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. The Cevennes–Vivarais region in southern France is prone to heavy rainfallthat can lead to flash floods which are one of the most hazardous naturalrisks in Europe. The results of numerous studies show that besides rainfalland physical catchment characteristics the catchment's initial soil moisturealso impacts the hydrological response to rain events. The aim of this paperis to analyze the relationship between catchment mean initial soil moisture θ ini and the hydrological response that isquantified using the event-based runoff coefficient ϕev inthe two nested catchments of the Gazel (3.4 km 2 ) and the Claduegne(43 km 2 ). Thus, the objectives are twofold: (1) obtaining meaningfulestimates of soil moisture at catchment scale from a dense network of in situmeasurements and (2) using this estimate of θ ini to analyze its relation with ϕev calculated for many runoffevents. A sampling setup including 45 permanently installed frequency domainreflectancy probes that continuously measure soil moisture at three depths isapplied. Additionally, on-alert surface measurements at ≈10  locations in each one of 11 plots are conducted. Thus, catchment meansoil moisture can be confidently assessed with a standard error of the meanof ≤1.7  vol % over a wide range of soil moisture conditions. The ϕev is calculated from high-resolution discharge andprecipitation data for several rain events with a cumulative precipitation Pcum ranging from less than 5 mm to more than 80 mm. Because of the highuncertainty of ϕev associated with the hydrograph separationmethod, ϕev is calculated with several methods, includinggraphical methods, digital filters and a tracer-based method. The resultsindicate that the hydrological response depends on θ ini : during dry conditions ϕev is consistentlybelow 0.1, even for events with high and intense precipitation. Above athreshold of θ ini = 34  vol % ϕev can reach values up to 0.99 but there is a high scatter. Somevariability can be explained with a weak correlation of ϕev with Pcum and rain intensity, but a considerable part of the variabilityremains unexplained. It is concluded that threshold-based methods can be helpful to preventoverestimation of the hydrological response during dry catchment conditions.The impact of soil moisture on the hydrological response during wetcatchment conditions, however, is still insufficiently understood and cannotbe generalized based on the present results.

How does initial soil moisture influence the hydrological response? A case study from southern France