Abstract
Abstract. With the aim to understand the spatial and temporal variability of groundwater recharge, a high-resolution, spatially distributed numerical model (MIKE SHE) representing surface water and groundwater was used to simulate responses to precipitation in a 2.16 km2 upland catchment on fractured sandstone near Los Angeles, California. Exceptionally high temporal and spatial resolution was used for this catchment modeling: hourly climate data, a 20 m×20 m grid in the horizontal plane, and 240 numerical layers distributed vertically within the thick vadose zone and in the upper part of the groundwater zone. The finest practical spatial and temporal resolutions were selected to accommodate the large degree of surface and subsurface variability of catchment features. Physical property values for the different lithologies were assigned based on previous on-site investigations, whereas the parameters controlling streamflow and evapotranspiration were derived from calibration to continuous streamflow at the outfall and to average hydraulic heads from 17 wells. Confidence in the calibrated model was enhanced by validation through (i) comparison of simulated average recharge to estimates based on the applications of the chloride mass-balance method to data from the groundwater and vadose zones within and beyond the catchment, (ii) comparison of the water isotope signature (18O and 2H) in shallow groundwater to the variability of isotope signatures for precipitation events over an annual cycle, and (iii) comparison of simulated recharge time series and observed fluctuation of water levels. The average simulated recharge across the catchment for the period 1995–2014 is 16 mm yr−1 (4 % of the average annual precipitation), which is consistent with previous estimates obtained by using the chloride mass balance method (4.2 % of the average precipitation). However, one of the most unexpected results was that local recharge was simulated to vary from 0 to >1000 mm yr−1 due to episodic precipitation and overland runoff effects. This recharge occurs episodically with the major flux events at the bottom of the evapotranspiration zone, as simulated by MIKE SHE and confirmed by the isotope signatures, occurring only at the end of the rainy season. This is the first study that combines MIKE SHE simulations with the analysis of water isotopes in groundwater and rainfall to determine the timing of recharge in a sedimentary bedrock aquifer in a semiarid region. The study advances the understanding of recharge and unsaturated flow processes and enhances our ability to predict the effects of surface and subsurface features on recharge rates. This is crucial in highly heterogeneous contaminated sites because different contaminant source areas have widely varying recharge and, hence, groundwater fluxes impacting their mobility.
Highlights
Assessment of groundwater recharge is fundamental to create strategies for management of water resources and to estimate volumetric groundwater flow through contaminated sites
Manna et al.: Spatial and temporal variability of groundwater recharge in a sandstone aquifer regions where dominance of evapotranspiration (ET) limits water resources
The average recharge value is 16 mm yr−1, which is consistent with previous estimates at the site and with those obtained for other sandstone aquifers in semiarid areas in the US (4 % – Heilweil et al, 2006) and other studies in semiarid regions around the world (0.2–35 mm yr−1 equal to 0 %–5 % of the average precipitation, Scanlon et al, 2006)
Summary
Assessment of groundwater recharge is fundamental to create strategies for management of water resources and to estimate volumetric groundwater flow through contaminated sites. Elevated tritium in precipitation derived from atmospheric releases during nuclear tests in the 1960s and transported into the subsurface has been an invaluable tracer to determine modern recharge and mechanisms of flow in both vadose and groundwater zones (Cook and Böhlke, 2000; De Vries and Simmers, 2002) These geochemical and isotopic techniques are based on the interpretation of hydrologic process influences on the distribution of tracers in the subsurface but cannot show the dynamic, short-term temporal effects nor provide a continuous spatial representation of these processes at the catchment scale. This provided spatially variable, long-term recharge values ranging from 4 to 23 mm yr−1 and indicated that, on average, 80 % of the flow in the vadose zone occurs as intergranular flow in the rock matrix and 20 % as fracture flow These chloride-based methods lump together hydrologic processes providing long-term recharge estimates for only few locations across a large site. The MIKE SHE simulations contributed to the conceptual model concerning the role of surface feature variability (e.g., topography and vegetation) on the hydrological processes, whereas the Cl-based studies informed the flow mechanisms in the underlying portion of the system
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