Modeling Rain Isotopic Composition under Orographic Control: A Landscape Approach for Hydrogeological Applications

Oxygen isotopic composition is useful for individuating recharge areas of groundwater
bodies by the comparison with those of local rainfalls. While on a global scale general relationships, such as the isotopic vertical gradient or continentality effects, efficiently describe spatial variations of the isotopic signature, hydrogeological applications need spatial models that are more focused on the effects of local topographic structures and/or subsoil geology. This work presents a case study in northeastern Sicily (Italy) characterized by complex geological and orographic structures, in which isotopic composition of rainfalls is governed by orographic effects and the varying initial composition of humid air masses. We used a black box approach, comparing the average isotopic composition of rain collected from a network of eight samplers with their spatial descriptors (elevation,
latitude and longitude). We obtained the best correlation with the simultaneous use of all these variables, applying their multiple linear correlation equation to transform the 1 × 1 km digital elevation model (DEM) of the study area into a digital isotopic model (DIM). The reliability of the DIM was confirmed by its good agreement with the oxygen isotopic composition contour map of the local groundwater.