4.1. Water requirements for paddy irrigation

Under these conditions, the field water condition including flooding period and depth is controlled considerably. For some periods, they drain water intentionally resulting in no submergence for some periods, which is to be called intermittent irrigation or flooding [11]. If this water management is practiced, water movement in the area would be accelerated, and it

The fact that rice is cultivated under water ponding condition in most cases implies the water ponding has the advantages even if it requires much water. Main advantages of the water

Most of these advantages come from stable water ponding on the field and could be potentially replaced with other materials or methods than water except stable "water" supply listed

On the other side, the water ponding induces some adverse effects on rice production and local

1. soil reduction due to longer submergence resulting in shortage of oxygen and emission of

3. much water requirement for maintaining ponding resulting in water resource development,

6. influence on local climate due to much evapotranspiration and modified ground surface

Consequently, taking both merits and drawbacks of water ponding into account integrally in addition to field irrigation and drainage conditions, actual water management in the fields for

4. difficulty on introduction of heavier machineries due to increased soil water contents,

3.2. Advantages and disadvantages of water ponding in paddy fields

4. control of temperature of rice and field (warm up and cool down),

5. supply of nutrients and control of fertilizer effects,

7. avoidance of adverse effects of continuous cultivation,

9. enhanced productivity of soil cultivation or plowing.

undesirable gases like hydrogen sulfide and methane,

5. growth of undesirable insects like malarial mosquito, and

2. requirement of much works to maintain ponding in the fields,

affects local hydrological regime.

148 Irrigation in Agroecosystems

ponding are listed as follows: 1. stable water supply to rice,

2. suppression of weeds,

3. control of harmful insects,

6. supply of necessary minerals,

as No. 1 above.

environment. They include:

temperature.

each growing stage is performed.

8. leach out of accumulated salts, and

Water ponding in the fields requires much water. Water requirements of paddy fields compose mainly of transpiration of rice plant, evaporation from ponding water or soil surface, and percolation into soil profile. In some cases, requirement to reestablish water layer after intentional drainage and to implement flow-through irrigation for saving management labor or for control of temperature might be included.

For planning and designing the irrigation facilities and the water use plan, water requirement is basically estimated base on evapotranspiration of rice field. The actual water lost in the field through other paths, including seepage into the deeper soil profile under the root zone part and run off or spill out into the drain through the field outlet, is often recognized as the "loss," rather than "requirement."

Table 2 shows the total water requirements for one irrigation season reported by JSIDRE [12]. The total requirements range from 500 mm in Senegal to 3900 mm in Kazakhstan or 4500 mm in East Africa. This wide range is caused fundamentally by the significant difference in the seepage rates, which are estimated as none at minimum and more than 30 mm/day at maximum. The effects of water management on water requirements are to be regarded. Water requirement of paddy field in dry area is sometimes much and sometimes less than paddy field in the humid region. For example, as mentioned above, water requirement of paddy field in Egypt or Kazakhstan is relatively much, where consumption for evapotranspiration is large with drier climate, while limited water availability constrains increased water use.


Table 2. Water requirement of paddy field per irrigation season [11].
