Uncovering patterns of near-surface saturated hydraulic conductivity in an overland flow-controlled landscape

Abstract

Saturated hydraulic conductivity (Ks) is an important soil characteristic affecting soil water storage, runoff generation and erosion processes. In some areas where high-intensity rainfall coincides with low Ks values at shallow soil depths, frequent overland flow entails dense drainage networks. Consequently, linear structures such as flowlines alternate with inter-flowline areas. So far, investigations of the spatial variability of Ks mainly relied on isotropic covariance models which are unsuitable to reveal patterns resulting from linear structures. In the present study, we applied two sampling approaches so as to adequately characterize Ks spatial variability in a tropical forest catchment that features a high density of flowlines: A classical nested sampling survey and a purposive sampling strategy adapted to the presence of flowlines. The nested sampling approach revealed the dominance of small-scale variability, which is in line with previous findings. Our purposive sampling, however, detected a strong spatial gradient: surface Ks increased substantially as a function of distance to flowline; 10m off flowlines, values were similar to the spatial mean of Ks. This deterministic trend can be included as a fixed effect in a linear mixed modeling framework to obtain realistic spatial fields of Ks. In a next step we used probability maps based on those fields and prevailing rainfall intensities to assess the hydrological relevance of the detected pattern. This approach suggests a particularly good agreement between the probability statements of Ks exceedance and observed overland flow occurrence during wet stages of the rainy season.