How full are southern Africa’s city dams? Nine cities’ reservoirs, measured from orbit

Every city in southern Africa lives on a few reservoirs, and almost nobody outside the water utilities can say how full they are. Utilities publish levels when they choose to, in different formats, and often only when there is a crisis. A bank lending to a brewery, an investor weighing a hotel, a developer planning a housing estate or a council planning its budget usually has to take the water supply on trust.

So we measured it. A satellite passes over every one of these reservoirs every few days, and open water looks very different from dry land. We mapped 23 reservoirs that supply nine SADC cities, Harare, Bulawayo, Gaborone, Windhoek, Lilongwe, Maputo, Cape Town, Durban and Maseru, and measured the area of water in each of them on every clear pass since January 2017: more than 9,000 measurements in all.

The record answers three questions a business would ask before committing to a city: how full its reservoirs are now, how low they have gone in the past decade and how often, and whether the storage has kept up with the city around it.

The answers differ sharply from city to city, and this year the weakest position is not where most people would look first.