Subsurface lateral flow from hillslope and its contribution to nitrate loading in streams through an agricultural catchment during subtropical rainstorm events

摘要

Subsurface lateral flow from agricultural hillslopes is often overlookedcompared with overland flow and tile drain flow, partly due to thedifficulties in monitoring and quantifying. The objectives of this studywere to examine how subsurface lateral flow generated through soil pedonsfrom cropped hillslopes and to quantify its contribution to nitrate loadingin the streams through an agricultural catchment in the subtropical regionof China. Profiles of soil water potential along hillslopes and streamhydro-chemographs in a trenched stream below a cropped hillslope and at thecatchment outlet were simultaneously recorded during two rainstorm events.The dynamics of soil water potential showed positive matrix soil waterpotential over impermeable soil layer at 0.6 to 1.50 m depths during andafter the storms, indicating soil water saturation and drainage processesalong the hillslopes irrespective of land uses. The hydro-chemographs in thestreams, one trenched below a cropped hillslope and one at the catchmentoutlet, showed that the concentrations of particulate nitrogen andphosphorus corresponded well to stream flow during the storm, while thenitrate concentration increased on the recession limbs of the hydrographsafter the end of the storm. All the synchronous data revealed that nitratewas delivered from the cropped hillslope through subsurface lateral flow tothe streams during and after the end of the rainstorms. A chemical mixingmodel based on electricity conductivity (EC) and H+ concentration wassuccessfully established, particularly for the trenched stream. The resultsshowed that the subsurface lateral flow accounted for 29% to 45% oftotal stream flow in the trenched stream, responsible for 86% of totalNO₃−-N loss (or 26% of total N loss), and for 5.7% to7.3% of total stream flow at the catchment outlet, responsible for about69% of total NO₃−-N loss (or 28% of total N loss). Theresults suggest that subsurface lateral flow through hydraulicallystratified soil pedons have to be paid more attention for controllingnon-point source surface water pollution from intensive agriculturalcatchment particularly in the subtropical areas with great soilinfiltration.