Monsoon rains on the Kenyan savannah: researchers have developed a new model to track the path of water, from where it falls as rain, back through the atmosphere to its origin as evaporation. Photo: O. Henriksson/Azote

What goes up must come down

New method tracks path of water, from where it starts as evaporation to where it falls as rain

Over 500 million people globally live in dry regions and rely solely on rainfall to grow their food. Any changes to this rainfall can have dramatic consequences for their livelihoods.

An increasing amount of research shows that human-caused changes to landscapes can significantly impact the amount of water that is evaporated into the atmosphere.

Travelling backwards through the atmosphere
In a study recently published in Biogeosciences, centre researcher Line Gordon has together with researchers from The Netherlands and Germany developed a method to locate the region from which evaporated water starts its travel through the atmosphere, and where it later falls as rain or snow.

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The method is based on a computer model that backwards track the path of the water, from where it falls as rain, back through the atmosphere to its origin as evaporation.

Gordon and her colleagues call the evaporation area a precipitationshed. This can be understood as the area upwind of a specific location that contributes to most of the rainfall. In other words, the precipitationshed can be understood as a 'watershed of the sky', where evaporated water flows through the atmosphere to a specific location, falling out as rain.

Seven rainfed regions identified
The researchers identified seven regions that are dependent on rainfed agriculture, and that receive most of their rainfall from elsewhere. These seven regions were located in northeast China, the border between Pakistan and India, the Western and Eastern Sahel, southern Africa, and part of Argentina.

The researchers then located the upwind precipitationshed for each of these seven regions. Once this was done, they were in a position to describe in detail the characteristics of each precipitationshed, including types of land use, the population and potential future land use changes.

"Ultimately, this knowledge is valuable to people who are dependent on rain for their livelihoods, such as poor farmers in dry areas like the Sahel in Africa, or northeast China," says Line Gordon.

The baseline for new management efforts
The study can prove to be the start of new regional administrative units which can coordinate regional land use activities and data sharing so that residents throughout the region do not negatively impact one another.

"The precipitationshed concept represents an important step toward bridging science and policy," says lead author Patrick Keys.

"It provides the baseline for establishing management efforts between areas where people are changing evaporation through land use, and the impacts those changes have on areas where people rely on the rainfall."

Source: Keys, P.W., van der Ent, R.J., Gordon, L.J., Hoff, H., Nikoli, R., and Savenije, H.H.G. (2012). Analyzing precipitationsheds to understand the vulnerability of rainfall dependent regions.  Biogeosciences, 9, 733-746.  doi:10.5194/bg-9-733-2012

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Reference

Citation

Keys, P.W., van der Ent, R.J., Gordon, L.J., Hoff, H., Nikoli, R., and Savenije, H.H.G. (2012). Analyzing precipitationsheds to understand the vulnerability of rainfall dependent regions. Biogeosciences, 9, 733-746. doi:10.5194/bg-9-733-2012

Line Gordon's research centres around interactions among freshwater resources, ecosystem services and food production, with a focus on how resilience thinking can enable better management of these resources. Her work ranges from the global to the local (mainly sub-Saharan Africa) scale.

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