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CH4 emission, but not necessarily brings the minimum GWP depending on other management practices.

14.3.3 Alternate Wetting and Drying 14.3.3.1 Role for Saving Irrigation Water

Agriculture accounts for 70% of the global water use (FAO 2006). Worldwide, water for agriculture is becoming increasingly scarce. Because of the competition with industry and domestic usage, it will be more difficult to secure a stable and adequate supply of irrigation water for paddy fields. To sustain a growing and rice-eating population in Asian countries, it is necessary to establish field management that will ensure current or higher levels of rice productivity with higher efficiency of water use without detrimental environmental impacts.

Several methods of water-saving irrigation have been developed according to the water availability, such as AWD (Bouman and Tuong 2001; Belder et al. 2004), continuous soil saturation (Borrell et al. 1997), and aerobic rice (Bouman et al. 2005; Xiaoguang et al. 2005) (Fig. 14.6). These methods reduce water inputs and increase the water productivity (rice yield/water use). Among them, AWD is suggested by the International Rice Research Institute (IRRI) to farmers, with a saving of about 15-30% of irrigation water input without reducing rice yield (Bouman et al. 2007).

FC S

FC S

Soil condition

Fig. 14.6 Schematic presentation of yield responses to water availability and soil conditions in different rice production systems and their respective technologies to reduce water inputs (Tuong et al. 2005). SSC - saturated soil culture, FC - field capacity, S - saturation point, AY - change in yield

14.3.3.2 Method of AWD

The early stages of AWD were conducted as the several-day intervals of flooding and drainage (Tabbal et al. 2002; Belder et al. 2004). It is similar to the intermittent irrigation. The current method of AWD commonly practiced by farmers is based on the groundwater depth and/or dry conditions of surface soil. On the other hand, at the experimental level, AWD is conducted on the basis of soil moisture tension, an objective indicator for water content in the soil.

Soil moisture tension is measured by a tensiometer usually inserted to a depth of 15 cm. Figure 14.7 shows the method of water management for AWD. When the moisture tension decreases to the pre-determined criterion by drainage, the field is flooded to about 5 cm of surface water depth. The repetition of this routine is the method of AWD. The criterion is variable, and its determination depends on the water availability, soil type, target rice yield, etc. Similar to the conventional water management in the temperate region, the field is flooded to keep sound growth just after the transplanting and during the heading stage.

Fig. 14.7 Schematic diagram of the AWD. The -20 and

-70 kPa are examples of the wet criterion for soil moisture tension. Horizontal arrow indicates the forced flooded period

Continuous flooding---20 kP a--70 kP a

Continuous flooding---20 kP a--70 kP a

Fig. 14.7 Schematic diagram of the AWD. The -20 and

-70 kPa are examples of the wet criterion for soil moisture tension. Horizontal arrow indicates the forced flooded period

Time passage

14.3.3.3 Effects on CH4 Emission

Drastic changes in water use by AWD will affect various aspects of paddy-field environment including GHG emission. Hosen (2007) reported the effects of AWD on CH4 emission, using two criteria (-20 and -70kPa). At the IRRI field in the Philippines, -20 kPa is used as a limit not to inhibit rice growth, which was derived from the previous studies (Hira et al. 2002). The -70 kPa is expediently set as a fairly severe limit for rice growth that would reduce rice yield. As compared to continuous flooding, the AWD, using the criterion of -20 kPa, decreased more than 70% of CH4 emission during the dry-season growing period under the conditions of receiving fresh straw (4tDWha-1). On the other hand, the AWD using -70 kPa decreased CH4 emission by nearly 90% under the same conditions.

The AWD has a high potential to decrease CH4 emission from irrigated paddy fields. However, in the AWD, the priority order of mitigating GHG emission seems to have been lower than water saving and increase in the water productivity. Further studies are necessary to evaluate the effects of AWD on the GHG emission.

14.4 Eh Control, Water Management Based on Soil Eh 14.4.1 Background for Development and Its Implications

In the temperate region, the timing and duration of mid-season drainage and intermittent irrigation depend on farmer's empirical knowledge and customary practices. However, these subjective decisions cannot cope with unusual weather conditions, such as cool temperature and intermittent rainfall, causing insufficient drainage and thus careless CH4 emission. Moreover, CH4 emission would occur before the pre-determined first drainage. These phenomena have a possibility to increase the uncertainty of the decreasing effects of water management on CH4 emission.

Therefore, there is a room to improve the conventional water management to positively decrease CH4 emission. One of the most useful indicators for predicting CH4 emission from a paddy field is soil Eh. In order to positively decrease CH4 emission without reducing rice yield, Minamikawa and Sakai (2005) have proposed the water management based on soil Eh, which named 'Eh control'.

Originally, water management is practiced to obtain suitable rice yield in various climate regions, but other aspects that must be considered are arising with the climate change. How is the new water management conducted? The problem is how to drain and flood, and thus it is necessary to establish a certain criterion to conduct the water management. One probable way to solve this problem is the use of an objective indicator for the factor controlling the issue. The AWD and Eh control are just the water management practices, using an objective indicator.

The use of an objective indicator is also to solve the secondary problem. The conventional water management finally depends on subjective decisions by farmers in any countries. Of course, the current method is useful to achieve a certain aim, but does not necessarily bring the best result. There is a room to reconsider and improve the current method more efficiently.

Therefore, an objective indicator is very useful to achieve the generalization and simplification of the conventional water management. Although it takes much to spread the new water management, it will be the mainstream in the future.

14.4.2 Method of Eh Control

The Eh control is now the water management at the experimental level. Figure 14.8 shows the schematic diagram of Eh control. The Eh control keeps the soil Eh between the pre-determined lower and upper limit by drainage and flooding. After the irrigation for rice transplanting, the flooded soil is drained when the soil Eh decreases to a lower limit. Thereafter, the drained soil is re-flooded when the soil

Fig. 14.8 Method of water management for the Eh Oxidative control. Soil Eh is controlled between the upper and lower

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