Question

types of water scaricity

Answer 1

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In Asia, more than 50% of all water used for irrigation is for rice (Barker et al., 1999). About 55% of the rice area is irrigated and accounts for 75% of the total rice production in the world (Bouman, 2001). Tuong and Bouman (2003) estimated that by 2025, 2 Mha of Asia's irrigated dry season rice and 13 Mha of its irrigated wetland rice may experience “physical water scarcity” and the rest of the approximately 22 Mha of irrigated dry season rice in South and Southeast rice may suffer from “economic water scarcity.” Seasonal water inputs in lowland flooded rice may vary from 660 to 5280 mm depending upon soil texture and climatic conditions (Table 1). Efforts are therefore underway to develop water saving technologies such as alternate wetting and dry (AWD; Bouman and Tuong, 2001), continuous soil saturation (Borell et al., 1997), irrigation at fixed soil moisture tensions varying from 0 to 40 kPa (Sharma et al., 2002; Singh et al., 2002), or irrigation at an interval of 1–5 days after disappearance of standing water (Chaudhary, 1997). Such water management systems are partial aerobic rice systems (PARSs). True ARS is a new production system in which rice is grown under nonpuddled, nonflooded, and nonsaturated soil conditions (Bouman, 2001; Tuong and Bouman, 2003). Thus in ARS, soils are kept aerobic almost throughout the rice growing season. In addition to lesser water availability, other factors in ARS include soil mechanical impedance, increased oxygen supply to roots, accumulation of ethylene and carbon dioxide in root tissue and availability of nitrogen as nitrate in place of ammonium (the dominant N ion under flooded conditions), and a changed soil fauna (Sanchez, 1976; Vosenek and van der Veen, 1994).

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