fundamentals of irrigation and on-farm water management: volume 1 || crop water requirement and...
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Crop Water Requirement and IrrigationScheduling
With increasing scarcity and growing competition for water, judicious use of water
in agricultural sector will be necessary. This means that exact or correct amount
and correct timing of application should be adopted. In addition, it will need more
widespread adoption of deficit irrigation, especially in arid and semi-arid regions.
Recent advances in new irrigation technologies will help to identify irrigation
scheduling strategies that minimize water demand with minimal impacts on yields
and yield quality, leading to improved food security. This chapter discusses the
detail method for crop and irrigation water requirement and latest concepts of
irrigation scheduling and strategies involved in irrigation scheduling. Numerous
examples are incorporated to understand the procedure to calculate irrigation water
requirement and irrigation scheduling. In addition, techniques for command area
development are discussed along with sample illustrations of designing parameters
for command area.
9.1 Crop Water Requirement
The amount of water needed to compensate the evapotranspiration loss from the
crop field is termed as crop water requirement. The value of crop water requirement
is identical to evapotranspiration. Crop water requirement varies with time and
space, as the evapotranspirative demand varies with local climate and crop condi-
tion. Crop water requirement represents the evapotranspiration (ET) under ideal
crop growth condition. The ET from a crop with insect and pest-affected condition
and low population (crop density) will be different from that of a well-established
and well-grown crop.
M.H. Ali, Fundamentals of Irrigation and On-farm Water Management: Volume 1,DOI 10.1007/978-1-4419-6335-2_9,# Springer ScienceBusiness Media, LLC 2010
9.1.2 Factors Affecting Crop Water Requirement
Weather and crop influence in determining water requirement for crop. The weather
elements which influence crop evapotranspiration (ET), and the way they influence
ET are described in Chap. 3 (Weather).
126.96.36.199 Crop Factor
Different crops use different amount of water during its growing period. Crop
factors influencing crop water requirement include the following:
l Type of cropl Cultivar/speciesl Growing stagel Leaf areal Leaf type, stomatal behaviorl Root length, root density
Due to difference in growth pattern, different crops or even different cultivar of
the same crop show different evapotranspiration demand. Length of crop duration
has a direct effect on total water requirement. A wheat variety with maturity period
of 150 days will use more water than 120 days variety. Crop planting time or the
season has an impact on crop water demand due to differential energy pattern for
evapotranspiration. Some crops are grown both in winter and summer/spring.
Summer season crop will surely need more water than that of winter. Younger
plants require less water than the mature one (Fig. 9.1). Besides, leaf area (evapo-
rative surface) and stomatal closure behavior influence on ET. Under similar
environmental condition, a plant having little leaf area and root system would
0 20 40 60 80 100% Growth period
Fig. 9.1 Typical pattern of ET demand during the growth period of cereal crop
400 9 Crop Water Requirement and Irrigation Scheduling
require much less water than a plant having higher leaf area and dense root system.
In addition, crop population may influence ET demand decrease in population
will result in less ET.
Other factors determining total crop water requirement are growing season
(winter or spring) and length of growing period (short or long duration crop
9.1.3 Calculation/Estimation of Crop Water Requirement
Crop water requirement can be calculated from the climate and crop data. Crop
water requirement for a given crop, i, for the whole growing season:
CWRi ETi Xm
t0ET0t Kct; (9.1)
where CWRi is the crop water requirement for the growing period, in mm, ETi is the
crop evapotranspiration for the growing period, in mm, t is the time interval in days,m is the days to physiological maturity from sowing or transplanting (total effectivecrop growth period), in numbers, ET0t is the reference crop evapotranspiration of
the location concern for the day t, in mm, and Kct is the crop coefficient for thetime t day.
Note: Actually, the crops do not need water up to the harvest date. Physiologicalmaturity is the status of maturity after which the weight of the grains does not
change/increase. Normally it reaches a week (7 days) ahead of traditional harvest
time in cereals, and 35 days in pulses and oilseeds.
