Intensive water content and discharge measurement system in a hillslope gully in China

Abstract

An automatic monitoring system was constructed to intensively study the water–profile–runoff interactions during rain events, of a hillslope gully. The system was installed on the Loess plateau in the Shaanxi province in northern China. The primary goal was to obtain a better understanding of soil erosion processes in this district to aid in better management. To accomplish this, infiltration and runoff data for running, calibrating and validating the LISEM erosion model was needed. The system presented here consisted of 29 Time Domain Reflectometry (TDR) water content sensors that monitored the water content variation in time. Sensors were installed in different subsystems throughout the gully geometry such as in the cropland surrounding the gully, in some gully slopes and in the gully floor. At the outflow point of the gully, an H-flume was installed in order to measure the actual discharge of the gully system and to measure directly the sediment concentration in the discharge. The monitoring system was programmed in this way so that more measurements were recorded when a rainstorm occurred. Installation took place in April 1998 and it was used until September 2000. In 1998, five major rain events were recorded, which generated runoff and erosion, in 1999 and 2000 only one. Cropland measurements showed high water contents and high infiltration rates all through the measuring period and also showed a strong reaction to precipitation. The sidewalls of the gully showed much lower water contents where only very shallow parts showed some infiltration during rain showers, while steep parts of the gully, such as pipes and vertical walls, showed an extremely low water content and practically no water content variation with time. Simulation results confirmed the suspicion that these areas are an important source for Hortanian overland flow within the gully system. Model simulations further showed that most of the sediment being eroded from the gully system was coming from rills and smaller gullies being cut into the main gully bottom. Saturation excess overland flow proved to be dependent on the characteristic of the rain event as well as on the water content of the soil at the beginning of the event.