Coupled surface water and groundwater model to design managed aquifer recharge for the valley of Santo Domingo, B.C.S., Mexico

The Valley of Santo Domingo represents the most important agricultural region in the state of Baja California Sur. The uncontrolled extraction of groundwater, and especially the over-exploitation of the Santo Domingo aquifer from 1957 on, has caused modifications to the natural flow system and induced a lateral inflow of seawater from the Pacific coast. As a result, the groundwater quality in the Santo Domingo Irrigation District (066) is deteriorating. Seawater intrusion and irrigation return water, combined with the mobilization of deeper groundwater, have been identified as important sources of salinization. Due to a reduction of the permitted extraction volume (to only one-third), an equality between discharge and recharge volume was achieved from 2003 on, but the deterioration of the groundwater quality still continues. To plan different scenarios of artificial recharge, a regional groundwater model was created, using the Modflow2000 software. The observed groundwater levels (from 1996 on) were used to calibrate the groundwater flow model, taking into account the extraction rates of more than active 500 wells. To simulate infiltration of surface water, runoff data were introduced to the Streamflow package, based on calculations from an HEC-1 model. In the Valley of Santo Domingo, the natural recharge and infiltration of irrigation return water generate an average annual recharge of 188 million m(3). As a first step toward an aquifer management plan, results from hydrological and hydraulic models were incorporated into the regional groundwater flow model, to plan scenarios of additional artificial recharge facilities that would prevent deepening of the cone of depression. The elaborated Water Resources Management plan proposes the capture of an annual volume of 30 million m(3) of surface water from the Sierra de la Giganta Mountains, which could be accumulated in four water retention dams. The captured surface water then would flow to the four smaller recharge dams with a total storage volume of about 2 million m(3), located downstream, where it would recharge the aquifer. As the model indicates, within a period of 3 months, an annual volume of at least 21.5 million m(3) of surface water could be infiltrated via the riverbeds and recharge dams. Although this volume may not be enough to prevent saline intrusion from the coast line, it would stop the water level decline in the center of the cone of depression and improve groundwater quality.