An ecohydrological modelling approach for assessing long-term recharge rates in semiarid karstic landscapes

An ecohydrological water balance method based on the hydrological equilibrium hypothesis was developed to estimate long-term annual recharge rates in semiarid karstic landscapes. Recharge was predicted from the difference between long-term annual precipitation and evapotranspiration rates. A multiple regression interpolation approach was used to compute precipitation. Evapotranspiration was quantified from the deviations between the observed local value of the normalised difference vegetation index (NDVI) and, the predicted minimum and maximum NDVI values for two hydrologically- well defined reference conditions representing the minimum and maximum vegetation density given a local long-term water availability index. NDVI values for the reference conditions (NDVImin and NDVImax) were estimated from an empirically-based boundary analysis. Evapotranspiration rates for the reference conditions were estimated using a monthly water budget model that integrates the roles of the soil water holding capacity and a climate-driven evaporative coefficient (k) representing the mean annual conductance of the vegetation canopy. The methodology was tested in Sierra de Ga´dor (SE Spain), where predicted evapotranspiration and recharge rates compared well with local and regional scale estimates obtained from independent methods. A sensitivity analysis showed that NDVImax and k are the parameters that mostly affect our model's evapotranspiration and recharge estimates.