3.1 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 2009 were well (R2= 0.98–0.99) approximated by exponential functions (Figure 3), which were continuous trends shown for the same gully networks between 1957 and 2005 (Mukai, 2017). The mean area-specific gully erosion rate (AEM ) for the 12 catchments from 2005 to 2009 was 73.3 Mg ha-1 y-1; 116.3 Mg ha-1 y-1 for the Rift margin and 51.9 Mg ha-1 y-1 for the Valley bottom. The AEM for the Rift margin was comparable to those measured in sub-humid Ethiopian highlands. The AEM for the 12 catchments from 1957 to 2009 was 21.2 Mg ha-1y-1, which increased from that from 1957 to 2005, 16.2 Mg ha-1 y-1 (Mukai, 2017).