Aquatic Ecology and Water Quality Management Group, Department of Environmental Sciences, Wageningen University &Research, P.O. Box 47, 6700 AA Wageningen, the Netherlands; College of Agriculture and Environmental Sciences, Blue Nile Water Institute, Bahir Dar University, P.O. Box 1701, Bahir Dar, Ethiopia.
Water Systems and Global Change Group, Department of Environmental Sciences, Wageningen University & Research, P.O. Box 47, 6700 AA Wageningen, the Netherlands.
Sci Total Environ. 2020 Aug 20;731:139199. doi: 10.1016/j.scitotenv.2020.139199. Epub 2020 May 6.
Rivers are exporting increasing amounts of nitrogen (N) to lakes, which is leading to eutrophication. However, the seasonality apparent in nutrient loading, especially in tropical areas, is thus far only partially understood. This study aims to better understand the seasonality and the sources of dissolved inorganic nitrogen (DIN) inputs from sub-basins to tropical lakes. We integrated existing approaches into a seasonal model that accounts for seasonality in human activities, meteorology and hydrology, and we applied the model to the sub-basins of a representative tropical lake: Lake Tana, Ethiopia. The model quantifies the river export of DIN by season, source and sub-basin and also accounts for open defecation to land as a diffuse source of N in rivers. Seasonality parameters were calibrated, and model outputs were validated against measured nitrogen loads in the main river outlets. The calibrated model showed good agreement with the measured nitrogen loads at the outflow of the main rivers. The model distinguishes four seasons: rainy (July-September), post-rainy (October-December), dry (January-March) and pre-rainy (April-June). The river export of DIN to Lake Tana was about 9 kton in 2017 and showed spatial and temporal variability: It was highest in the rainy and lowest in the dry seasons. Diffuse sources from agriculture were important contributors of DIN to rivers in 2017, and animal manure was the dominant source in all seasons. Our seasonal sub-basins and rivers model provides opportunities to identify the main nutrient sources to the lake and to formulate effective water quality management options. An example is nutrient application level that correspond to the crop needs in the sub-basins. Furthermore, our model can be used to analyse future trends and serves as an example for other large tropical lakes experiencing eutrophication.
河流向湖泊输送的氮 (N) 不断增加，导致湖泊富营养化。然而，目前人们对营养物负荷的季节性，尤其是在热带地区，只有部分了解。本研究旨在更好地了解热带湖泊子流域中溶解无机氮 (DIN) 输入的季节性和来源。我们将现有的方法整合到一个季节性模型中，该模型考虑了人类活动、气象和水文学的季节性，并将该模型应用于一个具有代表性的热带湖泊——埃塞俄比亚塔纳湖的子流域。该模型按季节、来源和子流域量化了 DIN 的河流输出，并考虑了将粪便露天排入河流作为 N 的一个扩散源。我们对季节性参数进行了校准，并根据主要河流出口处的实测氮负荷对模型输出进行了验证。校准后的模型与主要河流出口处的实测氮负荷吻合较好。该模型区分了四个季节：雨季（7 月至 9 月）、雨后季（10 月至 12 月）、旱季（1 月至 3 月）和雨前季（4 月至 6 月）。2017 年，DIN 向塔纳湖的河流输送量约为 9 千吨，表现出空间和时间上的变化：雨季最高，旱季最低。2017 年，农业的扩散源是河流 DIN 的重要贡献者，在所有季节中，动物粪便都是主要来源。我们的季节性子流域和河流模型为识别湖泊的主要营养物来源和制定有效的水质管理方案提供了机会。例如，根据子流域的作物需求，确定养分的应用水平。此外，我们的模型可用于分析未来的趋势，并为其他面临富营养化的大型热带湖泊提供范例。
