Response of the Rio Grande and shallow ground water in the Mesilla Bolson to irrigation, climate stress, and pumping

The El Paso-Ciudad Juarez metropolitan area obtains its water from the Rio Grande and intermontane-basin aquifers, Shallow ground water in this region is in close communication with the surface water system. A major problem w ith both systems is salinity. Upstream usage of the water in the Rio Grande for irrigation and municipalities has led to concentration of soluble salt s to the point where the surface water commonly exceeds drinking water stan dards. Shallow ground water is rei charged by surface water (primarily irri gation canals and agricultural fields) and discharges to surface water (agr icultural drains) and deeper ground water. The source of water entering the Rio Grande varies seasonally. During the i rrigation season, water is released from reservoirs and mixes with the retu rn flow from irrigation drains. During the non-irrigation season (winter), flow is from irrigation drains and river water quality is indicative of sha llow ground water. The annual cycle can be ascertained from the inverse cor relation between ion concentrations and discharge in the river. Water-quali ty data indicate that the salinity of shallow ground water increases each y ear during a drought, Water-management strategies in the region can affect water quality. Increas ing the pumping rate of water-supply wells will cause shallow ground water to flow into the deeper aquifers and degrade the water quality. Lining the canals in the irrigation system to stop water leakage will lead to water qu ality degradation in shallow ground water and, eventually, deep ground wate r by removing a major source of high quality recharge that currently lowers the salinity of the shallow ground water.