Monitoring of suspended sediment load and transport in an agroforestry watershed on a karst plateau, Southwest China

Abstract

The timing and distribution of soil erosion in agroforestry landscapes in karst regions remains poorly understood at the watershed scale, despite the recognized deleterious impacts of soil erosion. The aim of the research presented here is to understand multi-scale soil erosion and its relationship with agricultural activities in an agroforestry catchment developed on limestone bedrock in southwest China. Soil erosion monitoring was carried out for runoff plots on the hillside of the basin, and high-frequency suspended sediment and discharge monitoring was carried out for the surface and underground rivers at the outlet of the basin. The results show that the annual sediment transport modulus in this catchment is extremely low (5.1 Mg km−2 a-1). Sediment fluxes in the underground and surface rivers account for 19.7 % and 80.3 % of the total flux, respectively. Soil leakage is an important way but not the main way of soil erosion in this typical karst watershed. There is no obvious soil erosion on the hillsides (less than 1 Mg km−2 a-1), and the annual soil erosion modulus in the paddy field is about 53.7 Mg km−2 a-1. Agricultural activities dominate the distribution of soil erosion. Rainfall that generates surface runoff in the watershed is the key factor affecting soil erosion. This kind of rainfall often occurs in the summer (June to August) during crop cultivation. This study reveals that preventing soil loss from lowland farmland during heavy rainfall events should be prioritized in karst watersheds where agroforestry is practiced.