Non‐Point Source Pollution of Dissolved Phosphorus in the Ethiopian Highlands: The Awramba Watershed Near Lake Tana

Abstract

Intensifying agriculture in Africa is degrading the water quality of rivers and lakes, thereby, threatening the sustainable use of water resources. In Lake Tana in the Ethiopian highlands, the first signs of eutrophication were recently observed. Since relatively little is known about the non-point source pollution in sub-Saharan Africa, the objective was to examine non-point sources phosphorus contribution of an agricultural watershed near Lake Tana. The 7-km 2 Awramba watershed was selected.Dissolved phosphorus concentration (DPC) in groundwater and soil-available phosphorus was measured at three landscape positions and discharge, sediment, and DPC at the outlet. The results indicated that DPC in groundwater was most elevated in the periodically saturated valley bottoms while available P was the greatest in the cropped fields at mid-slope. The DPC increased with discharge. During base flow the concentration in the stream was similar to that in ground water. The results were consistent with other hydrological watershed studies in which the major source of runoff, erosion, and phosphorus originated from valley bottom lands. Hence, converting the periodically saturated areas in the stream corridor to buffer zones with reduced P input can be an important tool in reducing P from rural watersheds in the Ethiopian highlands.

Modeling discharge and sediment concentrations after landscape interventions in a humid monsoon climate: The Anjeni watershed in the highlands of Ethiopia

“…While it is well known that the relationship between rainfall, runoff, and sediment is highly nonlinear (de Vente & Poesen, ; de Vente et al, ; Nyssen et al, ; Parsons, Brazier, Wainwright, & Powell, ), it is expected that on this catchment scale (1 km 2 ), the net effect of increasing erosion processes, connectivity, and sediment storage will lead to increased sediment transport (de Vente & Poesen, ). The assumption that runoff and erosion can increase proportionally is supported by the experimental work of Hairsine and Rose () and the regional assessment in Gebremicael et al () as well as rating curve studies (Guzman et al, ; Moges et al, ; Vanmaercke et al, ), though seasonality and scale play a role. Above 10 km 2 , the effect of sediment sinks, and (re)deposition within the landscape often becomes more dominant than sediment sources resulting in gradually declining sediment yield for increasing area (de Vente & Poesen, ).…”

“…However, these types of models are not without problems either. Water balance-type models do not always perform well at a daily timescale, and different parameter sets are required for different basin sizes (Kim & Kaluarachchi, 2008 The PED approach was selected because (a) relatively few adjustable parameters are needed (nine for the hydrology and four for the erosion model; Table 1), (b) its representation of process-based mechanisms for (sub) humid, semi-monsoonal hydrology, and (c) daily discharge and sediment concentrations are predicted with similar or better Nash Sutcliffe efficiencies (NSE; Nash & Sutcliffe, 1970) as other more complicated models Guzman et al, 2017;Moges et al, 2016;Tesemma et al, 2010;Tilahun, Guzman, et al, 2013a;Zimale et al, 2016, forthcoming). Although a high NSE does not necessarily mean a valid model (de Vente et al, 2013;Schaefli & Gupta, 2007), the model predicts consistently close to observational data.…”

Groundwater Quality in an Upland Agricultural Watershed in the Sub-Humid Ethiopian Highlands

“…Monitoring and detailed case studies of groundwater quality can provide an early warning system before cleanup is needed. Therefore, in this study, we investigated the spatial and temporal variation of shallow groundwater quality in the Tikur-Wuha watershed, Lake Tana basin where the first signs of eutrophication have been noted [16]. The Tikur-Wuha watershed was selected because it is typical for other watersheds in the highlands with intensive agriculture and the availability of shallow groundwater wells that provide water for household consumption.…”

A Drivers-Pressure-State-Impact-Responses Framework to Support the Sustainability of Fish and Fisheries in Lake Tana, Ethiopia

“…According to Goshu [63], fecal coliforms and Escherichia coli concentrations significantly increased and analysis of Biological Oxygen Demand (BOD5) indicated organic pollution [64] in the Gulf of Bahir Dar. Additionally, a high concentration of the cyanobacterial genus Microcystis [65] leading to eutrophication [66] was observed in the Gulf of Bahir Dar. According to a discussion with local fishers, the taste of the flesh of O. niloticus, which primary feeds on algae, seemed to have changed noticeably in 2012.…”