Subsurface lateral flow from hillslope and its contribution to nitrate loading in the streams during typical storm events in an agricultural catchment

Compared with overland flow from agricultural hillslopes, subsurface lateral flow is often overlooked partly due to monitoring difficulties and the lack of quantitative identification its role in nutrient delivery to surface water. The objectives of this study were to examine how subsurface lateral flow generates from hillslopes to streams and to quantify its contribution to nutrient loading in streams. Hillslope hydrology and stream hydrology were simultaneously monitored during two typical storms and subsurface flow was separated by chemical mixing model. Positive soil water potential at the soil depths from 0.60 to 1.50 m was observed at the middle course of the storm events, suggesting soil water was saturated following the storms and the drained after the end of the storms. The hydro-chemographs in the stream in a trench below a hillslope showed that suspended sediment, particulate N and P were dominant in the stream during the storms, while after the end of the rainstorms the nitrate concentration and electricity conductivity (EC) in the stream increased with time on the recession limbs of the hydrographs. Meanwhile, a rebound or delayed curve appeared on the recession limbs for several hours immediately after the end of rainstorms. All the synchronous data confirmed nitrate was delivered from the hillslope through subsurface lateral flow to the streams even after the end of rainstorms. A chemical mixing model based on EC and pH showed that the subsurface lateral flow during the rainstorm events accounted for 29% to 45% of the stream flow and about 86% of total NO3−-N loss (or 26% of total N loss) from the peanut hillslope and for 5.7% to 7.3% of the stream flow about 69% of total NO3−-N loss (or 28% of total N loss) from the catchment outlet. The results suggest that subsurface lateral flow generated within a shallow soil profile have to be paid more attention for controlling non-point source surface water pollution from intensive agricultural catchment.