Runoff formation from experimental plot, field, to small catchment scales in agricultural North Huaihe River Plain, China
Abstract. Runoff formation at an experimental plot (1600 m2), a field (0.06 km2), and a small catchment (1.36 km2) with a shallow groundwater table and a dense drainage system in the agricultural North Huaihe River Plain (China) was analysed based on the observed rainfall, runoff, and groundwater table data of 30 storm events that occurred during the 1997 to 2008 flood seasons. The surface runoff was collected and measured at the outlet of the furrow of the experimental plot, whereas the total runoff was collected and measured at the outlets of the drainage ditches of the field and the small catchment. The present study showed that the relatively narrow range of rainfall amounts resulted in significantly different runoff amounts at the 3 scales. When the groundwater is close to the surface, the runoff amount is a large percentage of the total rainfall. The difference in rainfall and runoff amounts was regressed against changes in the groundwater table, and a significant linear relationship was determined. Significant rainfall-runoff relationships were indicated for the events divided into 3 groups according to the initial groundwater table depths (as indicators of the antecedent moisture conditions): less than 0.5 m, more than 2.1 m, or between 0.5 m and 2.1 m. These findings suggest that saturation excess surface flow dominated the runoff response, particularly when the groundwater table was shallow. For almost all events, the groundwater table rose above the bottom of the drainage ditch. The total runoff amounts were larger both at the field and at the catchment than at the plot with only the surface runoff collected, which shows a considerable contribution of subsurface flow. Groundwater table depth, which indicates antecedent moisture conditions and influences lateral sub-surface flow to the drainage ditches, is an important parameter that influences runoff formation in catchments, including the study area with a shallow groundwater table and a dense drainage system.
