Paddy Field Characteristics in Water Use: Experience in Asia - Water and Agriculture: Sustainability, Markets and Policies - page 219

Table 4. Lines, length and authority of rivers controlled by the River Act

Class Lines Authority Length Ratio Minimum flow

Class 1 River National Government 10,553km 7% Regulated

13,979

Local Governments 77,008km 54% Regulated

Total 87,560km 61%

Class 2 River 7,071 Local Governments 35,934km 25% Regulated

Quasi-class River 14,113 Municipalities 20,032km 14% Regulated

TOTAL 143,528km 100%

Source: River authority in Japan (2002).

Agricultural water users are controlled by the River Act in Japan when they take
water from a river. Moreover, the greater part of water taken from a river for rice paddy
irrigation is not consumed, i.e. it is evapo-transpired. Figure 5 illustrates the water
balance in typically irrigated rice paddies. The water taken from a river and not consumed
in paddy fields contributes to enhancing ecosystem services in two ways: a) water in the
total paddy irrigation and drainage system serves as a network of wetlands and water
ways, and creates another excellent secondary natural environment outside the river with
an enriched flora and fauna; b) water drained from paddies and returning to the river
reinforces the ecosystems inside the downstream rivers and marshes.

Figure 5. Water balance in paddy (Japan)

Source: Maruyama, T., R. Nakamura et al. (1998).

Paddy fields stretching along a river serve as a retardant reservoir that receives
outflow from the mountainous hinterlands and irrigated water drawn from the river, and
that gradually supplies the water to groundwater aquifer and the downstream river.
Figure 6 shows a schematic drawing and Figure 7 shows a diagram explaining the
contribution system of rice paddy irrigation to ecosystem services. This system is widely
observed in humid regions such as the Asian monsoon region. Figure 8 shows a diagram
explaining the typical arguments advocating a competitive relation between agricultural
water use and ecosystems where water is constantly scarce. It contrasts the contribution
system in humid regions with the competitive nature of water use in arid and semi-arid
regions.