Paddy Field Characteristics in Water Use: Experience in Asia - Water and Agriculture: Sustainability, Markets and Policies - page 218

3. Analysis on advantages of water use in paddy fields to enhancing ecosystem
services: Case studies in Japan

3.1. Diverting water from rivers in harmony with downstream ecosystems
The River Act in Japan provides that a minimum river flow should be regulated when
a water right is authorised to water users. Water users should release the amount of water
designated as a minimum river flow to the downstream river when they take water from
the river. The minimum river flow comprises flow for maintaining an appropriate
downstream river flow function for fishery, bio-diversity and navigation, as well as flow
for permitted water rights of the downstream water users. Figure 4 illustrates a system of
conditionality for obtaining a water right. When a river authority entitles a water right to a
water user for extracting water from a river, the authority strictly limits the amount of
water which can be drawn from the river, with a calculation subtracting the minimum
river flow from the 355 th largest river flow of 365 daily flows in the drought year that
statistically appears once per decade. In consequence, almost 100% of the length of rivers
in Japan has respectively been assigned, under the River Act, an amount of minimum
river flow for maintaining an appropriate downstream river flow function for ecosystems,
including bio-diversity (Table 4).

Figure 4. Conditionality for allocating a water right in rivers in Japan

Rainy season Typhoon season and rainy front in fall

￿ ￿ ￿ ￿
￿

River flow

Drought season
( from July to Augt.)

Reference minimum river flow
(The 355th minimum flow of 365 flows
which apears once in a decade)

Instream flow to be kept

? Flow for downstream water rights

Q of flow

Flow of maintaining appropriate downstream river
functions for fishery, biodiversity, and navigation .

May July Sept. Nov.

Months

Source : River authority in Japan (2002).