The Okavango Delta in northern Botswana is one of the world’s largest inland deltas. It is known for its annual flooding, which happens between February and May as a wave of water from seasonal rainfall traverses about 20,000 square kilometers of wetlands. But just as water makes a regular appearance in this part of the Kalahari Desert, so too does fire.

The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on NASA’s Aqua and Terra satellites acquired this series of images between April 28 and May 23, 2018. The images were composed from a combination of visible and shortwave infrared light (MODIS bands 7-2-1). The burn scar appears dark brown; vegetation is bright green; bare ground is light brown; and water is dark blue.

Notice how water appears to be moving from the areas of permanent swamp and filling the fingers of the so-called seasonal swamp. “The annual flood pulse is reaching the distal fringes of the delta about now,” said Michael Murray-Hudson, a wetlands ecologist at the University of Botswana’s Okavango Research Institute. At the same time, a slow-moving fire front (bright orange) is advancing toward the southeast, leaving a dark brown burn scar in its wake.

Also notice how the path of the fire appears to follow the path of the floodplain. Channels inundated with floodwater can generate a huge amount of vegetation that is prone to burning. But there is a sweet spot: researchers have shown that floodplains inundated with water on an intermediate basis—about every other year—have the highest potential to burn.

While the floodwaters help to generate the fuel needed for burning, the fires ultimately have a human origin. “Almost all of the fires are anthropogenic,” Murray-Hudson said. “People set them when they can, for example, when the landscape will carry a fire. It’s a pretty normal phenomenon, although the extent and frequency might be increasing as the human ecological footprint in the delta grows.”

Previous research suggests that fires can affect the ecosystem by changing the quality of floodplain water and by removing aquatic shelter for young, vulnerable fish. But the authors of that paper point out: “The amount of seasonal flooding has a larger ecosystem impact than fires and is the primary factor in the wetland’s productivity.”

NASA Earth Observatory image by Joshua Stevens, using MODIS data from LANCE/EOSDIS Rapid Response. Story by Kathryn Hansen.