Recharge to alluvial valley aquifers from overbank flow and excess infiltration

Recharge is an important parameter for models that simulate water and conta minant transport in unconfined aquifers. Unfortunately, measurements of act ual recharge are not usually available causing recharge to be estimated or possibly added to the calibration procedure. In this study, differences bet ween observed water-table elevations and water-table elevations simulated w ith a model based on the one-dimensional Boussinesq equation were used to i dentify both the timing and quantity of recharge to an alluvial valley aqui fer. Observed water table elevations and river stage data were recorded dur ing a five-year period from 1991 to 1995 at the Ohio Management Systems Eva luation Area located in southcentral Ohio. Direct recharge attributed to ov erbank flow during and shortly after flood conditions accounted for 65 perc ent of the total recharge computed during the five-year study period. Recha rge of excess infiltration to the aquifer was intermittent and occurred soo n after large rainfall events and high river stage. Specification of consta nt recharge with time values in ground-water simulation models seems inappr opriate for stream-aquifer systems given the strong influence of the river on water table elevations in these systems.