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

  • Uber, M.
  • Vandervaere, J.P.
  • Zin, I.
  • Braud, I.
  • Heisterman, M.
  • Legout, C.
  • Molinié, G.
  • Nord, G.
Publication Date
Jan 01, 2018
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The Cévennes-Vivarais region in southern France is prone to heavy rainfall that can lead to flash floods which are one of the most hazardous natural risks in Europe. The results of numerous studies show that besides rainfall and physical catchment characteristics the catchment's initial soil moisture also impacts the hydrological response to rain events. The aim of this paper is to analyze the relationship between catchment mean initial soil moisture e-ini and the hydrological response that is quantified using the eventbased runoff coefficient ev in the two nested catchments of the Gazel (3.4 km2) and the Claduègne (43 km2). Thus, the objectives are twofold: (1) obtaining meaningful estimates of soil moisture at catchment scale from a dense network of in situ measurements and (2) using this estimate of e-inito analyze its relation with ev calculated for many runoff events. A sampling setup including 45 permanently installed frequency domain reflectancy probes that continuously measure soil moisture at three depths is applied. Additionally, onalert surface measurements at 10 locations in each one of11 plots are conducted. Thus, catchment mean soil moisturecan be confidently assessed with a standard error of the meanof 1:7 vol% over a wide range of soil moisture conditions.The ev is calculated from high-resolution discharge andprecipitation data for several rain events with a cumulativeprecipitation Pcum ranging from less than 5mm to more than80 mm. Because of the high uncertainty of ev associated with the hydrograph separation method, ev is calculated with several methods, including graphical methods, digital filters and a tracer-based method. The results indicate that the hydrological response depends one ini: during dry conditions ev is consistently below 0.1, even for events with high and intense precipitation. Above a threshold of e ini D 34 vol% ev can reach values up to 0.99 but there is a high scatter. Some variability can be explained with a weak correlation of ev with Pcum and rain intensity, but a considerable part of the variability remains unexplained. It is concluded that threshold-based methods can be helpful to prevent overestimation of the hydrological response during dry catchment conditions. The impact of soil moisture on the hydrological response during wet catchment conditions, however, is still insufficiently understood and cannot be generalized based on the present results.

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