USING A TRANSIENT INFILTROMETRIC TECHNIQUE FOR INTENSIVELY SAMPLING FIELD-SATURATED HYDRAULIC CONDUCTIVITY OF A CLAY SOIL IN TWO RUNOFF PLOTS

Point measurement of soil properties allows to explain and simulate plot scale hydrological processes. An intensive sampling was carried out at the surface of an unsaturated clay soil to measure, on two adjacent plots of 4×11 m2 and two different dates (May 2007 and February-March 2008), dry soil bulk density, rb, and antecedent soil water content, qi, at 88 points. Field-saturated soil hydraulic conductivity, Kfs, was also measured at 176 points by the transient Simplified Falling Head technique to determine the soil water permeability characteristics at the beginning of a possible rainfall event yielding measurable runoff. The rb values did not differ significantly between the two dates but wetter soil conditions (by 31%) and lower conductivities (1.95 times) were detected on the second date as compared with the first one. Significantly higher (by a factor of 1.8) Kfs values were obtained with the 0.30-m-diam. ring than the 0.15-m-diam. one. A high Kfs (> 100 mm h-1) was generally obtained for low qi values (< 0.3 m3m-3) whereas a high qi yielded an increased percentage of low Kfs data (1-100 mm h-1). The median of Kfs for each plot/sampling date combination was not lower than 600 mm h-1 and rainfall intensities rarely exceeded 100 mm h-1 at the site. Occurrence of runoff at the base of the plot needs a substantial reduction of the surface soil permeability characteristics during the event, probably promoted by a higher water content than the one of this investigation (saturation degree = 0.44 – 0.62) and some soil compaction due to rainfall impact. An intensive soil sampling reduces the risk of an erroneous interpretation of hydrological processes. In an unstable clay soil, changes in Kfs during the event seem to have a noticeable impact on runoff generation and they should be considered for modeling hydrological processes.