SPATIAL AND TEMPORAL ANALYSIS OF GROUNDWATER RECHARGE WITH APPLICATION TO SAMPLING DESIGN

A numerical experiment of flow in variably saturated porous media was performed in order to evaluate the spatial and temporal distribution o f the groundwater recharge at the phreatic surface for a shallow aquif er as a function of the input rainfall process and soil heterogeneity. The study focused on the groundwater recharge which resulted from the percolation of the excess rainfall for a 90-days period of an actual precipitation record. Groundwater recharge was defined as the water fl ux across the moving phreatic surface. The observed spatial non-unifor mity of the groundwater recharge was caused by soil heterogeneity and is particularly pronounced during the stage of recharge peak (substant ial percolation stage). During that stage the recharge is associated w ith preferential how paths defined as soil zones of locally higher hyd raulic conductivity. For the periods of low percolation intensity the groundwater recharge was exhibiting more uniform spatial characteristi cs. The temporal distribution of the recharge was found to be a functi on of the frequency and intensity of the rainfall events. Application of sampling design demonstrates the joint influence of the spatial and temporal recharge variability on the cost-effective monitoring of gro undwater potentiometric surfaces.