2.2 Field measurements
Aerial photographs of the study area taken in 1957, 1972, and 2005
(1:50,000 scale) were used for the analysis. Geometric rectification and
photogrammetric restitutions were performed using ground control points,
and a DEM (10 m pixel size) for the 2005 orthophotograph with positional
accuracy in terms of root-mean-square errors (RMSExyz)
of 2.4, 3.7, and 3.5 m was constructed. The geometric rectification
performed by co-registration with the 2005 orthophotograph resulted in
root-mean-square errors (RMSExy) of 3.7–3.8 and
3.2–3.4 m for the 1957 and 1972 aerial photographs. In the field
surveys conducted in 2005, the dimensions of gully cross-sections and
the length of each gully section were measured for 127 gully points
selected from the Rift margin catchments and 139 gully points selected
from the Valley floor catchments.
After the field measurement in the 12 study catchments, relationships
between the volume of a gully network (V , 103m3) and the length of the gully network (L, km)
(a V-L relation) was calculated as V =
0.870L 1.406 (n = 12,r 2 = 0.963). This power function was used to
estimate the volumes of gully networks in 1957 and 1972. The
area-specific volume of a gully network (Va ,
103 m3 km-2) was
estimated by the equation of Va = V /A where A (km2) is the catchment area of
the gully network. A gully erosion rate in a mass unit for each of the
catchments (EM ; 103 Mg y-1)
was estimated by the equation of EM =
(V end BD end −V start BD start) /
(Y end – Y start) whereBD (Mg m-3) is the approximation of soil bulk
density and Y is year (1957, 1972, and 2005); the subscriptsstart and end represent the starting and ending years of
estimation. Similarly, an area-specific gully erosion rate in a mass
unit (AEM ; Mg ha-1 y-1) was
estimated by the equation of AEM = (V endBD end / A end −V start BD start /A start) / (Y end –Y start). Information on land use/cover in the 12
catchments was collected from the 2005 field survey, interviews, and
aerophoto-interpretation.
Some geomorphic indices were used to analyse the temporal changes in
areal and relief aspects of the study catchments in 1957, 1972, and
2005: (i) compactness coefficient (CC ; Gravelius, 1914);wherePe (km) is catchment perimeter. (ii) Form factor (FF ;
Horton, 1932); FF = A /HL 2π where HL (km) is maximum
catchment length. (iii) Relief ratio (RR ; Schumm, 1956);RR = HDC / HL where HDC (km) is a height
difference between the outlet (Hmin ) and the
highest point in the catchment (Hmax ). The
absolute elevation difference within the catchment (HDC )
represents the potential energy available for soil erosion (Verstraeten,
& Poesen, 2001). A high relief ratio corresponds to a more pronounced
topography and thus to a higher erosion risk (Verstraeten, & Poesen,
2001). (iv) Lemniscate ratio (LR ; Chorley, Malm, & Poaorzelski,
1957); LR = HL 2π / 4A . (v)
Hypsometric integral (HI ; simplified equation of the
elevation-relief ratio proposed by Pike & Wilson (1971) was used);HI = (Hmean − Hmin )
/ (Hmax − Hmin ) whereHmean is the mean height in a catchment. The
lower values of LR and CC and the higher value ofFF indicate the more compact shape of the catchment and hence the
lesser time of concentration for runoff and the more soil erosion
(Morgan, 1996). Strahler (1952) found that a catchment at a younger
evolutionary stage is highly susceptible to erosion and has a largeHI value, but it decreases as the landscape is denuded towards a
stage of maturity and old age. The HI value can be used as an
estimator of erosion status of catchments (Singh, Sarangi, & Sharma,
2008), such as the watershed is old and fully stabilized (HI ≤
0.3); equilibrium or mature stage (0.3≤ HI ≤0.6); and
disequilibrium or young stage (HI ≥ 0.6), in which the watershed
is highly susceptible to erosion (Strahler 1952). Values of the
parameters that used to express the geomorphic indices, such asA , Pe , ground heights, HL , were obtained from the
DEM for the 2005 orthophotograph using ArcGIS 10.1 (ESRI, Redlands) and
the 2005 field survey.
Gully topographic thresholds, the relationships of the slopes at the
gully heads (s ) that were formed before 1957, between 1957 and
1972, and between 1972 and 2005 and the upslope drainage areas of the
gully heads (a ) were investigated for the main gully channels of
the sub-areas. Land use/cover in the 12 catchments was categorised into
forest, grassland, and farmland on 1957 and 1972 aerial images and 2005
orthophotographs, and each polygon area was obtained by using ArcGIS.