EVALUATING LONG-TERM HYDRAULIC RELATIONSHIPS BETWEEN A MAJOR RIVER AND REGIONAL AQUIFER TO IDENTIFY IMPROVED WATER RESOURCE MANAGEMENT PRACTICES: AMISTAD RESERVOIR AND THE RIO GRANDE

Understanding hydraulic connections between the Rio Grande watershed, local aquifers in direct contact with Amistad Reservoir on the Rio Grande near Del Rio, Texas and Ciudad Acuna, Coahuila, and a major regional aquifer is important for sustainably managing water resources shared by Mexico and the United States. Amistad Reservoir is located along a gaining reach of the Rio Grande where the mean annual flow increases from 1.1 to 1.6 M acre-feet. Initial filling and subsequent changes in water level in Amistad Reservoir over the past 50 years provide a long-duration data set that yields important information about hydraulic connections between the Rio Grande, local aquifers, and the karstic Edwards-Trinity aquifer. Most useful are time series records of river stage, reservoir water surface elevations, groundwater elevations, inflow to and outflow from the reservoir, and the lag and attenuation between reservoir water surface elevation changes and the responses in nearby water wells. Initial filling of Amistad Reservoir in 1969 raised the level of the river and adjoining local aquifers by as much as 30 m. Subsequent fluctuations due to drought and local groundwater recharge have exceeded 10 m. Changes in water levels in the river and reservoir and their effects on local and regional groundwater system water levels can be used to infer local and regional aquifer properties. Several analytical approaches have been used to estimate aquifer properties and elucidate the relationship between the river and the aquifers. These approaches include classic aquifer hydraulics and gray-box time series models. Analyses have improved understanding of the hydraulic connection between the reservoir and the regional aquifer allowing for improved water resource management.