Irrigated rice is a heavy user of water. It takes about five liters of water to produce one kilogram of rice. In Asia, which contains over half of the world's estimated 253 million hectares of irrigated land, there has been a sharp decline in the rate of growth of such areas.
Scientists believe the future of rice production in Asia will depend heavily on the development of water-efficient measures to produce more rice for each unit of water used.
More than half the water expended in rice production is often used to prepare the land. And most of this is lost in the process through percolation and seepage. Rice is usually grown in clay soil, and alternate soaking and drying produces deep and wide cracks in it.
Scientists at the International Rice Research Institute in the Philippines found that in fields with porous subsoil, up to 60 percent of the water applied for soaking flows through these cracks. About 30 percent of the flow recharges the water table below, while 70 percent is lost through drainage. A major research effort at IRRI is now underway to reduce these losses.
Experiments in the Philippines have shown that shallow surface tillage after harvesting of the previous rice crop can save water during land soaking and preparation. The tilled layer decreases deep crack formation and surface soil particles block water flowing into the cracks.
The main factor in water use efficiency in a ricefield is the reduction of percolation loss. Puddling causes the formation of a semi-impermeable layer just beneath the topsoil, thus reducing water loss. IRRI studies show that even a small area of nonpuddling soil increases percolation losses by a factor of five. A further two to five-fold increase in percolation water loss results from runoff from flooded fields into the bunds and into the water table.
IRRI's work shows that maintaining a saturated soil throughout the growing season can save up to 40 percent of water in clay loam soil without a reduction in rice yields. Weed control is possible through chemical, mechanical, or manual means. Farmers prefer to flood their land continuously, however, as insurance against future water shortages, and to control weeds.
Research shows that where weed growth is a serious problem, continuous flooding up to the formation of the rice plants' panicles, and continuous saturation after that, uses 30-35 percent less water than the traditional practice of continuous flooding -- and without any increase in weeds or reduction in rice yield.
(IRRI news release)
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