Alemsha Bogale

GIS and remote sensing approach is an effective tool to determine the morphological characteristics of the basin. Gilgel Abay watershed is stretched between latitude 10.56° to 11.22° N and longitude 36.44° to 37.03° E which is one major contributing river of Lake Tana which is the source of Blue Nile. The present study addressed linear and areal morphometric aspect of the watershed. The study deals with emphasis on the evolution of morphometric parameters such as stream order, stream length, bifurcation ratio, drainage density, stream frequency, texture ratio, elongation ratio, circularity ratio, and form factor ratio. The morphometric analysis of the basin revealed that Gilgel Abay is firth-order drainage basin with total of 662 drainage network, of which 511 are first order, 111 are second order, 30 are third order, 9 are fourth order, and 1 is fifth-order stream. The total length of stream is longer for first order and decrease with increasing stream order. The mean bifurcation ratio is 5.16 which is greater than the standard range, and it indicates that basin is mountainous and susceptible to flooding. Low drainage density is observed which is 0.6 km⁻². It indicates that basin is highly permeable and thick vegetation cover. Areal aspect of the morphometric analysis of the basin revealed that the basin is slightly potential to flooding and soil erosion, indicating that runoff generated from the upland area of the watershed is significantly infiltrated at the gentle downstream part and contributing to groundwater potential. Further studies with the help of GIS and remote sensing with high-resolution remote sensing data integrating with ground control data in the field are more effective to formulate appropriate type of natural resource management system.

Lake Tana Basin is located in upper Blue Nile Basin which is comprises a total area of 15,096 km²of which 3063 km²is covered by the Lake which is the source of Blue Nile river. Lake Tana Basin and Blue Nile River provide various benefits also for downstream countries. The basin is highly degraded by different natural and manmade problems and it influence both Ethiopia and downstream countries. The main cause of basin degradation is inappropriate LULC. Huge area of cultivated land using without suitable management is the major basin problem. It is due to insure food security coming from unprecedented population growth rate. Forested land has encouraged the infiltration capacity and permeability of the land. It helps to increase the recharge capacity contribute to base flow whereas it is vice versa for cultivated land which is quick overland flow and significant soil erosion have observed. Besides, the soil erosion from agricultural land is the main source of nutrient enters to the lake which is the cause for eutrophication. To combat such problems both up and downstream countries should discuss together and design and implement appropriate basin management strategies to sustain the biodiversity and hydrological system of the basin.

Gully erosion has many negative impacts on both cultivated and grazing lands in Ethiopian highlands. The present study was conducted in Chentale watershed, Ethiopia, to quantify the contribution of gully erosion, and to assess its temporal changes. Within the Chentale watershed, we selected a sub-watershed (104.6 ha) and nested gully catchment, and gauged for stream flow and sediment concentration data in 2015 and 2016. We measured gully dimensions before and after the onset of the rainy season in 2016 to determine soil loss due to gully erosion. The temporal changes of gully expansion were determined by digitizing gully plain area from Google earth images taken in 2005 and 2013. The results support that gullies were expanding at higher rate in recent years. Area covered by gullies in the watershed increased from 1.84 to 3.43 ha between 2005 and 2013, indicating that the proportion of the watershed covered by gullies was nearly doubled in the investigated period. The estimated soil loss from the main watershed and gullies catchment was 6 and 2 t ha⁻¹ year⁻¹ in 2015, and was 7 and 9 t ha⁻¹ year⁻¹ in 2016, respectively. The results support that gullies were the main contributors of soil erosion in the watershed, and that integrated soil and water conservation measures are required to reduce soil erosion.