Paddy Field Characteristics in Water Use: Experience in Asia - Water and Agriculture: Sustainability, Markets and Policies - page 221

3.2 Stabilising downstream river flow through constant return flow to rivers
Rice paddies surrounded by levees store water from rainfall as well as irrigated water
and discharge them into drainage canals. The irrigated water comprises water from rivers,
irrigation ponds and streams. Some of the drained water directly returns to the
downstream river while other water recharges irrigation canals in lower ground. The
water is reused for irrigation in the downstream farmlands and for domestic water use.
For analysing such complicated water balance in a basin scale, a water budget model
divided into the sub-basin scale using the Complex Tank Model (CTM) was developed.
Following two studies quantify the amount of return flow from paddy fields in basin scale
using this methodology.
Nakagiri et al. (1998, 2000) studied the Kino River Basin located in the Kinki Region
in the western part of Japan. It has a catchment area of 1750 km 2 . The upper mountainous
area has an average annual precipitation of more than 3000 mm while the lower part
consists of a long and narrow plain along the main river stream with an average annual
precipitation of 1350 mm. There exist three reservoirs in the mountainous area and six
main diversion works in the plain, mainly for paddy irrigation. While the area of farmland
is only 15% of the whole basin area, the total intake water for irrigation is about 80% of
total water use. Figure 9 shows a ground plan of the Kino River Basin and division
system into 25 sub-basins. Figure 10 illustrates a diagram of the irrigation and drainage
network of sub-basins in the Kino River Basin. Figure 11 illustrates a diagram of CTM in a
sub-basin.

Figure 9. Plan of the Kino River Basin and division into sub-basins

Tsuburo Reservoir

River flow gauge

Shimobuchi D W

Nishiyoshino D W
Oda D W
Fujisaki D W

Diversion work (D.W.)

Suda

Iwade D W

Shinrokka D W

Funato

(Totsu River Basin)

Kino River

Osako Reservoir

Sarutani Reservoir

Waka River

Totsu River

Yamada
Reservoir

km