The effects of rain and evapotranspiration statistics on groundwater recharge estimations under semi-arid environments

Abstract. Better understanding the effects of rainfall and evapotranspiration statistics on groundwater recharge requires long time series of these climate variables. However, long records of the relevant variables are scarce. To overcome this limitation, time series of the rainfall and evapotranspiration are often being synthesized using different methods. Here, we attempt to study the dependence of estimated groundwater recharge on the synthesis methods. We focus on regions with semi-arid climate conditions and soil types. For this purpose, we used long records of climate data that were measured in two semi-arid locations with similar seasonal and annual potential evapotranspiration rates but different seasonal rain distributions. Stochastic daily rain and potential evapotranspiration time series were synthesized according to the monthly empirical distributions. This suggested synthesis method preserves only certain aspects of the measured statistics. Therefore, different correction methods were considered to match the synthesized time series to the measured annual or monthly statistics. Groundwater recharge fluxes were calculated using the 1D Richards equation, for several typical semi-arid soil types, and by prescribing the synthesized rain and potential evapotranspiration as atmospheric conditions. The estimated groundwater recharge fluxes are sensitive to the synthesis method. However, the ratio between the groundwater recharge and the total rain does not show the same sensitivity. The effects of the synthesis methods are shown to be independent of the data used as observations. These findings suggest that assessment of groundwater recharge under current and future climate conditions depend on the synthesis method used for rain and evapotranspiration.