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

Spatially representative estimates of saturated hydraulic conductivity, K-s are needed for simulating catchment scale surface runoff and infiltration. Classical methods for measuring K-s are time-consuming so sampling campaigns need to be designed economically. Important insights can be obtained by experiments directed to understand the controls of K-s in an agricultural setting and identify the minimum number of samples required for estimating representative plot scale K-s. 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 K, across the catchment suggests K-s to be only slightly influenced by physical and topographical soil characteristics while land use is the main control. The highest values of K-s 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 K-s measurements necessary for estimating the geometric mean of K-s 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 K-s 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.