Impact of water management

As indicated above, CH4 production is a process occurring under anaerobic conditions in strictly reduced (anoxic) conditions. For such conditions to establish, soils usually have to be flooded or completely waterlogged for at least several days without interruption. During drier periods oxygen enters the soil, redox potentials rapidly increase again and CH4 production ceases. This is often the case in rainfed rice production, where CH4 emissions are on average only about one third of those in irrigated systems (Abao et al, 2000; Setyanto et al, 2000; Yan et al, 2005). A traditional management practice in irrigated rice paddies is drainage on one or more occasions during the growing season. Drainage is much applied in Japan and China to enhance yields (Greenland, 1997) and is also popular in northern India (Jain et al, 2000), whereas in Vietnam, for example, continuous flooding is the norm (Table 8.1).

An additional benefit of drainage is that of disturbing the life cycle of water-dependent vectors of human disease (malaria, Japanese encephalitis and others) (Greenland, 1997). To relieve the mosquito-induced stress on local inhabitants, intermittent drainage is prescribed for fields surrounding towns in the rice-growing area of northern Italy (S. Russo, personal communication). Estimated CH4 reductions compared to continuous flooding are between 7 and 80 per cent (Wassmann et al, 2000b, and references therein) and 26-46 per cent (Zheng et al, 2000). Intermittent irrigation can be as effective (Husin et al, 1995; Yagi et al, 1996), or even more effective than mid-season drainage alone (Lu et al, 2000). Based on an analysis by Yan et al (2005) of more than 1000 seasonal measurements from over 100 sites, the 2006 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC, 2007a) adopt values for the reduction in CH4 emissions from a single mid-season drainage of 40 per cent and for multiple drainages of 48 per cent, compared to continuous flooding.

Some of the gains achieved by drainage in terms of global warming potential can be offset by enhanced N2O emissions (Cai et al, 1997; Akiyama et al, 2005). The IPCC guidelines estimate that, on average, 0.31 per cent of the nitrogen fertilizer applied to rice paddies is emitted as N2O (IPCC, 2007a). This emission factor was based on an analysis conducted by Akiyama et al (2005), in which they calculated a N2O emission factor of 0.22 per cent for continuously flooded rice paddies and an emission factor of 0.37 per cent for intermittently irrigated rice paddies. Yan et al (2009) estimated that 27 million hectares of the global rice area is continuously flooded. Assuming an average fertilizer application rate of 150kg N ha-1, if these continuously flooded rice fields were all drained more than once during the rice-growing season the N2O emission from rice fields would increase by approximately 9.5Gg. Even though the GWP of 1kg of N2O is approximately 12 times higher that of 1kg of CH4 (IPCC, 2007b), they calculate that the increased GWP resulting from this extra N2O emission would be only approximately 2.7 per cent of the reduction in GWP that would result from the 4.14Tg reduction in CH4 emissions, and therefore draining the fields is beneficial in terms of net climate forcing. However, it should be emphasized that only where fields are currently under continuous flooding and where flooding can be controlled, is drainage a mitigation option. Yan et al (2009) estimate that the global mitigation potential through water management is the same as that associated with off-season straw incorporation (i.e. 4Tg CH4 yr-1).

The effects of both applying rice straw to the land outside the growing season, and drainage during the season, are shown in Figures 8.5 and 8.6, and Table 8.2.

40 60 80 100

DAYS AFTER FIRST FLOODING

Figure 8.5 Effect of water management on CH4 emission from a rice paddy field

40 60 80 100

DAYS AFTER FIRST FLOODING

Figure 8.5 Effect of water management on CH4 emission from a rice paddy field

Note: The arrows indicate the period of mid-season drainage in the intermittent irrigation plot and the timing of final drainage in both of the plots. Source: Yagi et al (1997)

Figure Cultivation Rice
Figure 8.6 Distribution of potential mitigating effects by (a) applying rice straw off-season where possible; (b) draining all continuously flooded rice fields; and (c) adopting both options

Note: Negative values indicate an emission reduction. Source: Yan et al (2009)

Table 8.2 Mitigation potential (per cent) of methane emission from rice cultivation in major rice-producing countries by applying rice straw off-season where possible, draining all continuously flooded rice fields, and adopting both options simultaneously

Country

Rice straw

Draining

Both

off-season

rice field

options

China

12.8

15.6

26.4

India

16.3

13.6

27.5

Bangladesh

22.4

4.4

25.9

Indonesia

8.4

21.7

28.6

Vietnam

5.7

36.6

40.7

Myanmar

15.9

19.8

33.2

Thailand

20.2

4.7

24.2

The Philippines

9.0

22.7

30.0

Pakistan

25.1

28.7

46.7

Japan

33.6

15.6

43.9

US

35.2

21.8

49.3

Cambodia

27.9

6.6

33.4

South Korea

26.7

12.0

35.5

Nepal

19.0

16.7

32.6

Nigeria

19.6

6.3

24.7

Sri Lanka

18.5

24.5

38.8

Brazil

27.7

17.0

39.9

Madagascar

22.7

2.8

24.8

Malaysia

16.4

23.5

36.6

Laos

21.7

5.2

26.0

Globe

16.1

16.3

30.1

Source: Yan et al (2009)

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