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

Abstract

Abstract. 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 θ̃ini and the hydrological response that is quantified using the event-based runoff coefficient ϕev 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 θ̃ini to analyze its relation with ϕev 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, on-alert surface measurements 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 ≤1.7 vol % over a wide range of soil moisture conditions. The ϕev is calculated from high-resolution discharge and precipitation data for several rain events with a cumulative precipitation Pcum ranging from less than 5 mm to more than 80 mm. Because of the high uncertainty of ϕev associated with 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 θ̃ini: during dry conditions ϕev is consistently below 0.1, even for events with high and intense precipitation. Above a threshold of θ̃ini=34 vol % 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.

Detection and attribution of flood trends in Mediterranean basins

“…While much work has been done to estimate future climatic conditions, changes in hydrological variables, including surface conditions, can also strongly modulate climatic trends (Knighton et al, 2017). In particular, it is known that in many catchments the initial soil moisture conditions prior to flood events play a key role in flood generation (Brocca et al, 2008;Tramblay et al, 2010;Raynaud et al, 2015;Woldemeskel and Sharma, 2016;Wasko and Sharma, 2017;Uber et al, 2018;Wasko and Nathan, 2019), and its temporal change has not been much analyzed up to now. Between two episodes of rain, the base flow of the perennial rivers originates from the draining of the water contained in the soils and for some basins from the aquifers.…”

Evaluation of Soil Water Index of distributed Tank Model in a small basin with field data

“…Though the total values of rainfall of the Event VI and VII were similar (104.59 mm and 139.12 mm, respectively), the values of the initial soil water content were slightly different (0.55 cm 3 /cm 3 in Event VI and 0.41 cm 3 /cm 3 in Event VII), which might result in the differences in the peak flow between these events (0.15mm in Event VI and 0.09mm in Event VII). Zehe et al (2007) and Uber et al (2018) found significant effects of initial soil moisture condition on discharge values.…”

Soil Moisture Sensor Network Design for Hydrological Applications

“…In hydrology, soil moisture is the key component for the partitioning of rainfall into evapotranspiration, infiltration and runoff (Vereecken et al, 2008;Brocca et al, 2017;Rajib et al, 2016;Fuamba et al, 2019). In particular, the antecedent soil moisture condition of a catchment is among one of the most important factors for flood triggering (Uber et al, 2018;Zhuo and Han, 2017). For hydrological modelling, soil moisture is a vital state variable.…”