As high-income countries around the world increase environmental protections, ecosystem services are deteriorating where most needed - people in low- and middle-income countries rely heavily on ecosystem services. In Africa, human activity is changing water use, as land is put under cultivation and dams developed for energy and irrigation. Most often, ecosystem health is ignored in the course of development. For example, Lake Chad once sustained the livelihoods of people in eight countries, but fifty years of unchecked development reduced Lake Chad from 26,000 to 1,500 square kilometers with associated decline in ecosystem services.
Lake Tana: Ethiopia’s Largest Freshwater Body
The Lake Chad story serves as a warning for Lake Tana, the largest freshwater body in the Ethiopian highlands and second most populous area in Africa. The population has increased fivefold in 60 years. Conversion of forests and bush land to agricultural use has increased soil erosion. Continuous tillage and increased crop cultivation has depleted soil organic matter and further degraded soils (Tebebu et al., 2016). Agricultural practices have created a hardpan that altered the hydrological processes, reducing deep percolation and increasing direct runoff, soil erosion, and loss of nutrients (Fenta et al., 2022). Agricultural expansion may spur economic growth but also puts other ecosystem services at risk.
UNESCO registered the 3000km2 Lake Tana as a biosphere reserve, recognizing its valued ecosystem and cultural importance. Lake Tana provides the full range of ecosystem services: water supply and hydrological regulation of Blue Nile River, wind control, support for biodiversity and protection of river water quality by serving as a sediment sink. It supports 27 fish species (20 endemic), 215 bird species, surrounding wetlands with papyrus reed, and 37 islands with human settlements, monasteries and churches serving as a guard for culture over centuries. (Vijverberg et al., 2009). Currently, the risks to these ecosystem services require solutions and actions.
Since 1989, cultivated land in the Lake Tana area increased by 20%, now covering 68% of the Tana basin (Getachew and Manjunatha, 2022). Annual soil erosion ranges from 5 to 50 tons per hectare – a doubling of the sediment transport from 1980s to 2020s (Kebedew et al., 2021). Water use increased with expanded irrigated land area from 540km2 in 1980s to 1200km2 in 2020s (Abera et al., 2021). Moreover, phosphorus concentration in the lake increased from 0.01 mg P/l in 2003 to near 1.8 mg P/l in 2020, and nitrogen concentration increased from near zero to 2 mg total N/l after 2016 (Dersseh et al., 2022). Expanded agro-chemical use and uncontrolled water use cause water flow to the lake to evaporate at the end of the dry season, allowing for growth of invasive water hyacinth.
Restorative measures need to be designed for the specific location and climate, not applied as blanket actions. Bahir Dar University (BDU) research provides insight into potential actions to improve the ecological services in the highlands.
Ethiopia’s hydrology is based on a monsoon climate that creates distinct challenges for designing measures to improve ecological services. Rainfall during the wettest part of the monsoon period can be the same as the annual rainfall in a whole year in a temperate region. In addition, parts of the Ethiopian highlands consist of volcanic soils with geological faults in which a large portion of the water can flow in the subsurface and appear in springs downstream. Geological variability creates watersheds where the runoff coefficient is low and other watersheds that are high. Figure 1 shows that the runoff coefficient is higher in Group 2, indicating a higher need for watershed management investments than Group 1. All these factors need to be considered in landscape management.
Figure 1: Measuring runoff coefficient, percent of rainfall, and watershed area size and watershed types (Adem et al., 2020).
BDU’s research shows that runoff in the landscape is generated in areas saturated during rainfall events, in both valley bottoms and hillsides with hardpan. However, restorative practices have often been designed for temperate regions and assume runoff occurs when rainfall intensity exceeds the infiltration capacity. A direct conclusion is that any practices in the upland to improve ecological services downstream should also be directed to the bottomlands and only hillsides with hardpan.
Figure 2: Plantation of indigenous trees in the hillside of Lake Tana contributing areas
The Ethiopian government has invested in reforestation through the Green Legacy Program since 2019. However, the program prescribes blanket action for all types of landscapes. The program emphasizes planting indigenous trees on all hillslopes (Figure 2). For hillsides where hardpan forms, planting trees helps to break the pan and allows water to infiltrate below the surface. Yet, the hillside focus may not be appropriate near Lake Tana, because valley bottoms also need trees to transpire excess water. Bahir Dar’s research shows the program needs more consideration on the type of trees and where to plant to yield the most positive impact. Once the trees establish, the leaf fall and root turnover can then improve organic matter and restore the hydrologic path to previous hydrologic conditions.
Figure 3: BDU papyrus plantation around the lake shores to recover wetlands
BDU has also begun to rehabilitate wetlands by creating buffering zones (Figure 3). Lake Tana’s shoreline was once covered with wetlands and papyrus plants but is being lost to urban development and agriculture. The BDU initiative is buffering the shoreline restoring papyrus reeds to rehabilitate the wetlands and remove nitrates, while allowing sediment and phosphorus to sink before it can enter Lake Tana. BDU has planted over 10,000 papyrus reeds as a pilot effort. While offering a technical fix, the full solution will require farmers to willingly give up land for such initiatives.
To save the lake from ecological risks, more actions are needed to reduce sediment and nutrient input from the rivers into the lake. Though fertilizer use in Ethiopia is relatively low, the high rainfall leaches nutrients from cropland areas and valley bottoms where livestock freely graze, then transports them to Lake Tana. New York City watershed management best practices are instructive, such as crop rotation, nutrient management, and livestock management (Hoang et al., 2019). Likewise, application of fertilizers should be managed to increase crop uptake, thus reducing the concentrations in the runoff to Lake Tana. But more research is needed on adapting such practices to a monsoon climate where a large portion of the watershed saturates.
For the future, a Payment for Ecosystem Services (PES) scheme could offer a long-term, holistic solution to manage and sustainably use Lake Tana’s ecosystem services. To achieve this, ecosystem service users and ecosystem service providers need to be identified and quantified to establish who should pay and be paid. This approach could benefit the surrounding smallholder farmers, as they are the main service providers and may incentivize cooperation. The value of economic services needs to be determined using contingent and economic methods to outline the amount that farmers would be paid and broader benefits. In this regard, progress is being made through a partnership for a joint sustainable future of the Lake Tana Watershed between BDU and the Nature and Biodiversity Conservation Union (NABO). While balancing economic growth and ecosystem services will remain a challenge, the risks are too high to ignore.
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Opinions expressed in this essay are those of the authors. They do not purport to reflect the opinions or views of the GCSE or its members.