Assessment of land degradation processes in the northwestern Ethiopian Highlands
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Hailu Kendie Addis. (31/12/2016). Assessment of land degradation processes in the northwestern Ethiopian Highlands. Vienna, Austria: Universitaet für Bodenkultur Wien, Center for Development Research (BOKU/CDR).
Abstract
Soil erosion in the northwestern Amhara region, Ethiopia has been a subject of anxiety,
resulting in a major environmental threat to the sustainability and productive capacity of
agricultural areas. In the present thesis, selected soil properties, hydrological and sediment
dynamics were assessed for a watershed, while predicting the spatial distribution of soil
properties was also done. The 53.7 km2 Gumara-Maksegnit watershed was divided into a
500 m by 500 m grid to sample bulk density (ρd), pH, soil organic carbon (SOC), total
nitrogen (TN), available phosphorus (AP) and texture of topsoil (roughly 10 to 25 cm
depth). Such properties were investigated with respect to the two main land-uses (forest
and agriculture) and three different slope steepness classes, 0–10 (%), 10–30 (%), >30 (%).
The result indicated higher SOC, TN, silt and sand content in forest soils compared to
agricultural soils, while ρd is lower in the forest soil. Overall an increase of SOC, TN, silt
and sand content from gentle to steep slopes have been observed for both land-uses. In
contrast, clay content and ρd seem to increase from steep to gentle slopes on agricultural
areas, which might be due to accumulation of particularly fine soil particles eroded from
the steep areas.
In the second part, the performance of ordinary kriging (OK), inverse distance weighting
(IDW) and radial basis functions (RBF) for predicting the spatial distribution of soil
texture, pH, soil organic carbon (SOC) and available phosphorus (AP) were done. The
performance of each interpolation method was assessed quantitatively in terms of NashSutcliffe
efficiency (E), coefficient of determination (R2
) and index of agreement (d). The
interpolated maps generated based on the highest value of E displayed OK was best
performed for SOC and sand. RBF was most suitable for mapping of AP and clay, while
IDW gave better result when applied to pH. Overall, the cross-validation statistics for each
interpolation method showed there was no single method that significantly outperformed
the others. Therefore, one of the interpolation methods could be applied for surfaces map
generation in future studies of similar regions. In the third part, the Soil and Water Assessment Tool (SWAT) model was used to model
hydrology and sediment dynamics of the watershed. Spatially distributed stone bund
impacts were applied in the model through modification of the surface runoff ratio and
adjustment of a support practice factor simulating the trapped amounts of water and
sediment at the SWC structure and watershed level. The resulting Nash-Sutcliffe efficiency
(NSE) for daily streamflow simulation was 0.56 for the calibration and 0.48 for the
validation period, suggesting satisfactory model performance. In contrast, the daily
sediment simulation resulted in unsatisfactory model performance, with the NSE value of
0.07 for the calibration and –1.76 for the validation period and this could be as a result of
high intensity and short duration rainfall events in the watershed. Meanwhile, the calibrated
model indicated 21.08 Mg ha-1 average annual sediment yield, which is far beyond potential
soil regeneration rate.
