Scaling soil erosion at a cleared forest hillslope in a humid region

This study aimed to examine the effect of plot size on soil loss under natural rainfall in a humid region. Six experimental plots with areas of 3 to 360 m 2 were established on a hillslope, north of Iran. The soil losses were measured for 14 events. The results showed that absolute soil loss (ASL, g) increased with increasing plot size, whereas specific soil loss (SSL, g m −2 ) decreased, both following a nonlinear pattern. Based on ASL data, soil erosion occurred primarily as sheet erosion in plots 3–22 m long, whereas clear signs of rill formation were observed when plot length increased from 30 to 60 m, which was also confirmed by field observations. While the decreasing trend of SSL was very gradual from plot length of 3 to 10 m and of 30 to 60 m, it decreased sharply by increasing plot length from 10 to 22 meters. Scaling the SSL by two power equations showed that the power varies from −0.58 to −0.29 with a positive correlation to rainfall depth. Both equations did not fit to high SSLs well, showing under-estimation for small plots and over-estimation for big plots. The results revealed a clear scaling relationship between plot length and soil loss variables. Soil loss rates tended to decrease with increasing plot length, indicating that sediment delivery efficiency declines as the flow path becomes longer. These findings highlight the important role of scale effects and sediment redistribution processes in controlling soil erosion on forested hillslopes.