Significance of preferential flow at the rock soil interface in a semi-arid karst environment

Abstract

In many semi-arid environments, the surface materials are frequently characterized by thin soils and large proportions of bare rock, both of which are expected to considerably influence infiltration and groundwater recharge processes. In this study, small plot scale irrigation experiments in combination with dye tracer application, as well as real-time and subsequent soil moisture measurements and soil particle size analysis, were conducted to investigate the significance of preferential flow at the rock soil interface. Large scale assessments of soil depth and surface type distribution were used to upscale the findings of the irrigation experiments to the scale of a complete hillslope and, further, to estimate percolation properties of a hypothetical rainfall event.The two experimental plots were each located in the West Bank Mountains close to Ramallah. Each irrigation experiment was designed with 50mm of precipitation, applied on an area of 1m2 during the course of ca. 2.5h. Each irrigation plot had a surface share of approximately 50% rock outcrop and 50% soil surface. Percolation properties were investigated by subsequent soil moisture measurements at high spatial resolution on vertical soil profiles on the irrigated plot. Dye tracer application on the rock outcrop during the sprinkling allowed for tracking of resulting outcrop runoff in the subsurface.The soil depth survey included 2100 measurement points on 7 transects, which allowed the estimation of local soil depth distribution. Results of the irrigation experiments showed that precipitation-induced runoff from rock outcrops continued below ground as preferential flow along the rock–soil interface, while water from the soil surface percolated mainly vertically and much more homogeneously. This was evident from the high density soil moisture measurements, as well as from the dye tracer patterns. Outcrop runoff percolated faster and to greater depths than water infiltrating directly from the soil surface, thus possessing a greater potential for groundwater recharge.