Paddy Field Characteristics in Water Use: Experience in Asia - Water and Agriculture: Sustainability, Markets and Policies - page 211

annual rainfall in excess of 1000-1500 mm, influenced by low pressure and monsoons
accompanied by westerly winds. Moreover, the seasonal monsoon concentrates 70-90%
of annual precipitation in a half year of the wet season (Figure 1).
Figure 1. Annual precipitation and precipitation in the wet season
In terms of climate classification, meanwhile, it should be seen as a high-precipitation
region and belongs to a warm climate zone (a climate classification encompassing
temperate, subtropical and tropical zones). The water balance (calculated by subtracting
annual potential evapotranspiration (ET), from annual precipitation) generally exceeds
500 mm. There are not too many regions of the world where there is a positive water
balance of over 500 mm per year. Of these, the Asian monsoon region is the largest in
land area and the highest in population.
The Asian monsoon region is physiographically composed of river basins that are
influenced by tectonic zones, if the world's continental land is broadly divided into
tectonic zones where orogenic movement is active and stable zones where the geology is
older. This brings general characteristics of steep topography and fast-flowing river
streams. Figure 2 contrasts the steep slope of major rivers in Japan with the gradual slope
of major rivers in the continent.
The Asian monsoon region is characterised by large seasonal and short-term
fluctuations in the supply of water resources, as is evident in the distinct dry and wet
seasons. For this reason, the basic norm is to plant and cultivate paddy rice during seasons
when water resources are abundant. When doing so, water is introduced to arable land, in
excess of the moisture to be consumed by crops, with standing water, through the use of
floodwater and irrigation.