A three-year drought is threatening to cause city officials in Cape Town, South Africa, to shut off the tap water. The growing city of nearly 4 million relies mostly on reservoirs for its water supply. While drought is part of the cause of the current crisis, an increasing population is also straining water resources.

The view from data acquired by Landsat 8 brings the crisis in Cape Town into focus. The January 2014 image shows the largest reservoir in its water supply system, Theewaterskloof, at full capacity. In January 2018, it is easy to see how much the surface area of the lake has diminished after three years of drought. The Western Cape Government lists the reservoir’s water capacity at just 13 percent.

Smaller reservoirs do not appear to change much compared to Theewaterskloof, but Cape Town only gets a small fraction of its water from them.

In 2015, rainfall in the area measured 325 mm (12.8 inches)—normal annual rainfall there is 515 mm (20.3 inches). After that, the drought only got worse. Rainfall totaled just 157 mm (6.2 inches) in 2017. Rainfall this low for three straight years is extremely rare. Even with water use restrictions, the city could run out of water by mid-April 2018.

If the rainy season, which is from April to September, brings adequate rainfall, then Landsat can help monitor the extent of the reservoir as it begins to refill.