INTEGRATED WATERSHED INTERVENTIONS IMPACT ON INSTITUTIONAL, BIOPHYSICAL AND HYDROLOGICAL ISSUES IN NORTHWESTERN HIGHLANDS OF ETHIOPA

Abstract

Sustaining watersheds developed through community mobilization has been a major challenge in Ethiopia. This paper seeks to investigate the role of institutional functionality on the biophysical and hydrological performance of watersheds in northwestern highlands. For institutional analysis, a semi-qualitative research methodology was used. Based on the new institutional economic theory, twelve effective institutional indicators were devised for the evaluation. Data was analyzed using Mann-Whitney test. To develop allometric model for above ground biomass and carbon stock estimation, 42 trees harvested and weighed from six dominant species of exclosure site and adjacent communal grazing land. Household surveys and key informant interviews were used to assess the perception of local communities on exclosure management. QSWAT model was used to simulate the hydrological process and sediment yield and successfully calibrated and validated for flow. Sequential uncertainty fitting, was used to calibrate and validate the simulation against measured river discharge. Based on the finding of institutional analysis significant differences were observed between the two watershed groups in relation to good interaction (p=0.03) and technical rationality (p=0.04). Most of the institutional characteristics and rationalities which led for better performance in the case of the Model watershed were lacking in Community watersheds. The result from biomass study showed that above ground biomass correlates significantly with diameter at stump height (R2 = 0.78; P<0.01), and tree height R2 = 0.42, P<0.01). Our final choice for regression analysis was Height-Diameter mixed-effects model (M6), which explained 81% of the variations in above ground biomass. The model developed in this study allow monitoring changes in C-stocks and C-sequestration following the establishments of restoration practices in north western Ethiopian highlands. The C content of the total biomass for the exclosure and the grazing land were estimated as, 5.2 and 3.6 Mg C ha-1, respectively. For impact studies, model simulation results indicated that all intervention measures significantly affect hydrological behavior and sediment yield at the catchment outlet. The scenario assessment showed that the application of contour farming; soil bund and fanya juu; crop residue reduce sediment yield by 24.5%, 62 % and 41.3 %, respectively. In addition, contour farming, soil bund and fanya juu and residue management decreased the surface runoff by 16%, 47%, and 25.5% respectively. For the given agro-climate and farming system, all watershed interventions investigated are able to result a positive impact in altering reducing peak flow and extending the groundwater flow, and reducing sediment yield for the catchment.