Empirical runoff generation threshold in a small catchment in Central Italy by integrating in-situ and satellite observations.

lood management has growing interest considering ongoing climate change in the Mediterranean area. Although the modelling of flood events is one of the most studied topics, the lack of reliable data (rainfall event depth, antecedent soil moisture, etc.) makes this approach challenging, especially in scarcely equipped catchments. Empirical methods often identify and describe the relationship between rainfall events depth (P) and surface runoff (R), defining runoff generation thresholds at catchment or plot scales. However, these methods require knowledge of surface soil moisture (SM), which is generally not measured in catchments; or else, only some spot measurements are acquired that may not represent soil moisture conditions over the basin. In recent years, the use of satellite products to estimate meteo-climate variables has increased in flood forecasting. A scarcely anthropized small-to-medium-sized catchment in Central Italy is investigated to define the empirical runoff threshold obtained by considering P and SM from satellite estimation and evaluating its performance. The Tatarena catchment (about 32 km2) has medium-to-low permeability rocks (about 90% of the area), and three small springs of a few l/s feed the stream in the headwater where limestone rocks outcrop (10%). The geological and geomorphological characteristics of the Tatarena stream make it susceptible to intense rainfall events, which caused several flood events. The analysis considers P and R data from the Hydrographic Service of the Umbria Region and SM data from the Copernicus Soil Water Index product (SWI). The note presents the empirical threshold of R as a function of the sum of P and an SWI. This approach can be extended to other basins with similar lithological characteristics, contributing to the knowledge of the runoff trigger.