Effects of soil and water conservation practices on runoff, sediment and n utrient losses
Rainfall is a major dynamic source of soil erosion and nutrient loss on slopes. Soil and water conservation practices and agricultural activities can change the soil surface morphology and thus affect erosion and nutrient losses. This study focused on the effects of several typical soil and water conservation practices and agricultural land, for the purpose of: (1) determining how these practices prevent erosion and nutrient loss and identifying the hydrodynamic mechanisms; and (2) determining the application conditions for different practices. Runoff, sediment, total nitrogen (TN) and total phosphorus (TP) in fish-scale pits, agricultural land, narrow terraces, shrub cover and bare land, under rainfall events in rainy seasons (from May to November) during the 2010–2015 period, were monitored. Slope hydrodynamic mechanisms and application conditions of these practices were also investigated. The results showed that compared with bare land, fish-scale pits performed the best in preventing runoff, sediment, TN and TP, followed by 30% shrub coverage, narrow terraces and agricultural land, successively. Total runoff, sediment, TN and TP losses in fish-scale pits site were 19.70%, 2.03%, 10.10% and 35.97% of those in bare land of the same area, respectively. Soil and water conservation practices could change the hydraulic characteristics of slopes, decrease Re (Reynolds) and Fr (Froude) numbers, thereby decreasing runoff, sediment, TN and TP losses. Fish-scale pits were suitable for the areas with small single rainfall and good water permeability. When rainfall was greater than 60 mm, narrow terraces had highest efficiency in reducing sediment loss; therefore, they were suitable for the areas with relatively high rainfall intensity and soils similar to the sandy loams of the study area. As to the practice of covering land with plants, the effect was sustainable due to the plants’ long-term growth. Agricultural land was not recommended since the losses on it were relatively higher due to the impact of human activities. In reality, these practices may be applied in combination so as to effectively control water, soil and nutrient losses.