Baseflow index regionalization analysis in a mediterranean area and data scarcity context: Role of the catchment permeability index

Low flow characteristics are affected by different physiographic factors such as climate, topography, geology and soils, and regional regression prediction models, to estimate low flow indexes at ungauged sites, mainly rely on these factors. The paper focuses on the baseflow index, one of the most important low flow characteristics for a catchment, and presents: (i) the analysis of baseflow separation algorithms for BFI evaluation and (ii) a regional approach to predict the BFI at ungauged sites in a Mediterranean region, for which only very poor data are available. The prediction of baseflow contribution to total streamflow is based on the introduction of a permeability index, at the catchment scale, and regional linear regression equations simply relate the latter to the BFI. For the studied area geological features have been found to be the major factor affecting baseflow and the permeability index estimation for a particular catchment, in an apparently over-simplified schematization, essentially reflects catchment lithology. As a matter of fact, an accurate catchment geology spatial variability description reduces the average long-term BFI index prediction error from 23% to 14% and above all increases the explained variance from 23% to 68%.