Hydrological drought index based on simulated surface runoff using empirical rainfall-runoff models

The growing challenges related to water resource scarcity highlight the importance of exploring new approaches for the quantitative modelling of water resources and the assessment of hydrological drought intensity. This study aims to quantitatively simulate annual surface runoff at the pixel scale for the analysis of hydrological drought conditions through a new runoff condition index (RCI) derived from surface runoff modelled by empirical rainfall–runoff models. The simulated surface runoff and the derived RCIs were significantly correlated with 11 hydrometeorological variables from the ERA5-Land reanalysis (Rmax = 0.93) and the measured streamflows from 13 hydrometric stations (Rmax = 0.8). Furthermore, from a quantitative point of view, the surface runoff resulting from the results of the four models is comparable to what exists in the scientific literature in the region. Considering the sequences of wet years, the estimated annual runoff is 204.2 mm according to the results of the Schreiber model, whereas the historical average is 202.2 mm from 1975–2001. However, during years of severe meteorological drought, surface runoff can decrease by up to 26 mm/year. Overall, the developed approach provides a new perspective for the quantitative modelling of the hydrological response and, consequently, the severity of the associated hydrological drought.