Estimation of recharge from floods in disconnected stream-aquifer systems

Stream-aquifer interaction has been the subject of much research for cases of good hydraulic connection (continuous saturated zone) between a river and an aquifer. Under these conditions, floods do not represent a very large net input to the aquifer because most of the water that enters the aquifer during the flood returns to the river when its stage recedes. The situation is different in disconnected stream-aquifer systems, where the streambed lies above the water level in the aquifer, thus preventing return flow from the aquifer. Under these conditions, floods may represent large, but hard to quantify, water inputs. Here, we present a methodology to estimate recharge from floods for disconnected stream-aquifer systems. Recharge is estimated as the product of a flood time function (dependent on the streamflow) and an unknown factor, which is obtained from calibrating a ground water flow model to aquifer heads. The approach can also benefit from concentration data, which can be very informative when river water concentrations vary over time. This methodology is applied to a field situation where recharge from river flooding is found to amount to nearly 15 million m(3)/year on the average, which represents 40% of the total aquifer inputs. Recharge from flooding helps explain major head recoveries, suggesting that basin water management programs should allow some floods to occur.