Assessment of factors controlling the runoff coefficient in the Mediterranean context: a case study in central Italy

Global warming is affecting hydroclimatic parameters determining changes in temperature and precipitation patterns. In addition, human-induced activities act on the land use and land cover (LULC) features of catchments. Runoff generation can be affected by these factors in both natural and anthropogenic basins. The aim of the current study is to investigate the relationship between the runoff coefficient (Rc), computed by exploiting long-term rainfall and streamflow records, and several features that can potentially affect it, namely meteorological parameters, soil water storage (SWS), and LULC changes through the wavelet coherence analysis. The method has been applied over the Upper Tiber basin at Ponte Nuovo outlet, in central Italy. To facilitate the understanding of the current catchment conditions in terms of surface water availability, a trend analysis has been performed using the Mann-Kendall (MK) test. For the long-term period of 1927-2020, the results reveal a decreasing trend of Rc. In addition, the MK test for seasonal temperature and SWS shows increasing and decreasing trends, respectively. Based on the wavelet analysis, a significant positive correlation is observed between Rc and SWS in the annual cycle with a phase shift of less than one month, while a strong negative correlation is observed between Rc and temperature in the annual period with a phase shift of 3-6 months. The study of the relationship between Rc and LULC changes shows a weak correlation. The lower phase shift between Rc and SWS indicate that Rc is susceptible to SWS in a faster way than other components. These results allows a better understanding of the main factors influencing the Rc over the pilot area; moreover, an extension to other Mediterranean basins is foreseen as a follow-up of this work.