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A cushion of air trapped under the rice fields of Senegal

30.04.2004


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.


The rice fields, located on clayey soils, were situated above a water table lying at between 1.50 and 2 m depth. The research team quantified water input and output throughout the cropping period, about 100 days. Most of the water input to the plot was consumed by the plant and the average infiltration rate was extremely low (below 0.1 mm/day). In order to obtain a more precise water budget, measurements were made of the soil water tension and capillary pressure, the water content profiles and the water table depth. These measurements showed that the water brought in by irrigation hardly filters down at all beyond 40 cm depth below the surface. A zone exists between 40 and 50 cm depth which does not become saturated during the cropping season. Moreover, calculations of the infiltration flux confirming the measured values at plot level indicated that the water table was essentially fed by leakage from the bed of the irrigation canal and not by infiltration of water from the s! urface layer.

These results prompted the research team to suggest the presence of an isolating body of air under the rice fields. This hypothesis was rapidly confirmed using digital models. Up to now, most soil water transfer models, such as Hydrus, considered that air escaped freely and did not affect the water infiltration. This theory, valid for many situations in the field, proved not to be so in the case under investigation. A model taking the presence of air into account showed the entrapment of air contained in the soil between two wet fronts: the wetting front of water infiltrating from above and the water table deeper down. This air under compression appears to inhibit downward penetration of water in that the air has to leave the soil pores if the water is to enter and take its place.

The great quantities of water pushed into the rice fields are therefore almost completely consumed by the crop plants. The lack of drainage at depth means that there no leaching occurs and salts are not diluted. Air entrapment is probably a feature common to all irrigated rice-growing areas on clayey soils throughout the world. This phenomenon could explain the extensive salinization affecting the North-East Thailand rice fields. Investigations are soon to be conducted there to quantify the influence of air entrapment on water budgets and salt levels. Eventually, cultivation practices that can overcome poor drainage and salt accumulation could be developed.



(1) The partners involved in this research work carried out in Senegal belong to the Federal University of Pernambuco in Brazil (A.C.D. Antonino of the Department of Nuclear Energy), and to IRD (P. Boivin, C. Hammecker and J.L. Maeght from research unit UR 67)


FOR FURTHER INFORMATION

Contact: Claude Hammecker – IRD UR 067 " Les sols cultivés à fortes contraintes physico-chimiques des régions chaudes " - 300 avenue Emile Jeanbreau, 34095 Montpellier cedex ? France. Tel.: +33 (0) 4 67 14 90 28. Fax.: +33 (0) 4 67 14 90 68. Email: Claude.Hammecker@msem.univ-montp2.fr

Contact IRD Communication: Bénédicte Robert (press officer), Tel.: +33 (0)1 48 03 75 19, Email:presse@paris.ird.fr

Reference:
C. Hammecker, A.C.D. Antonino, J.L. Maeght, P. Boivin, 2003 - Experimental and numerical study of water flow in soil under irrigation in northern Senegal: evidence of entrapment, European journal of soil science, vol. 54, n°3, pages 491-503 (13).

To obtain illustrations concerning this research
Contact Indigo Base, IRD picture library, Claire Lissalde or Danièle Cavanna, Tel.: +33 (0)1 48 03 78 99, Email: indigo@paris.ird.fr

Bénédicte Robert | IRD
Further information:
http://www.ird.fr/us/actualites/fiches/2004/199.htm

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