Experimental examination of solute transport by surface runoff from low-angle slopes

The removal of chemicals in solution, by overland flow from agricultural la nd has the potential to be a significant source of chemical loss from zero- till and surface mulched farming systems. The objective of this study was t o determine the magnitude of solute loss by surface runoff from agricultura l systems. Previous experiments have enhanced the understanding of the exch ange process, but the initial soil conditions together with the tracer appl ication method in these experiments have meant that in some cases the resul ts have limited applicability to field situations. In this study, two diffe rent sets of experiments were carried out to determine the magnitude of sol ute loss by surface runoff. These experiments entailed the surface applicat ion of bromide to (1) field scale plots 18 m long by 2 m wide and (2) repac ked soil cores 236 mm in diameter; followed by the application of simulated rainfall in both cases. The most substantial finding of the field experime nts was that the quantities of solute in surface runoff varied greatly with soil type and structure (0.07-14.9% of the applied bromide). Also, on some soils, large quantities of tracer were measured in the surface runoff even after several hours of infiltration. The experiments on soil cores showed that soil structure plays an important role in the quantity of chemical tha t may be transported in the surface runoff. These field results showed that , in certain systems, solute movement by overland flow is an important tran sport mechanism, which should be considered when budgeting for chemical los s. (C) 2000 Elsevier Science B.V. All rights reserved.