Mapping topsoil field-saturated hydraulic conductivity from point measurements using different methods

Topsoil field-saturated hydraulic conductivity, Kf_s, is a parameter that controls the partition of rainfall between infiltration and runoff and is a key parameter in most distributed hydrological models. There is a mismatch between the scale of local in situ Kf_s measurements and the scale at which the parameter is required in models for regional mapping. Therefore methods for extrapolating local Kf_s values to larger mapping units are required. The paper explores the feasibility of mapping Kf_s in the Cévennes-Vivarais region, in south-east France, using more easily available GIS data concerning geology and land cover. Our analysis makes uses of a data set from infiltration measurements performed in the area and its vicinity for more than ten years. The data set is composed of Kf_s derived from infiltration measurements performed using various methods: Guelph permeameters, double ring and single ring infiltrotrometers and tension infiltrometers. The different methods resulted in a large variation in Kf_s up to several orders of magnitude. A method is proposed to pool the data from the different infiltration methods to create an equivalent set of Kf_s. Statistical tests showed significant differences in Kf_s distributions in function of different geological formations and land cover. Thus the mapping of Kf_s at regional scale was based on geological formations and land cover. This map was compared to a map based on the Rawls and Brakensiek (RB) pedotransfer function (mainly based on texture) and the two maps showed very different patterns. The RB values did not fit observed equivalent Kf_s at the local scale, highlighting that soil texture alone is not a good predictor of Kf_s.