Soil Moisture On Rice

Distribution

Rice's center of origin is in Southeast Asia. Though rice ranks third in total production, it provides more than half of the daily dietary calories for the majority of the world's population. This is because most rice is consumed directly by humans in countries where it is produced. Generally, less than 5% of the global rice production is traded internationally. Although rice is cultivated from the equator to 55° latitude, one of the widest production zones of any crop, only 15 countries produce more than 90% of the annual total rice production. Less than 20% of total rice production lies outside of Asia (table 2.2).

The flood-irrigated lowland rice ecosystem accounts for 75% or more of the total rice production, though it occupies only 55% of the production area (IRRI, 2002). In contrast, rain-fed lowland and upland rice, which occupies about 38% of the total rice area, produces only 21% of the world's total rice production. Although there has been some success in efforts to improve the drought resistance of rice through breeding (Ingram et al., 1995), because of genetic limitations and because soils and soil management practices often restrict rice root growth, rice is arguably the most drought susceptible of the important food crops. Under tropical growth conditions rice plants have a transpiration WUE of about 0.010 Mg ha-1 mm-1 (Yambao and Ingram, 1988), and an evapotranspiration WUE about half of that, giving rice a WUE less than half of the most inefficient maize plants.

Drought in Rice Ecosystems

For the most part, irrigated rice only experiences drought when irrigation systems break down or when a regional drought restricts the amount of water available for irrigation. Though both rain-fed lowland and upland rice ecosystems suffer from frequent droughts, there are important differences between the two.

In the rain-fed lowland ecosystem, roots must be able to function under both flooded and drained soil conditions. Thus, roots of rain-fed lowland rice often have a mix of aerenchyma and non-aerenchymatous tissue (Ingram et al., 1994). Farmers generally puddle the soils of rain-fed lowland rice fields, like those of the irrigated lowlands. This puddling creates a hard soil layer about 10-15 cm below the soil surface. When these puddled soils drain, they often become very hard and develop deep cracks, thereby

Table 2.2 Global rice production and area harvested

Country

Area (%)

Production (%)

India

29.4

22.5

China

18.9

31.0

Indonesia

7.7

8.4

Bangladesh

7.2

5.9

Thailand

6.5

4.3

Vietnam

5.0

5.5

Myanmar

4.3

3.5

Others

21.0

18.9

World total

151 Mha

585 Tg

damaging roots and restricting root growth (Ingram et al., 1994). Although the roots systems of direct-seeded lowland rice plants appear to be better able to supply water to the plants during periods of drying and rewetting, many farmers continue to transplant rice to improve weed management (Ingram et al., 1995). Upland rice roots generally do not form aerenchmya and perform much like the roots of other cereal crops. Upland rice soils often have low pH and high aluminum saturation, both of which restrict rice root growth and increase susceptibility to drought. Because of frequent water and nutrient deficits, upland rice generally yields 1 Mg/ha or less, with genetic improvement coming largely through growing rice varieties with developmental rates that match the average length of the rainy season (Ingram et al., 1995).

Rice is most susceptible to drought during panicle emergence, largely because this is the crop stage at which the leaf area has reached a maximum and plants transpire the most water (Yambao and Ingram, 1988; Singh et al., 2001). Rice varieties differ in sensitivity to drought during panicle emergence, which may be explained by the ability of some varieties to store nonstructural carbohydrate in vegetative tissues before drought and remobilize those substrates to maintain seed set and growth during drought (Ingram et al., 1995).

Water Production Functions

Yambao and Ingram (1988) developed a drought stress index for rice based on growth stage and the cumulative difference between actual and potential transpiration. They found that most of the difference in growth stage sensitivity to water deficit could be explained by the differences in crop water demands. Rice is most sensitive to water deficit during panicle emergence largely because that is when potential transpiration is greatest. Building on this concept, Singh et al. (2001) tested a range of water production functions for rice. In general, those that considered differential sensitivity to water deficit at different growth stages performed better than those that did not, but results differed greatly among cultivars. As for other crops, there is a strong relationship between evapotranspiration reduction and yield reduction by drought, but these drought stress indices and production functions must be calibrated for each variety and location rather than having a general applicable relationship.

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