yr [16-18 in/yr]. Thus, any water considered to be in excess of sustainable yield will only be available during years when precipitation is great enough to allow for reasonably large recharge. An additional factor in the determination of sustainable yield of groundwater is whether there is adequate capacity for storage of water that can be retained during times of abundant recharge for use when water demand exceeds supply. Although engineered storage facilities, such as surface reservoirs or aquifer storage and recovery, are always possible, most storage occurs naturally in aquifers. Excess water is thereby limited when there is: (i) low average precipitation rates, (ii) the lack of naturally occurring or engineered storage capacity, or (iii) both low precipitation rates and limited storage. Unless there is both adequate recharge and adequate capacity to store water, the amount of water in excess of the sustainable yield is limited to whatever excess water is available during periods of drought. An analysis of conditions necessary for excess water in the Edwards Plateau is provided to highlight these limitations.