Soil hydraulic properties in a marly gully catchment (Draix, France)

Runoff is one of the main factors controlling gully development. Concentrated flow erosion occurs where flow discharge and velocity exceed critical values. In Southern French Alps, in black marl gully catchments, runoff from hillslopes occurs in the form of Hortonian overland flow because rainfall intensities exceed the infiltration capacity of the soil. In such a situation, spatial variability of infiltration capacities is critical for describing the runoff production.This study reports detailed field measurements of near-saturation and saturated infiltration properties: capillary sorptivity (S) and hydraulic conductivity (K). A total of 140 infiltration tests were conducted using tension disc and ponded ring infiltrometers on 6 different regoliths (weathering profile) encountered in a marly gully catchment. The main objectives were to improve our knowledge of the spatial variability of infiltration in black marl areas and to compare different field and data analysis methods. Hydraulic conductivity values K(h) at supply pressure heads (h) ranging from h = - 100 up to h = 0 mm were calculated using steady state flow and Wooding's equation [Wooding, R.A., 1968. Steady infiltration from shallow circular pond. Water Resour. Res. 4 (6), 1259-1273]. K(h = 0 mm) was also estimated from the ponded ring infiltrometer data set. The estimation of sorptivity was based on transient flow and the solution of Haverkamp et al. [Haverkamp, R., Roos, P.J., Smettem, K.R.J., Parlange, J.Y., 1994. Three dimensional analysis of infiltration from the disc infiltrometer. 2. Physical based infiltration equation. Water Resour. Res. 30 (11), 2931-2935]. Three methods were used to analyse tension disc infiltrometer data: multiple radii, multiple potential and single test.Only the multiple radii method was found not suitable in our context due to a large spatial variability in the infiltration properties over small distances. The other methods gave similar estimates of the hydraulic conductivity. The hydraulic conductivity K(h) ranged up to two orders of magnitude. This result confirms the role played by structural voids when the regolith is at near saturation. The dispersion of the estimates of field saturated hydraulic conductivity is larger than unsaturated values. The spatial differences found are explained by the topographical position and by the regolith structure observed for the 6 sites. An analysis based on the ponding time show that the regolith type plays an important role in the dynamic of the runoff production.