| dc.description.abstract | Soil erosion belongs to one of the most severe environmental problems in Ethiopia. Particularly in the highlands, land degradation by soil erosion is a concerning issue, due to steep slopes and high intensity rainfall. This while, according to the FAO, nearly all of the national agricultural and economic activity is derived from the highlands. Soil losses on the cultivated highlands were estimated by Hurni around 42 ton/ha/yr. To improve the economic growth of Ethiopia and reduce the threat of land degradation, several soil and water conservation (SWC) measures have been introduced over the years, one of which are contour bunds. Studies show that contour soil and stone bunds have positive effects on soil loss reduction and yield increase. The majority of this research however, is completed in the northern highlands.
In this study, the focus lies on the effectiveness of contour bunds as a soil water conservation measure in agricultural fields in the Bokole watershed, situated in Southwest Ethiopia. Two experimental fields covered with maize crops were studied during a field work between August and September 2016, but only one of these fields was sufficient for erosion and runoff assessment. Both fields were divided into six adjacent plots with total lengths of around 18 to 20 metres and variating widths of around 6 to 10 metres. Each field contained four plots with soil bunds, dividing the plots into an upper (A) and lower (B) section and two plots served as control plots, containing no bunds. On the control plots sediment concentrations were measured. Erosion was assessed on each plot for field 2 with the Assessment of Current Erosion Damage (ACED) method. An additional field with of 1000 m2 (20m x 50m) was used to assess erosion on land with contour stone bunds. Runoff and erosion was modelled with the rMMF method. To include the effect of bunds on runoff, an adapted version of the rMMF model was used.
The lower plots (B-plots) of the experimental field were more vulnerable to erosion, due to their steeper slopes in combination with greater plot lengths. In general, the control plots contained the most erosion features, with the exception of plot 2B. It shows that slope steepness plays an important role in land degradation. Overall, longer and wider rills were found on the field with stone bunds compared to the experimental field with soil bunds and the control plots, as these features were older than on the experimental field. Erosion of the two control plots was measured at 8.6 ton/ha for the months June up to August and annual modelled losses were in the region of 37 to 44 ton/ha/yr. Annual runoff on fields without any Soil Water Conservation measures was modelled around 500 to 580 mm. The modelled effect of soil bunds showed a reduction in runoff of 20-30% for the upper plots (A-plots) and around 50% for the B-plots. Erosion was reduced by 63-70%. Rainfall variability proved to have a great influence on erosion, varying from 5 ton/ha/yr for the driest year in the record to more than 91 ton/ha/yr for the wettest year.
SWC measures, such as contour soil bunds can be easily included in the adapted rMMF model. Therefore, a new function to compute soil losses for the adapted rMMF model is desired, so that the adapted model can be used for modelling runoff as well as erosion. | |
