Yoshida - 1981 - Fundamentals of Rice Crop Science

CLIMATIC ENVIRONMENT AND ITS INFLUENCE 95
Table 2.14. Water requirement of an irrigated rice crop. a
By water loss
Transpiration 1.5–9.8 mm/day
Evaporation 1.0–6.2 mm/day
Percolation 0.2–15.6 mm/day
Range of total
daily loss
5.6-20.4 mm/day
By field operation
Seed nursery
Land preparation
Field irrigation
Total
40 mm
200 mm
1000 mm
1240 mm/crop
a Kung (1971).
2.5.2. Water requirements
The water requirements of irrigated rice in 43 locations in China, Japan, Korea, the
Philippines, Vietnam, Thailand, and Bangladesh are summarized in Table 2.14.
Water is lost from irrigated rice during the crop season, through transpiration,
evaporation, and percolation, Water losses through percolation are the most
variable. The total water loss shown in Table 2.14 ranges from 5.6 to 20.4 mm/day,
but most of the observed values for total water loss range from 6 to 10 mm/day.
Thus, on the average, about 180–300 mm water/month is needed to produce a
reasonably good crop of rice.
In field operation, a total of 1,240 mm is an average water requirement for an
irrigated rice crop. The same figure is obtained if the assumptions are made that
the crop duration is 5 months, monthly water requirement is 200 mm, and land
preparation requires 200 mm water. If land preparation takes more than 30 days,
however, as much as 500–600 mm of water may be required.
Several different units are used in discussing the water needs of field crops:
1 liter/ha per second = 8.64 mm/day
= 86.4 m 3 /ha per day
1 md/day = 0.116 liter/ha per second = 10.0 m 3 /ha per day.
2.5.3. Water gains — precipitation
Precipitation (rainfall plus snow) is the primary source of water in agriculture.
Rainfed rice depends mainly on current rainfall, whereas water is supplied to
irrigated rice through a storage system, which collects precipitation and stores it
for later use.
The amount and pattern of rainfall varies widely from one place to another, and
from year to year. At the moment, there is no way to predict the amount and pattern
of rainfall for a given region with sufficient accuracy. What we know is average
rainfall based on long-term records.