Paddy Field Characteristics in Water Use: Experience in Asia - Water and Agriculture: Sustainability, Markets and Policies - page 212

Figure 2. Slope of major rivers in Japan and the world
Source: Takahashi, Yutaka (1990).
Most of the water introduced in excess of the moisture to be consumed by crops is
returned to groundwater and the downstream river via percolation and surface outflow to
drainage channels leading to the river. The proportion of water consumed for evapo-
transpiration differs from region to region, but in the example of Japan, it is said to be
25-50% of the water introduced into paddy fields and the rest of it, 50%-75% of the
water, returns again to the water cycle system in a river basin. In this manner, by
repeating the cycle within a river basin of initially extracting water from rivers,
temporarily inundating paddy fields via water supply channels, then slowly accumulating
groundwater or returning the water to rivers and reusing it downstream, water resources
can be retained on land for as long as possible and used efficiently. This use and reuse of
water is important in areas with many rivers with short courses and fast currents because
of steep topography, where water resources might otherwise be immediately released into
the sea without realising their full potential value.
2.2 Characteristics of paddy rice agriculture — substitutability in water use
In the Asian monsoon region, paddy field irrigation is managed using the benefits of
the wet environment as much as possible. “Wet environment” as used herein refers to two
environmental conditions that are present during the wet season. The first condition is the
fact that anybody who wishes to obtain water can ordinarily get it relatively easily and in
large amounts if rain falls normally. This is because high precipitation results in a large
amount of water resources available, and because the hilly terrain has many mountain
streams and networks of small- and medium-sized rivers that have easier accessibility
than large rivers. In economic terms, this means that, in most cases, water resources have
an extremely low shadow price.