A GIS tool for hydrogeological water balance evaluation on a regional scale in semi-arid environments

A GIS tool to evaluate hydrogeological water balance based on a mass-balance model applied to surface and subsurface systems is discussed. The tool is designed for managers responsible for groundwater resource planning during conditions of water shortage. In developing the tool, the natural groundwater recharge was evaluated through the application of a soil water balance equation, and defined as the difference between the inflows (rainfall, irrigation) and the outflows (plant evapotranspiration, surface run-off). A distributed approach was used in the soil water balance equation to account for the spatial variability of climate and landscape features. Conversely the groundwater balance was calculated on a watershed or aquifer scale, using a lumped water balance equation, in which withdrawals for different uses were estimated together with inflows from other water bodies and coastal outflows. The model was implemented on a GIS platform with an automatic routine that manages all the data sets required and allows for the forecasting of groundwater storage volumes. Furthermore, the model was able to evaluate agricultural water demands under different climatic and management scenarios. A tool which provides a summary of the results and performs a statistical analysis for any portion of the study area was also implemented. The model was applied to a coastal region of Southern Italy. The averaged groundwater balance calculated by the model was in agreement with the piezometric head and chlorine concentration trends measured in selected monitoring wells.