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. With the aim to understand the spatial and temporal variability ofgroundwater recharge, a high-resolution, spatially distributed numericalmodel (MIKE SHE) representing surface water and groundwater was used tosimulate responses to precipitation in a 2.16 km 2 upland catchment onfractured sandstone near Los Angeles, California. Exceptionally hightemporal and spatial resolution was used for this catchment modeling: hourlyclimate data, a 20 m×20 m grid in the horizontal plane, and 240 numericallayers distributed vertically within the thick vadose zone and in the upperpart of the groundwater zone. The finest practical spatial and temporalresolutions were selected to accommodate the large degree of surface andsubsurface variability of catchment features. Physical property values forthe different lithologies were assigned based on previous on-siteinvestigations, whereas the parameters controlling streamflow andevapotranspiration were derived from calibration to continuous streamflow atthe outfall and to average hydraulic heads from 17 wells. Confidence in thecalibrated model was enhanced by validation through (i) comparison ofsimulated average recharge to estimates based on the applications of thechloride mass-balance method to data from the groundwater and vadose zoneswithin and beyond the catchment, (ii) comparison of the water isotopesignature ( 18O and 2H ) in shallow groundwater to the variabilityof isotope signatures for precipitation events over an annual cycle, and(iii) comparison of simulated recharge time series and observed fluctuationof water levels. The average simulated recharge across the catchment for theperiod 1995–2014 is 16 mm yr −1 (4 % of the average annualprecipitation), which is consistent with previous estimates obtained byusing the chloride mass balance method (4.2 % of the averageprecipitation). However, one of the most unexpected results was that localrecharge was simulated to vary from 0 to >1000  mm yr −1 dueto episodic precipitation and overland runoff effects. This recharge occursepisodically with the major flux events at the bottom of theevapotranspiration zone, as simulated by MIKE SHE and confirmed by theisotope signatures, occurring only at the end of the rainy season. This isthe first study that combines MIKE SHE simulations with the analysis ofwater isotopes in groundwater and rainfall to determine the timing ofrecharge in a sedimentary bedrock aquifer in a semiarid region. The studyadvances the understanding of recharge and unsaturated flow processes andenhances our ability to predict the effects of surface and subsurfacefeatures on recharge rates. This is crucial in highly heterogeneouscontaminated sites because different contaminant source areas have widelyvarying recharge and, hence, groundwater fluxes impacting their mobility.

Spatial and temporal variability of groundwater recharge in a sandstone aquifer in a semiarid region