Rainbow water: new findings set to start policy dialogue
The Second Open Science meeting of the Global Land Project reviewed the latest findings on “rainbow water”, the atmospheric moisture from which rainfall derives. Scientists discussed evidence for a renewed focus on terrestrial vegetation and the special roles of trees in contributing to rainfall, says Meine van Noordwijk
The direction and distance that recycled moisture travels depends strongly on the part of the globe where trees grow. Previous ideas that all forests and all trees increase rainfall regardless of location are now replaced by findings of greater specificity. Recycling and, hence, tree cover along the pathway of air circulation, plays a clear role in the rainfall of the Western Amazon and the La Plata basin in South America, the rainfall of the Sahel region in West Africa, and in China’s primary food producing region in the Northeast.
Most water policies, and the predictions of a global water shortage, are based on the “blue water” in rivers, canals and reservoirs. But more than half of global rainfall does not become blue water, rather, it infiltrates the soil and is used by vegetation, with trees, especially deep-rooted ones, using more than other plants.
This has made fast-growing trees notorious for reducing stream flows. While the extra water-use helps to reduce floods—because it increases the water storage capacity of the soil—reduced stream flow is often not desirable for “downstream” communities. However, the additional contributions to rainbow water now balance this effect by benefits “downwind”. Whether or not downstream and downwind coincide depends on where you are on the globe.
By tracking where air masses that bring rain have been in the 10 days prior to rainfall, scientists have been able to find direct evidence that green vegetation does, indeed, increase rainfall downwind. Direct measurement of air movement and humidity, plus a simple water-balance model, suggests the relative importance of different parts of the continents as sources and sinks of rainbow water. In policy terms, the “sink” areas have a stake in maintaining or restoring tree cover in the “source” areas.
For such policy dialogues, it is important to understand the historical changes in rainfall in different parts of the world. A treasure house of such information can be found inside trees, especially older ones.
“By analyzing the rings of trees and especially the stable isotopes of carbon and oxygen deposited in the wood, we can reconstruct the past climate and even deduce the type of water that trees used”, said Dr Aster Gebrekirstos, the World Agroforestry Centre’s specialist in dendrochronology.
“The more often water has been recycled, the lower the fraction of the heavy oxygen-18 isotopes. We are currently measuring a long-term series for a site in Burkina Faso, for trees that have seen many changes in their lifetime.”
Rainfall recycling over shorter distances may well play a major role in the “water towers” of East Africa.
“We found evidence of a marked decrease in rainfall in the Rungwe Basin in Tanzania”, said Dr David Williamson, hydrologist at L’Institut de Recherche pour le Développement and the World Agroforestry Centre, “which coincides with a decrease in tree cover on the lower slopes and probably more runoff flowing into another watershed”.
At the Global Land Project Open Science meeting in Berlin, Germany, the scientists debated whether the new evidence was sufficient to draw the attention of climate-policy interest groups. In the current spirit of “early engagement”, the overall sense was that it is indeed important to start this dialogue.
Many details are yet to be clarified but early discussions in the policy arena may well help shape the debate, allow the involvement of more scientists scrutinizing the evidence, and help us from a one-way perspective of “teleconnections” towards a two-way “telecoupling”.
Open questions are:
- How does location and timing of evapotranspiration influence the likelihood of recycling as rainfall, especially as regards the start and end of rainy seasons?
- Can interactions of surface roughness, albedo, evapotranspiration on turbulence on “edges” be managed to modify triggering of rainfall, realizing the potential of rainbow water to contribute to the terrestrial water balance?
- What is the dose-response curve for tree cover in rainbow-water generation? Is partial tree cover over a larger area equivalent to forest over a smaller area?
- How can biophysical understanding of “teleconnections” lead to institutional “telecoupling”?
We’ll keep you posted on progress.
Van Noordwijk M, Namirembe S, Catacutan DC, Williamson D, Gebrekirstos A. 2014. Pricing rainbow, green, blue and grey water: tree cover and geopolitics of climatic teleconnections. Current Opinion in Environmental Sustainability 6:41–47.
The work links to the CGIAR Research Program on Forests, Trees and Agroforestry