ROLE OF MUCHING ON RUNOFF AND SOIL LOSS IN FIELD PLOTS UNDER NATURAL RAINFALL 

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Soil loss estimation using USLE

The Universal Soil Loss Equation (USLE) (Wischmeier and Smith, 1965, 1978) is the most widely known and used empirical soil loss model all over the world. Later in the 1980’s, the United States Department of Agriculture-Agricultural Research Service (USDA-ARS) modified the model to the Revised Universal Soil Loss Equation (RUSLE), which was an improved version of USLE for northeastern areas of the USA incorporating new approaches, new data from different locations, and corrections of the USLE limitations (Yoder and Lown, 1995; Smith, 1999). RUSLE is computer based, replaces the tables, figures, and tedious USLE calculations with simplified keyboard entry (Yoder and Lown, 1995) while maintaining the basic structure of USLE. Unfortunately, due to inadequate availability of input data for the study sites to comply with the input requirements of RUSLE, only USLE was used to estimate soil loss for the sites. The USLE computes sheet and rill erosion using values representing the four major factors affecting erosion, namely climate erosivity R, soil erodibility K, topography LS and land use and management CP (Kenneth et al., 1991). Like the SLEMSA, the USLE doesn’t estimate deposition, sediment yield at a down stream location and ephemeral gully erosion and does not represent fundamental erosion processes and interactions (Kenneth et al., 1991). Where A is the computed long term average annual soil loss per unit area, R is the rainfall factor, K is the soil erodibility factor, LS is the topographic factor, C is a cover management factor, and P is the support practice factor. The USLE has been used widely all over the world either in the same or modified forms (Tiwari et al., 2000). Hurni (1985) also used this model to assess soil erosion in Ethiopia. He even modified some factors of the USLE for the Ethiopian conditions. Three of the most significant modifications include R (rainfall erosivity index), C (land cover) and P (management factors) factors. This was a valuable input to the erosion and soil conservation research in Ethiopia since the 1980’s. However, the available information in this regard is still a gross oversimplification of the realities in different localities. There is a need to conduct a detailed and extensive assessment of erosion hazard taking the various site-specific erosion factors into consideration. The objective of this experiment was to assess the erosion hazard in selected areas of Harerghe using the USLE as was originally described by Wischmeier and Smith (1978) as well as taking some of the recommendations of Hurni (1985) for Ethiopian conditions into considerations. The results of this study was compared with that estimated using SLEMSA to have a general comparative overview of the erosion hazard indices in the study areas. Sensitivity analysis of the input variables were also conducted to see how a change in a given factor affects the magnitude of estimated soil loss. The soil loss values estimated by these models will help the extension agents and policy makers to recognize the relative severity of erosion in a given locality and will help to prioritise and suggest appropriate soil management strategies in accordance with the level of hazard.

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Estimated soil loss at the study sites using USLE

The estimated values of the various soil loss factors and the amount of soil loss in tons per hectare per year are presented in Table 3.3.
The estimated soil loss among the study sites varied from 1.74t ha-1yr-1 at AU Alluvial to nearly 135 t ha-1yr-1 at Gelemso. High soil loss was also estimated for Karamara Adele, Hamaressa, and Babile all of which are above 50 t ha-1yr-1. Some sites including AU alluvial, AU vertisol, and Diredawa have estimated soil losses of less than 10 t ha-1yr-1. These sites are characterised by low slope gradients resulting in low value of LS (topographic factors) factors and consequently low soil loss. In general, however, 80% of the studied sites have estimated soil losses of more than 10 t ha-1yr-1 which is beyond the tolerable limits given by Smith et al. (1997) for most soils. The results indicate that all soil erosion factors are important in determining the amount of soil loss. Gelemso, where the highest estimated soil loss was recorded, is characterised by the highest rainfall erosivity factor as well as high values of other factors.

CHAPTER 1 LITERATURE REVIEW 
1.1 Soil erosion mechanisms and processes
1.2 Soil surface sealing and crusting
1.3 Effect of soil texture on sealing an erosion
1.4 Effect of slope gradient on runoff and soil loss
1.5 Effect on rainfall intensity on runoff and soil loss
1.6 Soil erosion impacts
1.7 Soil erosion models
1.8 Role of crop residue mulching on soil properties and erosion control
CHAPTER 2 ERODIBILITY ASSESSMENT OF SOME SOILS OF HARERGE, EASTERN ETHIOPIA, BY USING RAINFALL SIMULATION 
2.1 Introduction
2.2 Materials and methods
2.3 Results and discussion
2.4 Conclusion
CHAPTER 3 PREDICTION OF SOIL LOSS USING SOIL EROSION MODELS 
3.1 Introduction
3.2 Soil loss estimation using SLEMSA
3.3 Soil loss estimation using USLE
3.4 Comparison of soil loss estimated by SLEMSA and USLE
3.5 Qualitative comparison of soil erodibility indices determined in the laboratory trials and soil loss estimated using the SLEMSA and USLE models
3.6 Estimation of tolerable soil loss and soil life for the study sites
3.7 Conclusion
CHAPTER 4 EFFECT OF SOIL TEXTURE, SLOPE GRADIENT AND FAINFALL INTENSITY ON RUNOFF AND EROSION 
4.1 Introduction
4.2 Materials and methods
4.3 Results and discussion
4.4 Conclusion
CHAPTER 5 CHANGES IN SOIL ERODIBILITY UNDER SIMULATED RAINFALL AS INFLUENCED BY MULCHING RATES AND APPLICATION METHODS
5.1 Introduction
5.2 Materials and methods
5.3 Results and discussion
5.4 Conclusion
CHAPTER 6 ROLE OF MUCHING ON RUNOFF AND SOIL LOSS IN FIELD PLOTS UNDER NATURAL RAINFALL 
6.1 Introduction
6.2 Materials and methods
6.3 Results and discussion
6.4 Comparison of measured and estimated soil losses under mulching treatments
6.5 Conclusion
CHAPTER 7 GENERAL CONCLUSIONS AND RECOMMENDATIONS 
7.1 Soil erodibility
7.2 Soil loss modelling
7.3 Soil conservation
7.4 General remarks
7.5 Research needs
ACKNOWLEDGEMENTS
REFERENCES
APPENDICES

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