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.