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 K_fs-values. The sensitivity of K_fs 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 K_fs is fairly small. Hence, although the pressure infiltrometer technique as applied here is useful to estimate K_fs 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 K_fs values of measurement locations having a positively calculated α using SEA. Using this field α, K_fs values for the other locations with a negative α are hence determined. Finally, the spatial correlation of K_fs on the hillslope is examined. Ranges of ln(K_fs) between 2.85 and 3.8 m were observed, respectively, for the omnidirectional case and the y direction along the hillslope μ.