The methods for determining ET0 and Kc have been described in details inChap. 7 (Field Water Balance). Crop water requirement for a particular growth
stage (or period) of the crop can be calculated using the Kc for that growth stage (orperiod).
9.1.4 Difference Between Crop Water Requirementand Irrigation Water Requirement
Crop water requirement (CWR) and irrigation water requirement (IWR) can be best
described with the following mathematical functions:
CWR f (weather, crop)IWR f (weather, crop, soil, rainfall, irrigation method, depth to water-table or
Distinguishing characteristics of crop water requirement and irrigation water
requirement are given below:
9.1 Crop Water Requirement 401
9.1.5 Water Requirement of Major Field Crops
Typical water requirement of some major crops are given in Table 9.1.
9.2 Irrigation Water Requirement
In the crop field, water is used in many pathways such as direct evaporation from
soil and plant surfaces, transpiration through plant leaves, assimilation into the
plant and plant fruits, and other beneficial uses such as salt leaching, crop cooling,
and freeze protection. The beneficial uses are applicable for particular circum-
stances, but the other needs are essential for all conditions and they are governed
by weather, soil and crop. Net irrigation water requirement is the quantity of water
required for successful crop growth.
Table 9.1 Water requirement of major crops
Crop Water requirement (cm) Crop Water requirement (cm)
Alfalfa 150160 Wheat (winter) 3040
Bean 3040 Wheat (spring) 4055
Corn 5070 Rice 110160
Cotton 70100 Sorghum /millet 4050
Lentil 1525 Soybean 3050
Maize 4060 Sugarcane 150200
Groundnut 3040 Sugar beet 7590
Onion 3040 Sunflower 4055
Note: The typical values are based on average climatic condition (semi-arid to semi-humid), fullirrigation strategy (no deficit), and flood irrigation method. The values for extreme climatic
conditions will vary from this estimate (lower for cold climate and higher for dry climate). It
may also vary depending on the season (some crops are grown in two seasons, winter and
summer), time of transplanting or sowing, and duration of the crop (short duration cultivar will
require less water than the long-duration one, and vice versa).
Crop water requirement Irrigation water requirement
1. It is a function of weather and crop 1. It involves additional factors other than the
weather and crop
2. Normally it is less than the irrigation
2. Normally it is higher than the crop water
3. It is not a function of soil and
3. It depends on soil type and irrigation method
4. It does not depend on rainfall (but on
temperature and humidity)
4. Irrigation water requirement decreases if there
is any rainfall
5. It is not a function of depth to water-
table or saturated layer
5. It decreases with the contribution of upward
flow from the water-table or saturated layer
402 9 Crop Water Requirement and Irrigation Scheduling
9.2.1 Factors Affecting Irrigation Requirement
The factors which influence crop water demand (ET demand), also influence the
irrigation water demand. Weather, crop, and soil all these three factors influence
in determining water requirement for irrigation. Crop and weather factors influen-
cing crop water demand have been described earlier.
188.8.131.52 Soil Factor
Soil factor does not affect irrigation demand directly (but a little by absorbing heat),
but affect total water requirement by its storage and release properties of water. In
addition to type of the soil, the organic matter content of the soil control a great deal
to its storage and release properties. Seepage and percolation rate of sandy soil is
much higher than that of the silt and clay soils. The rate of percolation follows the
order: sandy soil > silt > clay soil. Hence, the total irrigation requirement duringthe crop growing season is higher in sandy soil than the clayey soil.
Other factors affecting irrigation water requirement include effective rainfall,
water required for leaching of salt (leaching requirement), water required for land
preparation or land soaking, etc. Estimation of these components is described below.
9.2.2 Components of Total Water Requirement for Irrigation
Total irrigation water requirement is the amount of water required during the
cropping period for successful crop cultiva