USGS FIRM YIELD ESTIMATOR MODEL: EVALUATING THE INFLUENCE OF SMALL RESERVOIRS ON GROUNDWATER RECHARGE

In semi-arid regions at the edge of the Sahel in West Africa, groundwater serves as the primary source of drinking water for the local population. Despite the importance of groundwater resources, few studies have been done in the region to assess recharge or aquifer characteristics. Abundant small reservoirs in the region may contribute to groundwater recharge and thereby aid groundwater sustainability.

The USGS analytical model, the "Firm-Yield Estimator (FYE) model", was used to estimate the significance of reservoir and aquifer interactions. The FYE method determines how much reservoir storage recharges groundwater as monthly changes in reservoir stage. The model builds upon the Rorabaugh method to estimate the groundwater contribution to streamflow. Requirements of the model include that the reservoir drainage area be defined and the reservoir receives surface water inflow.

In the semi-arid field site in Ghana, the reservoir contribution to the local aquifer averaged only 4.7% of total annual recharge. Small reservoirs within the Ghana field area do not contribute significantly to groundwater recharge because of low transmissivity and storativity values characteristic of weathered granite aquifers. However, in general, water levels from shallow wells showed that the water table mounds near the reservoirs, suggesting that some recharge from the reservoirs does occur. In a few cases, water levels in wells located near small reservoirs fluctuated little, suggesting a substantial hydrologic connection between the small reservoirs and underlying aquifer. Reservoirs can have a significant influence on groundwater recharge, though the extent of influence depends on local geology.