Watershed prioritization for conservation planning using RUSLE and morphometric methods,Northwestern Ethiopia

Soil erosion and the subsequent sedimentation are serious environmental problems.Improper land management coupled with intense rainfall has complicated the problem in the Ethiopian highlands.Consequently,soil loss costs a profound amount of the national GDP and deters land productivity.Quantifying soil loss and prioritizing areas for conservation is imperative for proper planning and resource management.Therefore,this study aimed at estimating the mean annual soil loss rate of Gilgel Abay watershed which has critical implications on Lake Tana and Abbay River(Upper Blue Nile River),using RUSLE and morphometric analysis methods.Datasets including rainfall,soil,Digital Elevation Model and land use/land cover were used to generate important parameters required for the soil loss estimation.Data collected through field observation,consultation with experts and document analysis were used for validation purposes.The results of the study reveal that the watershed experienced an average soil loss rate of 39.8 t ha^(-1)yr^(-1).Subsequently,nearly 25%(37,038 ha)of the watershed is threatened by moderate to very severe soil loss.Among the four sub watersheds,sub watershed 3(S3)and sub watershed 4(S4),which consist of about 65%(105,000 ha)of the total area of the study watershed,experienced an annual soil loss rate of over 30 t ha^(-1)yr^(-1).The amount of soil loss that the study watershed has experienced is above the soil loss tolerance level of Ethiopia(2-18 t ha^(-1)yr^(-1))and the tropical region(10 t ha^(-1)yr^(-1)).Similarly,the results of morphometric analysis showed that the studied watershed has S-shaped hypsometric curve,an indication for maturity stage of landscape development.The studied watershed is also characterized by a non-circular shaped drainage with rugged and dissected topography that contributes to high flash flood,peak discharge and incidence of soil loss.The result of both models signified that S3 and S4 are highly susceptible to soil erosion.Therefore,these two sub watersheds need priority for soil and water conservation(SWC)measures.Furthermore,the study demonstrated the importance of the integration of different models in the identification of soil erosion prone areas and prioritization for the proper implementation of SWC measures.

Erosion risk assessment:A contribution for conservation priority area identification in the sub-basin of Lake Tana,north-western Ethiopia

Soil erosion is a serious environmental problem arising from agricultural intensification and landscape changes.Improper land management coupled with intense rainfall has intricated the problem in most parts of the Ethiopian highlands.Soil loss costs a profound amount of the national GDP.Thus,quantifying soil loss and prioritizing areas for conservation is imperative for proper planning and resource conser-vation.Therefore,this study has modeled the mean soil loss and annual sediment yield of the Gumara watershed.Landsat 5 TM,Landsat 7ETM+,and Landsat 8 OLI were used for land use land cover(LULC)change analysis.Besides these,other datasets related to rainfall,digital soil map,Digital Elevation Model,reference land use,and cover(LULC)ground truth points were used to generate parameters for modeling soil loss.The watershed was classified into five major land-use classes(water body,cultivated land,grazing land,built-up and forest and plantation)using a maximum likelihood algorithm covering a period of the last 30 years(1988-2019).The mean annual soil loss and sediment yield were quantified using RUSLE,Sediment delivery ratio(SDR),and Sediment Yields models(SY).The analysis result unveils that within the past 30 years,the watershed has undergone significant LULC changes from forest&plantation(46.33%)and grazing land to cultivated land(31.59%)with the rate of-1.42km^(2)yr^(-1) and-2.80km^(2)yr^(-1) respectively.In the same vein,the built-up area has expanded to cultivated and grazing land.Subsequently,nearly 15%(207 km2)of the watershed suffered from moderate to very severe soil loss.On average,the watershed losses 24.2t ha^(-1) yr^(-1) of soil and yields 2807.02t ha^(-1) yr^(-1) sediment.Annually,the watershed losses 385,157t ha^(-1)yr^(-1) soil from the whole study area.Among the admir-ative districts,Farta(Askuma,Giribi,Mahidere Mariam and Arigo kebeles),Fogera(Gazen Aridafofota and Gura Amba kebeles),East Este(Witimera kebele),and Dera(Gedame Eyesus and Deriana Wechit kebeles)districts which cover 50%of the watershed were found severely affected by soil erosion.Thus,to curve back this scenario,soil and water conservation practices should prioritize in the aforementioned districts of the watersheds.