The Influence of Soil Characteristics in Low Flows Regionalization

Problem statement: For the purpose of low flows regionalization, relevant issue for water resources management like environmental flows requirements definition, this study focused on the controls on the seasonal and spatial variability of q95 (i.e., the specific discharge that was exceeded on 95% of all the time) with particular reference to the role of soil characteristics, that, like soil infiltration rate, aquifers recharge, evapotranspiration and topography, usually play a relevant role in low flows seasonality and occurrence within a river. Approach: Piemonte and Valle d'Aosta Regions (North-Western Italy) were the investigated study area (30,027 km2) where 41 catchments were analyzed with the aim of robust regression models enabling the transfer of hydrological information from gauged to un-gauged sites. Results: The regionalization method consisted of multiple regression models between low flows and catchment characteristics. Twenty-five catchment descriptors were used, checking their relative influence with the multi-regressive procedure and a special attention was devoted to the selection of significant soil characteristics in the regionalization process. Seasonality indices were used to classify catchments into two sub-regions and separate multiple regressions was performed by checking the prediction performance with cross-validation. Also a global regression was fitted out but it yielded a lower performance. In the study domain land use, topography and Thornthwaite moisture index demonstrated to be the most significant variables in order to represent relationships between catchment soil characteristics and low flows regime. Conclusion/Recommendations: Results obtained in this study were comparable with other regionalization studies carried out in Austria and Switzerland. The interpretation of the identified regression models provided, at local scale, new tools for water management and environmental flows requirements and, from a wider point of view, useful insights into the general comprehension of low flows processes.