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Discussion papers | Copyright
https://doi.org/10.5194/hess-2018-28
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Feb 2018

Research article | 05 Feb 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Hydrology and Earth System Sciences (HESS).

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

Magdalena Uber1,3,a, Jean-Pierre Vandervaere1, Isabella Zin1, Isabelle Braud2, Maik Heistermann3, Cédric Legoût1, Gilles Molinié1, and Guillaume Nord1 Magdalena Uber et al.
  • 1Univ. Grenoble Alpes, CNRS, IRD, Grenoble-INP, IGE Grenoble, 38000, France
  • 2Irstea, UR RiverLy, Lyon-Villeurbanne Centre, Villeurbanne, 69625, France
  • 3Institute of Earth and Environmental Science, University of Potsdam, Potsdam, 14476, Germany
  • anow at: Univ. Grenoble Alpes, CNRS, IRD, Grenoble-INP, IGE Grenoble, 38000, France

Abstract. The Cévennes-Vivarais region in southern France is prone to high intensity and long lasting rainfalls 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 depth and intensity and catchment characteristics such as topography, geology, land use and hydraulic routing, 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 θ~ini and the hydrological response that is quantified using the event-based runoff coefficient ϕev in the two nested catchments of the Gazel (3.4km2) and the Claduègne (43km2). To this end, two research questions are addressed: (1) How heterogeneous are soil moisture patterns in space and time and do they correlate with land use? (2) How does soil moisture at the event onset affect the hydrological response?

The estimation of soil moisture at catchment scale is hindered by high spatial and temporal variability. A sampling setup including 45 permanently installed frequency domain reflectancy probes that continuously measure volumetric soil moisture at three depths is applied. Additionally, on-alert measurements of soil moisture in the topsoil at ≈10 locations in each one of 11 plots are conducted. Thus, catchment mean soil moisture can be confidently assessed with a standard error of the mean of ≤1.7vol% over a wide range of soil moisture conditions.

ϕev is calculated from high-resolution discharge and precipitation data for several rain events with a cumulative precipitation Pcum ranging from less than 5mm to more than 80mm. Because of the high uncertainty of ϕev associated to the hydrograph separation method, ϕev is calculated with several methods, including graphical methods, digital filters and a tracer based method. The results indicate that the hydrological response depends on θ~ini : the seasonal as well as the within-event discharge dynamics follow that of soil moisture. During dry conditions ϕev is consistently close to zero, even for events with high and intense precipitation. Above a threshold of θ~ini=34vol% ϕev can reach values up to 0.99 but there is a high scatter. Some variability can be explained with a weak correlation of ϕev 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.

Magdalena Uber et al.
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Short summary
We investigate how rivers in a flash flood prone region in southern France respond to rainfall depending on initial soil moisture. Therefore, high resolution data of rainfall, river discharge and soil moisture was used. We find that during dry initial conditions, the rivers hardy respond even for heavy rain events. For wet initial conditions, however, the response remains unpredictable: For some rain events almost all rainfall is transformed to discharge, whereas this is not the case for others.
We investigate how rivers in a flash flood prone region in southern France respond to rainfall...
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