Evaluating the effect of diffuse and point source nutrient transfers on water quality in the Kombolcha River Basin, an industrializing Ethiopian catchment

Abstract

Many catchments in sub‐Saharan Africa are subject to multiple pressures, and addressing only point sources from industry does not resolve more widespread diffuse pollution from sediment and nutrient loads. This paper reports on a preliminary study of nutrient transfers into rivers in two catchments in the industrializing city of Kombolcha, North Central Ethiopia. Sampling of rivers and industrial effluents was done over two sampling periods in the wet season of 2013 and 2014. Catchments boundaries and land use map were generated from remote sensing and ground data. Higher total nitrogen (TN) concentrations were found from sub‐catchments with largest agricultural land use, whereas highest total phosphorus (TP) was associated with sub‐catchments with hilly landscapes and forest lands. Emissions from brewery and meat processing were rich in nutrients (median TN: 21–44 mg L−1; TP: 20–58 mg L−1) but contributed on average only 10% (range 4–80%) of the TN and 13% (range 3–25%) of the TP loads. Nutrient concentrations in the rivers exceeded environmental quality standards for aquatic life protection, irrigation, and livestock water supply. In Ethiopia, more than 85% of farmers operate on less than 2 ha of land, with concomitant pressure for more intensive farming. Land is exclusively owned by the State, reducing a sense of land stewardship. As the City of Kombolcha moves to agricultural intensification and increased industrialization, attention is needed to fill gaps in monitoring of nutrient pollution in rivers and use information to reconcile development with land use and its degradation.

Estimating combined loads of diffuse and point-source pollutants into the Borkena River, Ethiopia

“…export coefficient of each land-use category) that calibrate the model with lowest sum of error percentages. Other dependencies factors affecting export of TN and TP are hard to apply, as related data such as soils, hydrology and topography are scant and problematic to measure in the areas (Zinabu et al, 2018).…”

“…In comparison with the errors in the calibration process, the model validation for TN resulted in lower sum of errors (Table 5.5.d), but this could be consequential on higher discharge during the wet season (i.e. June to September, 2014) from which the validation data set is derived (Zinabu et al, 2018).…”

“…Kombolcha, in the North central part of Ethiopia, is bordered by moderately intensive agriculture, grasslands and natural and plantation forest lands in the north-west and south-west, and industrial areas in the center and north-east (Zinabu et al, 2018). The semi-arid climate has a rainy season from mid-June to the end of September.…”

“…(Ding et al, 2010;Wood and Beckwith, 2008;Yetunde, 2006;Lin, 2004;EPA, 2001). The land use and topography of Kombolcha, comprises areas of forest, grassland and crops on hill sides, mountainous areas and relatively flat lands (Zinabu et al, 2018). Use of applicable coefficients were further informed from consultations with the local Bureau of the Water and Agricultural Office on soil, topography, imperviousness, and vegetation cover.…”

“…Use of applicable coefficients were further informed from consultations with the local Bureau of the Water and Agricultural Office on soil, topography, imperviousness, and vegetation cover. Details of the methods used to measure water flows in the river are reported by Zinabu et al (2018). Nutrient concentrations in the Leyole and Worka rivers were measured in two monitoring campaigns when there was sufficient water flowing from 15 June to 30 September in both 2013 (C1) and 2014 (C2).…”