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Evapotranspiration studies could help keep Africa’s Sahel green

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Young boys working in a newly cropped field in Africa.

USGS geographer Michael Marshall has been studying the effects of evaporative processes on land – water emitted into the atmosphere from the soil and from plants – in East Africa and in the Sahel belt that crosses north-central Africa. Using output from a land surface model, Marshall found substantial drying over much of the Sahel and East Africa during the growing seasons.

“The Sahel is getting drier, and the land process that buffers the decline in rainfall is breaking down under increased warming,” said Marshall, a postdoctoral scholar with the USGS Western Geographic Science Center based in Flagstaff, Ariz. Marshall studies evapotranspiration – the combined effects of water entering the atmosphere via evaporation from the soil or from a plant surface, as well as transpiration, in which plants open their stomata (pores) to take in carbon from the atmosphere for food.

Most research into the effect of evaporation on global climate change deals with the oceans, since they form more than 75 percent of the earth’s surface. But evaporative processes on land are also important, because these processes can more easily be influenced by human activity and because, in some places, they can have a local effect on climate. Evapotranspiration triggers convection – the movement of columns of air – and rainfall. In the Sahel, evapotranspiration and localized convection are especially important because of the distance from the ocean or other large water bodies.

Using remote sensing satellite data and surface reanalysis data, downscaled from global general-circulation models, Marshall and his team analyzed evaporation in Africa over a 31-year period. In contrast to many Intergovernmental Panel on Climate Change (IPCC) simulations, which predict increased moisture over the Sahel, they identified substantial drying over much of the Sahel during the growing season, particularly in the last 10 to 12 years. To explain the discrepancy, Marshall suggested that the negative impacts of warming temperatures may be overcoming any possible rainfall recovery across the Sahel. He hypothesized that lands converted from natural vegetation to agriculture might have contributed to the change.

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A herder moves cattle through a barren landscape in eastern Africa.

“The climate models for Africa tend to be uncertain in general, due to a lack of station data in Africa and the complexity of climate in the tropics. There is still much to be learned,” Marshall said.

Marshall suggested that the new agricultural crops, which have smaller root systems and are in the ground for a shorter time than the native perennials, may be so stressed by their harsh conditions that they are releasing less water into the atmosphere and thus affecting the climate. Very few African farmers have access to irrigation. Thus, any disruption of the process that creates convection and rainfall would greatly affect them in the long term.

Marshall aims in further research to use higher-resolution data to compare satellite and ground information from Africa with data from the central United States and from India, where the movement of air produced by evapotranspiration can also induce rainfall.  He hopes his work will give land managers tools to better understand these regions.

Marshall’s research supports the Famine Early Warning Systems Network (FEWS NET), which helps target more than $1.5 billion of food-related assistance to more than 40 countries each year. The USGS is actively involved in FEWS NET, which is sponsored by the U.S. Agency for International Development (USAID) Office of Food for Peace. FEWS NET examines the populations of the developing world with the most food insecurity, identifying critical situations in which food aid will be needed. Often the populations most in need are those whose livelihoods are tied to subsistence rain-fed agriculture and pastoralism.

Marshall’s findings, published in a recent issue of Climate Dynamics, are available online.

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