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

Leaching risks of surface-applied agrichemicals in water repellent soils ca n only be quantified with an acceptable degree of accuracy if knowledge of the underlying processes, principles and an appropriate simulation model ar e available. The present study aimed to investigate water flow and solute t ransport processes in the unsaturated zone of a water repellent sandy soil, and to indicate consequences for modeling. Soil blocks with a length, widt h and depth of 1.2, 0.6, and 0.52 m, respectively, were sampled in the Oudd orp water repellent sandy soil to investigate three-dimensional soil water content distributions. Preferential how patterns were clearly visible in so il blocks sampled after distinct rain events. Additional TDR measurements r evealed that preferential pathways develop rapidly during severe rain storm s, causing infiltrating water to be preferentially transported to the deepe r subsoil. Further, preferred pathways recurred at the same sites during al l rain events. Simulations with a two-dimensional flow and transport model indicate that preferential how paths will only form during infiltration int o dry water repellent soils, i.e. in the range below the critical soil mois ture 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. (C) 2000 Elsevier Science B.V. All rights r eserved.