Surface water in Dr Ruth Segomotsi Mompati

Open water seen on the ground from space: dams, lakes, rivers wide enough to see, pans, wetlands with open water and flooded fields. Each year, the Landsat satellites look at every 30 m spot many times; where the sky is clear, each look says water or not. The average water area counts each spot for the share of its clear looks that were wet. Permanent water was wet in more than 80% of the clear looks; seasonal water was wet at least once but less often than that.

What it isn’t: not how full a dam is (that is the Department of Water and Sanitation’s weekly level, by volume), not groundwater, not rain, and not water in pipes or tanks. Narrow rivers, water under trees or bridges and anything smaller than about 30 m are missed. A year with few clear looks (cloud, or the early years when fewer satellites flew) is less sure, and 1988 is missing everywhere because Digital Earth Africa does not have it: a missing year is left out, never counted as dry.