Soil Moisture Dynamics and Effects on Runoff Generation at Small Hillslope Scale

Soil moisture variability has a major impact on runoff generation. In this study, the dynamics and variability of soil moisture at small hillslope transects were analyzed by artificial rainfall simulation experiments in which the influence of crop type and slope angle were considered. Soil moisture variability and dynamics during the rainfall-runoff process were monitored by capacitance sensors (EC-5) with high temporal resolution (1min) and continuously measured after rainfall ceased to investigate soil moisture dynamics and patterns by the method of data visualization. Relationships between soil moisture and runoff (surface and subsurface flow) and response lag times of soil moisture at different slope positions and soil layers were investigated. The dynamics of soil moisture during the entire crop growth stage were also analyzed. Results showed that soil moisture exerted strong influence on runoff generation, but the relationships were strongly affected by crop type. There was an obvious threshold (approximately 30% vol soil moisture content) between soil moisture and runoff coefficients/surface flow for the wheat crops for both slope angles, but the relationship between soil moisture and runoff was more scattered for spring maize. The response of subsurface flow to soil moisture exhibited hysteretic behavior attributable to soil moisture characteristics and a long recession process and was significantly influenced by slope angle. Lag times between soil moisture and the generation of subsurface flow were both controlled by antecedent soil moisture conditions. During wet conditions, differences in soil moisture response time at different soil layers were small, and obvious subsurface flow could be observed. Conversely, there were large differences between the response times (e.g.,the increase in soil moisture for 40-cm depth was close to 0) for rainfall events during dry conditions, and little lateral subsurface flow was observed. Understanding relationships between soil moisture and subsurface flows under different slop angles and vegetation covers will help to improve models that aim to predict the impact of land-use change on runoff and provide useful information for evaluating the effects of soil conservation measures on runoff and sediments in north China.