IRRIGATION AND WATER MANAGEMENT
Water is a major component of the rice plant. Sampling from dried leaf blade tissues, it was found that about 88% of the leaf blade is water and 12% is solid matter. Water is usually the limiting factor in the food manufacturing of the plant because light and air are always available. In rice production, water affects the physical character of the plant, the nutrient status and nature of the soil, and the extent of weed growth. The supply of water at the right stage of plant growth is equally important to achieve high grain yield (De Datta 1973, IRRI 1983).
Functions of water
Lack of water decreases the amount of manufactured food in the plant when the pores of the leaves close and air cannot enter. The plant uses carbon dioxide from the air to manufacture food; the brighter the light, the more light energy used to produce more carbohydrates. Carbohydrates and minerals are carried to different parts of the plant by water. With this function, 1 ha of rice plants uses at least 8 million liters of water (equivalent to 400,000 of the 5-gallon kerosene cans) during its life cycle (De Datta 1981). Water is absorbed by the roots from the soil and carbon dioxide from the air enters the plant through pores on the leaf surface. The light energy breaks down the water, which then combines with air (carbon dioxide) to produce carbohydrates. The green colorings of the chlorophyll are manufacturing units that collect the light energy (Tanaka 1966).
Another important function of water is to cool the leaves, just like perspiration cooling the human body. The water evaporates if temperature is too high; most of the water taken up by the rice plant is lost through evaporation. Without water, leaves droop and roll. With water, they are erect and fully expanded (IRRI 1966, Yoshida 1981).
Other benefits from water are suppression of weed growth, enhancement of nutrient absorption, and minimizing the loss of nitrogen or insecticides in the soil. Flooding on and off results in great losses of nitrogen in gaseous form. The less air in the soil, the less transformation there is of nitrogen into gas.
Water requirements
In Southeast Asia, the seasonal water requirement for rice cultivation ranges between 750 and 1,500 mm, with an average of 1,200 mm. It is assumed that the rice plant stays for 120 days in the field and an average requirement of 10 mm per day seems reasonable. The total water requirement includes water needed to raise the seedlings, prepare the land, and manage a crop of rice from transplanting to harvest (Caoili 1974).
The water requirements of irrigated rice in 43 locations in China, Philippines, Japan, Korea, Vietnam, Thailand, and Bangladesh are summarized below. Water is lost from irrigated rice during the crop season through evaporation, transpiration, and percolation. Water losses through percolation are the most variable, with values ranging from 6 to 10 mm/day. Thus, on the average, about 180-300 mm water/month is needed to produce a reasonable crop of rice (IRRI 1970, Mallick 1982). The water requirement of an irrigated rice crop is apportioned between water losses and field operation as follows:
By water loss
Transpiration 1.5-9.8 mm/day
Evaporation 1.0-6.2 mm/day
Percolation 0.2-16 mm/day
Range of total losses 5.6-21 mm/day
By field operation
Nursery 40 mm
Land preparation 200 mm
Field irrigation 1000 mm
Total 1240 mm/crop season
Different units are used in expressing the water needs of crops:
1 liter/ha per second = 8.64 mm/day or = 86.4 m3/ha per day (1 mm/day is equal to 0.116 liter/ha per second or 10.0 m3/ha per day)
Water depth and plant growth stages
As a general rule, these are the desirable water depth and irrigation management schemes for the different growth stages under continuous flooding (Caolli 1974):
- At transplanting
The puddled field should be covered with only a shallow film of water, preferably 2-3 cm deep. This will facilitate early rooting, and tillering, and promote firm root anchorage in the soil. A shallow depth maintained in the field re-quires less water for irrigation and results in better crop yield. How-ever, the land must be carefully leveled, and this may entail more expenses. - After transplanting
As soon as the transplanted seedlings establish themselves and vegetative growth starts, water in the field should be gradually increased to cover the
soil but not to drown the crop. Deep water would increase plant height and reduce the tillering capacity of the plants. - Tillering stage
Water depth is kept as shallow as possible up to maximum tillering stage
when reproductive growth starts. Weed competition from planting up to this stage should by now be completely eliminated. Water level is kept between 5 and 10 cm, depending on the height of the plants. It is clear that land preparation, nursery management, basal fertilizer application, plant spacing, and water management all contribute to achieve maximum tillering.
- Reproductive stage
This stage includes the panicle primordial development, booting, heading, and flowering. Abundant water (between 5 and 10 cm deep) must be kept in the field because the rice plant at this time is very sensitive to water stress. Two factors should be considered at this stage (IRRI 1976, 1977; De Datta 1981).
Drought will cause severe damage if it occurs from pani-cle initiation to flowe-ring stage. There will be increased panicle sterility, impeded pa-nicle formation, and delayed heading and flowering. Topdress-ing of nitrogen ferti-lizers might not be effecttive at all. On the other hand, excessive water at the reproductive stage will decrease culm strength and increase lodging susceptibility.
- Ripening stage
The last phase of the grow-ing period includes the milk, dough, and full ripening stages. Very little water is needed at this time and after the yellowing or ripening stage, no standing water is required. This allows draining of the field about 2-3 weeks before har-vest. When using a combine harvester, the field is drained much earlier to harden the field and prevents the equipment from sinking deep into the soil. Once the field is drained, there is risk of rodent damage.
Units of measurement
The units commonly used in water measurement are those expressing rate of flow-e.g., gallons per minute (gpm) or cubic feet per second (cfs). The table shows the base flow unit and the equivalent flow value.
| Base flow unit | Equivalent flow |
| 1 cubic foot per second | 1.9835 acre-feet per day or 0.9913 acre-inch per day |
| 1 gallon per minute | 0.0022 cubic foot per second |
| 1 million gallons per day | 1.5472 cubic feet per second |
| 3.0700 acre-feet per day | 694.4400 gallons per minute |
| 1 acre-inch per hour | 452.6000 gallons per minute |