Areal groundwater recharge to the aquifer occurs due to infiltration of precipitation and/or irrigation return flow. The estimation of current and future recharge rates is an essential element of climate change impact assessments. Relative changes in recharge rates are of interest, and how these changes affect the groundwater levels.
There are several methods to quantify groundwater recharge; Scanlon et al.  provide a useful review on choosing the appropriate technique to determine recharge. The methods can be divided into physical, chemical (tracer) and numerical modeling approaches. One of the well-known physical methods is the water table fluctuation method , which is based on the premise that groundwater level rises in unconfined aquifers are due to recharge water percolating through the vadose zone. Groundwater dating and the chloride mass balance methods are examples for tracer methods to determine recharge. Moreover, numerical modeling can be a useful and robust approach to quantify recharge. Spatially-distributed recharge estimates can be obtained by inverse modeling of groundwater flow models. Another method is to use soil-vegetation-atmosphere-transfer (SVAT) models, which are 1-D process-based models that calculate unsaturated groundwater flow through the vadose zone. One example for a SVAT model is the 1-D Hydrologic Evaluation of Landfill Performance model , HELP, which simulates vertical leakage of water through the soil profile. It accounts for precipitation in any form, surface storage, runoff, evapotranspiration, snowmelt, vegetative interception and growth, unsaturated flow and temperature effects. Since the model is 1-D, like most SVAT models are, recharge rates are estimated for specific combinations of soil type and depth, vadose zone conductivity and water table depth. Hence, the model outcome needs to be upscaled and extended to all points of the study area. The recharge boundary condition for a finite-difference groundwater flow model can be obtained by linking the HELP model with a GIS (Geographical Information System). Thereby, the spatial distribution of recharge rates can be obtained, where recharge rates are determined for each grid cell of the groundwater flow model (e.g. [8, 9]).
Precipitation-runoff models are used to estimate lumped recharge rates at the basin or sub-basin scale. They provide a single recharge estimate for the entire watershed and generally yield groundwater recharge estimates as a residual term in the water budget equation. There are also groundwater-centered recharge estimate approaches; groundwater flow model calibration is another technique to predict recharge rates from information of observed hydraulic heads, hydraulic conductivity and other parameters (e.g. ).
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