Response of Groundwater to Rainfall and the Recession Constant of Antecedent Precipitation in a Slope with Residual Soil

Rainfall infiltration is an essential factor triggering shallow landslides in the slopes with weathered residual soil. However, the response of groundwater in the residual soil layer to rainfall is a complex process. Meanwhile, the recession constant of antecedent precipitation remains difficult to determine. In this study, in-situ monitoring and laboratory tests were conducted on a slope with residual soil in Ningde City, Fujian Province. The response of groundwater and soil volumetric water content to rainfall in a hydrological year were analyzed and discussed. In addition, a calculation method based on the hydrological response of the soil layer on a slope to rainfall was proposed. The recession constant was calculated, discussed, and verified. The results show that the rainfall intensity threshold triggering a response of volumetric water content on top of soil was approximately 5 mm/d. From the descending period to a stable period, despite heavy rainfall, the groundwater level continued to decline until it stabilized. During the rising period, the minimum rainfall intensity triggering the rapid rise of groundwater level was 17.8 mm/d. The highest water level and the rise amplitude of groundwater level had a linear positive correlation with rainfall intensity. The weathered granitic soil layer has good permeability and the response speed of groundwater to rainfall is fast in this period. In addition, the average value of the total recession constant was 0.78 with an antecedent days of 2–7 days. This study presents useful information for understanding the response of groundwater to rainfall and provides a new approach towards determining the recession constant, which can be used as a key parameter to determine the critical rainfall threshold of shallow landslides.