| dc.description.abstract | A methodology was developed for assessing soil erosion hazard in the Upper Ewaso Ng'iro
basin of Kenya, using Geographic Information Systems (GIS), the Universal Soil Loss
Equation (USLE) and the European Soil Erosion Model (EUROSEM). The USLE was
used in a GIS environment by creating thematic maps of R, K, L, S, C and P and then
calculating soil loss by raster-grid modelling with ArclInfo GRID.
The rainfall erosivity factor (R) was derived from relationships between rainfall amount and
erosivity using erosion plot data from within the catchment. The nature of the relationship
was found to be a function of agro-climatic zones of the region. Mean annual erosivities
ranged from 145 to 990 J m-2 hr-I. For a given amount of rainfall, erosivity was higher in
zone IV than in the wetter zones II-III. The soil erodibility factor (K) was estimated using
the USLE nomograph and data from laboratory analysis of field samples collected from
representative major soil mapping units. The K-values were low to medium, ranging from
0.10 to 0.25 over 84 percent of the basin. The topographic factor (LS) was obtained by
creating Digital Elevation Models (DEMs) of the basin with TOPOGRIDTOOL of
ArclInfo. These were then used to determine the slope steepness and length factor values,
calculated with raster-grid modelling. Although DEMs proved a useful tool, maximum
values of both steepness and length had to be set in this reconnaissance study to achieve
reasonable results. A finer resolution of input data and a smaller grid cell size are needed
for accurate determination.
The cover and management factors (C) were obtained by determining the land cover types
within the basin using remotely ser1sed data (SPOT 1 colour composite prints) and ground
truthing studies. The factor values were estimated from USLE guide tables and
measurements of cover from plots and test sites. Some 70 percent of the basin is covered
by rangelands. The conservation practice (P) factor values were estimated from USLE
guide tables and then applied to areas where soil conservation had been introduced
according to maps obtained from the Ministry of Agriculture. The USLE was validated
usingdata from erosion plots. A value ofR2 = 0.645 was obtained between predicted and
measured values but the standard error was rather high (e = 5.745 t ha-1 yr"). Using an
annual soil loss of9.0 t ha-1 yr' as tolerance level, some 36 percent of the basin was found
to experience unacceptably high erosion rates. Most of this area was communal grazing
land and cropland where soil conservation measures had not been applied. A critical land
cover type within the grazing land is shrubland, where vegetation cover is less than 40
percent and high erosion risk was predicted and confirmed by field surveys.
EUROSEM could not be integrated within a GIS in the time available for research. It was
therefore simulated outside GIS environment, where it was applied to Embori and
Mukogodo plot data using separate data sets for calibration and validation. Calibration was
used to obtain input parameters for saturated hydraulic conductivity, cohesion and
Manning's roughness coefficients. Validation gave correlation coefficients of 0.907 and
0.840 for predictions of storm runoff and soil loss respectively at Embori; the
corresponding values for bare soil plots at Mukogodo were 0.895 and 0.577. However,
EUROSEM predicted runoff poorly (R2 = 0.570) and failed to predict soil loss at all the
vegetated plots at Mukogodo. The model was applied to simulated vegetation covers of
barley, maize, grass and forest for a 36.7 mm rainstorm at Embori. The simulated soil
losses showed an exponential decrease with increasing cover. At a threshold cover of 70
percent, soil loss diminished to zero under grass and forest and decreased to a minimum
value under barley and maize. These results support the USLE simulations, which showed
that areas with more than 70 percent cover (such as forest) had a low erosion hazard, even
with steep slopes and high rainfall erosivities.
This research has demonstrated that GIS can be used with the USLE to assess and quantify
erosion hazard, giving results that can be used for conservation planning. EUROSEM can
be applied successfully to bare soil and cropland, but application to other land covers
requires further investigation. Land cover and topography are the main factors controlling
the spatial distribution of soil loss in the Upper Ewaso Ng'iro basin. Future conservation
activities should be concentrated on the rangelands | en |
