Improving flood management assessing seasonal flood regulating ecosystem services 

Changes in land use can influence flood hazard, as land cover is one of the main factors determining runoff during an extreme rain event. Flood-regulating ecosystem services (FRES) are generally evaluated to assess how much water the environment can hold in a storm event, providing a reduction of runoff and flood hazard in a given river basin. Agriculture areas can be seen as part of the ecosystem that provides flood regulation, as they decrease the runoff with respect to barren areas or urbanized surfaces.  Methodologies developed to evaluate FRES usually assess the reduction of runoff due to land use changes without considering differences in initial soil moisture conditions before the storm event depending on crop rotation or irrigation management during the year. Here we provide a methodology to assess FRES on a seasonal basis, evaluating crops and irrigation management practices that may exacerbate flood hazards in small agricultural river basins. We do so by coupling two hydrological models. Watneeds, an agro-hydrological model which determines the water demand in agriculture, is used to derive daily soil moisture conditions. Mobidic, a fully distributed rainfall-runoff model, sets the soil moisture conditions of Watneeds as initial soil moisture before an extreme event to evaluate the associated flood hazard. We test the methodology on the upper Ombrone Grossetano river basin (Tuscany, central Italy), as more than 60% of its land cover is represented by agricultural areas. Gridded dataset and ground measurements are used to inform and calibrate the models and to perform the analysis. Particularly, the Chirps dataset is bias corrected with ground observation and used to perform the hydrological balance in Watneeds. Rain gauge measurement provided by the Hydrological Regional Service are used in a frequency analysis of extreme rainfall to extract the rainfall quantiles to be modeled in Mobidic. Results show that each crop provides different soil moisture conditions under identical meteorological conditions, impacting the flood hazard accordingly. Indeed, different agricultural scenarios and practices may produce different responses to similar events in different seasons with potential management applications. This underlines how FRES should be evaluated seasonally in agricultural river basins and how crop selection, irrigation scheduling and crop calendarization in small agricultural river basins could be subject to policies that also consider potential impacts on flood risk management. This work is part of the FLORAES project, funded by the Premio Florisa Melone 2023, awarded by the Italian Hydrological Society to stimulate independent research and collaboration among young Italian Hydrologists.