Surface Runoff

Surface runoff, or overland flow, is a fundamental process of interest in hydrology. Surface runoff generation can occur at multiple scales, ranging from small pools of excess water that propagate downhill to stream networks that drain large catchments. Accurate quantification of runoff is vital to clarify the mechanisms and effects of overland flow and also indispensable to understand fundamental hydrological processes. In this chapter, four kinds of measurement techniques, including runoff plot method, curve number method, isotopic tracer method, and salt solution method, are introduced. Runoff plot experiments are often conducted to evaluate the rainfall–runoff processes and widely used to study runoff and/or sediment losses. The curve number method is used to estimate watershed direct-runoff volume by a curve number value which is developed based on measured watershed runoff and rainfall data. The isotopic tracer method is used to measure the surface runoff by separating its contribution from multicomponent based on the mass balance of stable isotopes. The salt solution method is usually used to measure the shallow water flow by detecting the movement of salt. Besides, models of surface runoff are also summarized in this chapter. The models can be classified into conceptual models and process-based models. The conceptual models are simple transfer functions describing a linear relationship between rainfall and surface runoff. While the process-based models take into account of the spatial variability of climate, soil, vegetation, and terrain, which are able to make a series of hydrological processes interconnected. Despite their complexities, the process-based models are very helpful to study the changes in hydrological processes caused by human activities. Furthermore, the vegetation has important impact on surface runoff. For instance, with the increase of vegetation coverage, surface runoff can be reduced effectively. And root induces macropores, which are of importance for runoff mitigation due to their large diameters and high connectivity, enhancing rapid rainfall infiltration and percolation to deeper soil layers. Finally, we put forward some challenges about the measurement and simulation of the surface runoff including the establishment of surface runoff observation network in different ecological system, the combination of land surface model and distributed hydrological model, and the coupling between ecological processes and the runoff process on different scales.