2.3 Long-term gully volume change
Changes in both mean volume of the gully networks (V ) and mean area-specific volume of the gully networks (Va ) for the two sub-areas from 1957 to 2005 were well approximated by exponential functions (R2 = 0.99; Mukai, 2017). During the entire study period, V of the gully networks in the two sub-areas were almost the same scale, whereas Vaof the gully network in the Rift margin sub-area was always higher than that in the Valley floor sub-area, and the gap between the two sub-areas widened as time passed. The mean area-specific gully erosion rate (AEM ) for the 12 catchments from 1957 to 2005 was 16.2 Mg ha-1 y-1; 25.1 Mg ha-1 y-1 for the Rift margin and 10.5 Mg ha-1 y-1 for the Valley bottom sub-areas.