3.1 Temporal and spatial dynamics of catchment geomorphic characteristics
All values of the catchment geomorphic parameters, HL (maximum catchment length), A (catchment area), Pe (catchment perimeter), and HDC (height difference between the outlet and the highest point) showed increasing tendencies as the gully networks expanded over time (Table 1). Over the three years, 1957, 1972, and 2005, no areal aspect of catchment geomorphic parameters, HL ,A , and Pe , showed any significant differences (p> 0.05) between the two sub-areas. The same was true for the catchment areal aspect indices, LR (lemniscate ratio),CC (compactness coefficient), and FF (form factor; Table 1). Besides, no notable increasing or decreasing tendencies inLR , CC , and FF were observed over the three periods. The relationships between HL and A (Figure 3 (a)) and Pe and A (Figure 3 (b)) showed the same spatial and temporal trends; (i) regardless of the year and sub-area, all the data appeared to be positioned on the linear lines that represented theHL -A and Pe -A relationships; and (ii) the Valley bottom stretched the HL -A and Pe -Alinear lines over time, i.e., the Valley bottom main gullies largely extended in terms of all of the HL , A , and Pe over time, whereas the temporal changes were limited for the Rift margin main gullies.
In contrast, the catchment relief aspect parameter HDC (height difference for catchments) showed a significant difference (p< 0.01) between the two sub-areas (Table 1). The same was true for the catchment relief aspect indices, RR (relief ratio) andHI (hypsometric integral). Over the three years, eitherHDC or RR values have not changed much for the Rift margin (0.38−0.40 or 0.12−0.14) or Valley bottom (0.08−0.11 or 0.05−0.06) sub-areas. However, the HDCs of the Rift margin in 1957 and 1972 were approximately 4.8 times higher than those of the Valley bottom, indicating the catchments in the Rift margin sub-area had more erosive runoff that had higher potential energy for soil erosion and, therefore, had a higher erosion risk than those in the Valley bottom sub-area since the gully incision. The HI values of the Rift margin shifted from 0.37 in 1957 to 0.33 in 2005, indicating the catchments were always at the equilibrium or mature stage, whereas those of the Valley bottom shifted from 0.60 in 1957 and 1972 to 0.47 in 2005, indicating the catchments recently have developed from disequilibrium/young stage to equilibrium/mature stage. The relationship between HL andHDC , which corresponds to RR (Figure 3 (c)) showed a contrasting trend between the sub-areas; (i) regardless of the years, all the data appeared to be positioned on the linear lines representing the HL-HDC relationships, i.e., maintaining almost the sameRR values, which were specific to each sub-area; and (ii) the Valley bottom main gullies largely extended in terms of both height difference (HDC ) and catchment length (HL ). In contrast, the temporal changes were limited for the Rift margin main gullies.