Numerical modeling of the groundwater flow system in a sub-basin of the Leon-Chinandega aquifer, Nicaragua

The Leon-Chinandega aquifer is located in an agricultural area in the northwest of Nicaragua. This shallow unconfined aquifer of 1300 km 2 is one of the most important groundwater reservoirs of the country. A sub-basin of 330 km2 was selected to study the groundwater flow system using a numerical groundwater flow model (visual MODFLOW). Transient and steady state models were constructed based on a comprehensive conceptual model. Two flow systems were identified: a deep regional system recharged in the cordillera and discharged in the central and lower plain; and a shallow local system, recharging in the central plain and discharging at lower elevations. Rivers are responsible for the relative stability of hydraulic heads in the system, but baseflow is very sensitive to any stress change. Decrease in river discharge induced by pumping becomes very critical during dry periods. It was found that the response time of the aquifer is about one hydrologic year, which allows developing management strategies within short time horizons. Simulations can be used to establish a balance between desirable groundwater development and reasonable levels of surface water depletion.