Unsaturated hydraulic conductivity of structured soils from a kinematic wave approach

Abstract

Measuring soil hydraulic conductivity in the immediate vicinity of soil water saturation remains challenging, particularly for structured agricultural soils. As a consequence, it is not possible to accurately predict infiltration and drainage rates near saturation with the Darcy-Richards model. Since these rates are accurately predicted by the kinematic wave approach near saturation in structured soils, we assume that the flux-water content relationship could be used to assess hydraulic conductivity within this range of soils and moisture content. Two types of experiments were performed on a structured loamy clay soil in the laboratory: (i) infiltration experiments on samples at laboratory capacity, in order to apply the kinematic wave theory, and (ii) evaporation experiments on saturated samples, in order to assess hydraulic conductivity using Wind's method. Estimating the relationship between the flux of drained water and macropore moisture content according to the kinematic wave theory requires the knowledge of two parameters: a macropore-flux distribution index a and a conductance term b. These parameters were fitted using both a current and a new method. Currently parameters are estimated from the falling limb of the drainage hydrograph whereas the new method uses from the evolution of the drained water flux versus macropore saturation after the end of rainfall. Results have shown that the new method is more reliable than the current one. The estimation of kinematic wave parameters is not influenced by rainfall intensity but depends on initial water content and temporal evolution of the soil sample. Hydraulic conductivity data assessed by both the kinematic wave theory and Darcy's theory seem consistent at values close to “laboratory capacity”, defined in this work as volumetric water content after one-day draining. Thus, the kinematic wave approach could be a reliable tool to assess the hydraulic conductivity of macroporeus soils near saturation.