Influence of rainfall intensity and slope on suspended solids and phosphorus losses in runoff

Suspended solids (SS) and phosphorus (P) losses in rainfall generated runoff can lead to the deterioration of surface water quality. Simulated rainfall experiments were conducted to investigate the effects of rainfall intensity (30, 50, 65, and 100 mm h^-1) and land slope (0°, 5°, and 10°) on SS and P losses in runoff from experimental rigs containing bare land soil and soil planted with grass (tall fescue). In addition, total phosphorus (TP), particulate phosphorus (PP), and dissolved phosphorus (DP) losses in runoff were also measured. Results showed that tall fescue could reduce loads of SS by 86-99.5%, PP by 92-98.5%, and TP by 55-89.8% in runoff compared with losses from bare soil; this is due to a combination reduced raindrop kinetic energy at the soil surface, reduced soil erodibility in the presence of plant roots and shoots, and an increase in roughness and consequently reduced overland flow velocity resulting in the trapping of particles. Linear relationships between losses of SS and TP and between TP and PP in runoff were significant (R² > 0.93) in both bare soil and grass. In addition, SS and TP losses increased greatly significantly with rainfall intensity and slope. The influence of rainfall intensity on SS and P losses was greater than the influence of slope. Simple linear regressions were constructed between losses of SS and P, the rainfall intensity (30 to 100 mm h^-1), and land slope (0° to 10°). The multiple regression equations of SS and P losses in runoff established in this study can provide a simple predicting approach for estimating the non-point source pollution load of SS and P arising from rainfall.