The use of aquifer response rate in the assessment of groundwater resources

In groundwater resources it is frequently necessary to decide how much of the recharge to a groundwater unit is required to maintain the baseflows in the rivers of the unit. Groundwater recharge enters the aquifer in a very irregular way both in terms of seasonal variation and variation between years. As the water is transferred to the rivers some of this variation is smoothed out so that the baseflows exhibit less severe variation. In an idealised one-dimensional aquifer, the ability of the system to smooth this variation is determined by the parameter, aquifer response rate. An analytic solution is used to develop a type curve which allows calculation of the minimum baseflows in the rivers in the annual cycle from the aquifer response rate. The extension of this theory to unconfined aquifers and to real irregular variation in recharge is investigated using a simple finite difference numerical model. A method of calculation of the response rate for real aquifers is proposed and this is applied to three examples of groundwater units. These units contain rivers which are gauged so that the predicted minimum baseflows can be compared with the observed minimum baseflows. The method is intended to have application to real problems of water resources, particularly in catchments where river gauges do not exist. (C) 1997 Elsevier Science B.V.