Preferential flow in water repellent sandy soils: principles and modeling implications

Leaching risks of surface-applied agrichemicals in water repellent soils can only be quantified with an acceptable degree of accuracy if knowledge of the underlying processes, principles and an appropriate simulation model are available. The present study aimed to investigate water flow and solute transport processes in the unsaturated zone of a water repellent sandy soil, and to indicate consequences for modeling. Soil blocks with a length, width and depth of 1.2, 0.6, and 0.52 m, respectively, were sampled in the Ouddorp water repellent sandy soil to investigate three-dimensional soil water content distributions. Preferential flow patterns were clearly visible in soil blocks sampled after distinct rain events. Additional TDR measurements revealed that preferential pathways develop rapidly during severe rain storms, causing infiltrating water to be preferentially transported to the deeper subsoil. Further, preferred pathways recurred at the same sites during all rain events. Simulations with a two-dimensional flow and transport model indicate that preferential flow paths will only form during infiltration into dry water repellent soils, i.e. in the range below the critical soil moisture content. Based upon the obtained results, indications are given on how to incorporate this preferential flow and transport process in current one-dimensional simulation models.