Science - January 26, 2006

Irrigation leads to more rain

Large-scale irrigation can increase local rainfall. Alterra researchers, together with their Japanese colleagues, have calculated that thirty percent more rain will fall in southwestern Saudi Arabia if an irrigated area of 325,000 hectares is established in the area.

The principle is simple. Through irrigation more moisture is added to the air from rivers or groundwater. If this moist air can rise high enough to condense, more rain will fall in the area. By applying this principle you can create a wetter local climate in dry areas, predicted Dr Ronald Hutjes and Herbert ter Maat of Alterra and their Japanese colleagues.
The researchers modelled a 325,000-hectare irrigation area in the mountainous province Asir in southwestern Saudi Arabia. It is the mountains in this area that are primarily responsible for pushing up the moist air, Hutjes explains. ‘The principle has been demonstrated by this research,’ according to Hutjes. How successful the method will be in practice, however, will of course depend on the local conditions. The rain in Asir, for example, will most likely not fall in the irrigated area but in the mountains. Nevertheless, the researchers believe that agriculture in the area could still benefit from the extra precipitation. Hutjes expects that a larger irrigated area will work even better.
The researchers presented their research last Monday at the large, international iLEAPS congress on the relation between ecosystems and the atmosphere held in Boulder, Coloroado, in the United States. It will later be published in Global and Planetary Change. / MW

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