Soil Moisture Distribution and Runoff Generation in a Gully System

摘要

Gullies are severe stages of water erosion and gully erosion is a very serious problem on the loess plateau of China. So far very few soil erosion models can predict gully erosion. To develop and validate a gully erosion model, it is necessary to understand the relationship between soil moisture and runoff generation that affects the gully erosion. An automatic raingauge, 29 TDR (Time Domain Refectometry) sensors, an H-flume together with an ultrasonic water level sensor and a turbidity sensor, and a computer system were installed in a small gully watershed on loess plateau of China. Rainfall, soil moisture, runoff and sediment were continually monitored and recorded in a 12-hour interval generally and 20-minute interval during rain events through 1998-2000. Soil moisture spatial distributions and temporal changes were different in the gully and in different depths. Crop fields in the upland had the highest soil moisture, then was gully floor, and the gully side slopes in order, the vertical gully side wall had the lowest values. Average soil moisture was 10.0%-11.7% at 15cm depth and 10.2%-13.1% at 30cm depth of the upland, around 7.1%-7.7% of the gully floor, 7.1%-7.5% of the gully side slope, and 4.5%-6.0% of the vertical cliff. Soil water content also varied with depth. The shallow layers were more active than deep layers, and thus /moisture in these layers was higher in a wet period or wet year and lower in a dry period or dry year than that of deep layers. Generally, runoff occurred when the maximum 5-minute rainfall intensity exceeded 30mm/hr. The duration of runoff was very short in this area, and was 10-20 minutes normally.