ESTIMATING POTENTIAL GROUND WATER RECHARGE USING SUBSISTENCE FLOWS IN THE RIO GRANDE / BRAVO, PECOS AND DEVILS RIVER SUBBASINS OF WEST TEXAS, USA

The area of interest for this study is the Rio Grande/Bravo and two of its major tributaries located in the Big Bend region of west Texas. It begins at the confluence of the Rio Conchos and continues downstream to Amistad Reservoir (BBTAR), and includes the Pecos River from Pandale, Texas to Langtry, Texas, and from Baker’s Crossing to Pafford’s Crossing on the Devils River. Ground water contributions emitting from the Edwards-Trinity Plateau Aquifer (ETPA) in the form of springs and seeps were used estimate potential recharge. The BBTAR ground water contributions average 6.5 cms with a drainage basin of about 32,750 km². The lower Pecos River ground water contributions average 1.5 cms with a contributing drainage basin of 1165 km². Ground water contributions for the Devils River average 1.9 cms with a contributing drainage basin of 2915 km². Recharge estimates were calculated ranging from 1.97% to 19% of overall average rainfall. In addition to base flow discharge as a method to estimate recharge, the Water Table Fluctuation Method (WTFM) was also used to estimate potential recharge in or near each subbasin using the following formula: R=S_ydh/dt = S_yDh/Dt , where R is recharge, S_y is specific yield, h is water-table height, and t is time. Results using the WTFM average only 41% to 68% of the above estimated recharge using base flow. This suggests significant ground water contributions may come from outside the surface drainage basin. The significance in quantifying ground water recharge is crucial in sustaining our water resources especially in the arid southwest.