Paddy Field Characteristics in Water Use: Experience in Asia - Water and Agriculture: Sustainability, Markets and Policies - page 214

Owing to the characteristics of rice, namely high tolerance to inundation and the
natural and economic condition allowing ample water use, multiple effects can be
expected from rice paddy irrigation, as mentioned later, including reducing labour for the
work of tilling the submerged paddy soil, reducing the proliferation of weeds (except
vascular plants), maintaining soil fertility (by replenishing nourishments and mineral
ingredients through irrigated water), preventing soil erosion (because it is surrounded by
levees), avoiding fall in yield by repeating mono-crop cultivation, and leaching (removing
salinity from soil). The upshot is that, as an internal economy for farming, paddy rice
cultivation in the Asian monsoon region, with ample water use, has the following
advantages which allow continuous cropping of rice on the same land for hundreds to
thousands of years (Table 1).

Table 1. Advantages of paddy rice agriculture with ample water use

Items of
advantages

Explanation on advantages of paddy rice agriculture with ample water use

Reducing
management in
distributing water
(off-farm)

Because ample water is available, it is possible to convey water to all parts of the field with
even poorly-built canals, and it is easy to manage water distribution at divergence points.
This means that the amount of investment in facilities and labour required for off-farm
water management can be reduced.

With the system called “plot-to-plot irrigation”, the paddy fields themselves serve as
irrigation canals. This method can be used to supply water to all of tens or hundreds of
paddy plots easily. By repeatedly using water (i.e., by introducing it into paddy fields that
are located in higher-elevation terrain and letting excess water flow to downstream paddy
fields), labour required for on-farm management of water, as well as investment in
facilities, can be reduced.

Reducing
management in
distributing water
(on-farm)

Reducing weed
control

Flooding can prevent growth of weeds, except vascular plants like reeds that normally
grow quickly and thickly when the soil is not submerged in the wet and warm climate.

Use of levees around rice fields and a standing pool of water reduce soil erosion losses
even during periods of heavy rain. In fact, rice paddies act as a settling basin for
suspended sediments in water.

Preventing soil
erosion

Organic matter in the soil, decomposing slowly through anaerobic decomposition when the
soil is flooded, maintains soil fertility. Organic nitrogen is transformed into ammonia
nitrogen while the soil is under reduced conditions, and nitrogen is easily taken up by
plants and attaches to soil particles. Less phosphate fertiliser is required for flooded soils
because soluble, plant-available phosphates are formed while the soil is in a reduced
state.

Reducing
fertilisation

Paddy rice cultivation in clay-rich soil involves a year-long process whereby flooding
expands and softens the soil (swelling) and drying shrinks the soil, forming cracks. This
process increases the pore space between grains of soil, which facilitates movement of
water, improves soil leaching that occurs with rainfall and prevents the build-up of salts in
the soil.

Reducing
ploughing

The soil is under reduced conditions when it is flooded and becomes oxidized when water
is drained. This process promotes alternation between anaerobic and aerobic microbes,
which maintains bacterial balance and soil fertility and prevents a fall in yield from
repeated cultivation of the same crop on the same ground.

Preventing a fall in
yield by repeated
cropping

Source: Yamaoka, Kazumi, Naoki Horikawa and Tatsumi Tomosho (2004).