Groundwater levels fluctuate as a result of changes in the relation between inflow and outflow to the aquifer, and the magnitude of the fluctuations are also depending on the geologic conditions in the aquifer. Of special interest when studying groundwater is to estimate the inflow to the aquifer, the recharge, as it is the renewable part of a water resource. Here a simple method for modelling the relation between inflow and outflow from groundwater level variations is described, a method that also relates to the geologic features of the aquifer. Input data are groundwater level observations from several years, data that are often readily available. From these data longer periods of decreasing groundwater levels are chosen, during which the recharge is assumed to be zero. An exponential recession curve is established by a least-square fitting to all the chosen periods, and from this curve a base level for zero groundwater runoff is evaluated, together with a geologic reservoir parameter that consists of the storage coefficient, the area of the aquifer, and a groundwater runoff coefficient. A recharge parameter that is proportional to the recharge is then evaluated.

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