Irrigation by surge flooding, a technique used essentially in rice cropping, involves the input of large volumes of water. In some regions, this water does not infiltrate to any depth. Poor infiltration like this can cause severe loss in soil quality and harm crops. Recent investigations on such a situation in a rice field in the River Senegal valley, involving water budget monitoring for 100 days, the length of a cropping season, have confirmed a lack of water infiltration below 40 cm depth. Scientists from the IRD and Pernambuco Federal University of Brazil jointly conducted the project. Mathematical models revealed that air, trapped in dry soil owing to the clay content, is confined and compressed between the wetting front which progresses from the surface and the water table below. The resulting cushion of air creates a blockage, slowing down then stopping the water’s filtration deeper into the soil. This investigation offers new lines of approach that might explain intense salinization of the soil that occurs in some regions of the world.
Rice cultivation uses great volumes of water, especially where the submerged-field method involving surge flooding irrigation is practised. Maintenance of a layer of water on the soil surface throughout the cropping period usually favours its infiltration deeper down. However, it has been known for many years that in some regions water often does not reach deep into the soil. This unusual feature, poorly understood up to now, becomes a problem in rice fields in arid areas because it can have harmful effects. Although from one point of view it conserves a mass of water, in that flow does not penetrate too deeply and remains entirely available for the rice to grow, it can lead to soil quality loss. Absence of infiltration lets mineral salts accumulate in the root zone, and an intense salinization sets in. That process can generate hydraulic stress which acts on the plants, limiting their growth or even killing them.
How can such a low rate of drainage be explained? An IRD team from research unit 67-ARIANE Les sols cultivés à fortes contraintes physico-chimiques des régions chaudes, working jointly with a Brazilian researcher (1), determined the water budget in rice farmers’ plots in the River Senegal valley. They employed mathematical models to define the water-flow events in the soil.
Bénédicte Robert | IRD
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