On the estimation of spatially representative plot scale saturated hydraulic conductivity in an agricultural setting

Spatially representative estimates of saturated hydraulic conductivity, Ks, are needed for simulating catchment scale surface runoff and infiltration. Classical methods for measuring Ks are time-consuming so sampling campaigns need to be designed economically. Important insights can be obtained by experiments directed to understand the controls of Ks in an agricultural setting and identify the minimum number of samples required for estimating representative plot scale Ks. In this study, a total of 131 double-ring infiltrometer measurements were made on 12 plots in a small Austrian catchment. A statistical analysis of Ks across the catchment suggests Ks to be only slightly influenced by physical and topographical soil characteristics while land use is the main control. The highest values of Ks were observed in arable fields, with a median of about 3 times and a coefficient of variation (CV) of about 75% of those in grassland areas. An uncertainty analysis aimed at determining the minimum number of Ks measurements necessary for estimating the geometric mean of Ks over a given area with a specified accuracy suggests that, beyond a specific and plot-size dependent number of measurements, the benefit of any extra measurement is small. The confidence interval of the geometric mean of Ks decreases with the number of measurements and increases with the size of the plot sampled. Applications of these findings for designing field campaigns are discussed.