Characterisation of the field-saturated hydraulic conductivity on a hillslope: in situ single ring pressure infiltrometer measurements

Spatial variability of surface hydraulic properties is an important factor for infiltration and runoff processes in agricultural fields. At 120 locations on a hillslope, steady-state infiltration rates were measured at two applied pressure heads with a single-ring infiltrometer. The solution of two steady-state infiltration equations for each location (the simultaneous-equation approach, SEA) yielded 41 negative α-values, 79 positive α values and 120 positive Kfs-values. The sensitivity of Kfs and α to small measurement errors was estimated using a Monte-Carlo simulation (MC). Results of this MC simulation showed that the uncertainty on α is extremely high while the uncertainty on Kfs is fairly small. Hence, although the pressure infiltrometer technique as applied here is useful to estimate Kfs at each measurement location, it is not suited for the estimation of an α-value at each measurement location. A new procedure is proposed for the simultaneous estimation of one overall ‘field α’ and the 79 Kfs values of measurement locations having a positively calculated α using SEA. Using this field α, Kfs values for the other locations with a negative α are hence determined. Finally, the spatial correlation of Kfs on the hillslope is examined. Ranges of ln(Kfs) between 2.85 and 3.8 m were observed, respectively, for the omnidirectional case and the y direction along the hillslope μ.