Effect of Antecedent Soil Water on Preferential Flow in Four Soybean Plots in Southwestern China

Antecedent soil water exerts considerable influence on the infiltration rate and distribution of preferential flow. Effects of antecedent soil water on preferential flow were investigated through dye experiments with Brilliant Blue FCF < C.I. Food Blue 2) in four soybean plots with low (LWC) and high < HWC) antecedent soil water levels. Changes in soil water weremeasured during the dye experiments. Horizontal and vertical soil slices were exposed every 10 cm approximately 24 h after the dye tracer application, and staining ratios (stained area to total area) were calculated from photographs. This study demonstrated that the dye tracer infiltrated to a mean depth of 56 cm at an average rate of 1,455 mm h(-1) in the LWC plots and to a mean depth of 34 cm and at an average rate of 523 mm h-1 in the HWC plots. Finger flow occurred in Layer B of the LWC plots, possibly resulting in their deeper infiltration depth. The staining ratio in each layer of the LWC plots was generally larger than the corresponding one of the HWC plots. In both the LWC and HWC plots, the average staining ratios decreased with soil depth and exhibited a sharp decrease from Horizon A to B. The largest effect of antecedent soil water on dye tracer movement was found in Layer B. The results indicated that preferential flow was substantial under conditions of low antecedent soil water content in the conditions found in southwestern China. These results could be helpful in improving crop yields and reducing pollution of shallow groundwater by leading farmers to irrigate and till the soil surface with the appropriate frequency.