3.2 Temporal changes in catchment geomorphic characteristics
All values of the catchment geomorphic parameters (HL , A ,Pe , and HDC ) showed increasing tendencies as the gully
networks expanded over time (Mukai, 2017). Over the three periods, 1957,
1972, and 2005, no areal aspect of catchment geomorphic parameters,HL (maximum catchment length), A (catchment area), andPe (catchment perimeter) showed any significant differences
(p > 0.05) between the two sub-areas (Mukai, 2017).
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 tendencies in LR ,CC , FF , and RR were observed over the three
periods. The relationships between HL and A (Figure 4 (a))
and Pe and A (Figure 4 (b)) showed the same spatial and
temporal trends; (i) regardless of the period and sub-area, all the data
appeared to be positioned on the linear lines that represent the
relationships between each pair of the two parameters; and (ii) the
Vallley bottom main gullies largely extended the HL -A andPe -A relationships 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 (Mukai, 2017). The same was
true for the catchment indices, RR (relief ratio) and HI(hypsometric integral; Table 1). Over the three periods, the Rift margin
showed higher RR values than the Valley bottom, showing the
catchments of the Rift margin had higher sediment loss per area. TheHI 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 and HDC (Figure 4 (c)) showed a
contrasting trend between the two sub-areas; (i) regardless of the
period, all the data appeared to be positioned on the linear lines
representing the HL-HDC relationships that are specific to each
sub-area; and (ii) the Valley bottom main gullies largely extended theHL-HDC relationship over time, whereas the temporal changes were
limited for the Rift margin main gullies.