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  • Chapter 9

    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

    9.1.1 Definition

    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

    399

  • 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).

    9.1.2.1 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

    1

    2

    3

    4

    5

    6

    0 20 40 60 80 100% Growth period

    ET

    (m

    m/d

    ay)

    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

    cultivar).

    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

    saturated layer)

    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

    water requirement

    2. Normally it is higher than the crop water

    requirement

    3. It is not a function of soil and

    irrigation method

    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.

    9.2.1.1 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

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