Section 5. Water requirements
Where plants are growing water must be added periodically to the
soil to overcome the effects of evaporation from the surface of the soil. During
early periods of plant growth, while much of the soil surface is exposed to sun
and wind, the moisture loss by evaporation predominates. At later stages of crop
maturity, much of the soil surface is shaded and protected from wind. Then
transpiration water requirements predominate. Evaporation losses are much larger
in climates where the relative humidity is low.
Evaporation losses can be estimated by using an open top
evaporative pan. A standard pan, used by the U.S. Weather Bureau, has been
adopted at meterological stations in many countries. This Class A circular pan
is metal, 1.83 m diameter and 25 cm deep. It is mounted on a wooden platform
elevated 15 cm above the ground.
All tall vegetation should be removed, or cut short, around the
pan. Smaller pan diameters may be used without significantly affecting the
results. Daily water loss can be determined by measuring either the depth or the
volume of water required to bring the water back to its original level. The
level should be low enough (5 to 7 cm below the rim) to keep water from blowing
from the pan by strong wind.
Table 5-1 shows constants which may be multiplied by the pan
evaporation rate to find the total evaporation-transpiration rate for typical
crops. For example, assume that on a day when 8 mm evaporated from a pan then
for maize water use would have been 8 m x (30 to 40 percent) or 2.4 to 3.2 mm.
Figures 5-1 and 5-2 show characteristic water-use patterns for
maize and sorghum for Kansas conditions. Water use for corn (maize) at about May
1 is low because of low air temperatures and high relative humidities. By about
June 1, when sorghum is just germinating (because it is planted about one month
later than maize) soil surface evaporation rates have increased with higher
temperatures and lower relative humidities. At the end of the season, water
requirements drop very low for corn about October 1. Water use does not drop so
low that early for sorghum because sorghum plants remain green, and
transpiration continues until the plant is killed by frost.
The water use characteristics of these two crops vary by regions
of the world but they indicate the wide variations in water consumption
throughout a growing season because of climatic and plant growth factors.
The collection of data required to prepare charts such as
Figures 5-1 and 5-2 is time consuming and expensive. Most published data show
only the seasonal water requirements for crops and is not: very accurate over a
wide range of climates If you use such data; use considerable judgment when
applying growing season data to particular periods, such as a month, to forecast
water requirements
Table 5-2 shows total water use by several major crops in
California for the growing season and for the whole year. Table 5-3 shows
similar data for western Oklahoma, a semi-arid area Table 5-4 shows water
requirements for other crops and areas.
Evaporative losses are a function of temperature, relative
humidity, and wind velocity, at the soil surface. Moisture moves upward in the
soil by capillary action so light shallow surface tillage will help reduce
evaporation losses. Other means of reducing evaporative losses include covering
the surface with mulches of leaves, straw and plant residues. Plastic sheets may
be used but are generally too expensive in most developing countries.
Table 5-1. Approximate range of- crop factors kc (%)
Source: From Doorenbos, J and Pruitt, W. O. Crop Water
Requirements, Irrigation and Drainage Paper No. 24, Food and Agriculture
Table 5-2. Evaporation, California (mm)
Source: California Department of Public Works, Bulletin No. 27,
1931.
Table 5-3. Water requirements, western Oklahoma (mm)
Source: D. O. Anderson, N. R. Cook, and D. D. Badger, Estimation
of Irrigation Water Values in Western Oklahoma, (Oklahoma State University,
Stillwater, Oklahoma, 1966).
Table 5-4. Plants' supposed water requirements
Source: Finkel, H. J. Handbook of Irrigation Technology, Vol. 1,
CRC Press,
Inc.
