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Young Zimbabwean researcher Muongeni Manda presented her findings on Harare’s threatened water supply at the International Aeronautical Congress 2024 held in October in Milan. Harnessing earth observation data from Digital Earth Africa, the Geoinformatics and Environmental Conservation postgraduate from Chinhoyi University of Technology, Zimbabwe has been able to map and predict concentrations of algal growth, providing vital insights to protect Lake Chivero and better manage ecological threats.
Lake Chivero is a reservoir on the Manyame River in Zimbabwe, situated downstream from Harare City and its surrounding settlements. It provides the main water supply for Harare and is used for irrigation, fishing, and serves as a habitat for birds. Chivero is home to a delicate ecosystem of shoreline vegetation and aquatic life. The surrounding Miombo Woodlands is home to white rhino, plains zebra, ground pangolins and numerous bird species. This delicately balanced ecosystem is prone to seasonal changes in the lake’s water levels, and the resultant water quality and salinity can have huge impacts on the water provided to the city of Harare.
The lake has faced increased ecological threats including pollution from urban growth, siltation and invasion of the fast-growing water hyacinth. These invasive aquatic plants are known to overrun bodies of water, resulting in congested waterways and the death of submerged native plant species. The subsequent decay of native plants that depletes dissolved oxygen in the water is related to fish deaths, and can have disastrous impacts on fishing communities.
”Lake Chivero’s water quality is declining due to increased levels of nutrient concentrations, including nitrogen and phosphorus, a condition known as Eutrophication. This results in algal blooms degrading the water quality and aquatic life by depleting oxygen in the water. The cause is likely to be urbanisation and agricultural activity,” says Manda.
Manda was able to use freely available Digital Earth Africa data to study the changes in water extent and quality over time, compared with rainfall data from Climate Hazards Group InfraRed Precipitation with Station (CHIRPS).
Digital Earth Africa’s Modified Normalised Difference Water Index (MNDWI) provided data on water extent from January 2017 to December 2023, in comparison with the rainfall data generated by CHIRPS, indicating drought in October 2020 and floods in early 2021, a direct result of tropical cyclones Eloise and Guambe in January and February of that year.
After measuring the water extent, water quality was estimated using the Normalised Difference Chlorophyll Index (NDCI) to observe the presence of algae and to predict chlorophyll-a concentration. High values of NDCI indicate the presence of chlorophyll-a, a pigment found in algae and cyanobacteria that plays a crucial role in photosynthesis. Chlorophyll-a is indicative of an algal bloom, which can lead to oxygen depletion and fish death.
The NDCI, implemented by Digital Earth Africa, provided a spatial comparison over six years of data from Copernicus Sentinel 2’s 13 spectral bands. While very high NDCI data was noted on Sentinel 2 imagery in March 2017, much lower NDCI data was indicated in December 2023. Variations in chlorophyll level were studied, with higher NDCI values during periods of drought in January 2018, October 2019 and November 2020.
“Extra water can dilute the salinity and alkaline levels of lakes, creating a less hospitable environment for algae – and therefore a reduced food source for flamingos,” explains Manda. “Where water extent and chlorophyll concentration have both increased, it is likely that the rainwater washed additional nutrients into the lake, strengthening algal populations.”
Manda has been supported by the Digital Earth Africa team in finetuning her research.
Dr Kenneth Mubea, Capacity Development Lead at Digital Earth Africa, says, “Manda’s findings have weighty relevance in protecting the important habitat of Lake Chivero and the water supply of Harare. They will also inform implications for climate risk management and sustainable development goals, and optimal use of earth observation data in analysing and predicting algal growth.”
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