Fundamentals of Irrigation and On-farm Water Management: Volume 1 || Crop Water Requirement and Irrigation Scheduling

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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


  • 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). 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

    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


    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. 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 cultivation. Total irrigation water requirement

    of a crop may be consisting of the following components:

    l Water for land preparation (as pre-sowing irrigation for dry-land crops)l Water for land soaking (specially for rice)l Crop water demand (includes ET and seepage and percolation)l Water for leaching requirement (specially in saline soil)l Water for special use such as crop cooling, freeze protection

    For each crop or for each area, all of the above items may not essential. For dry-land crops, a pre-sowing irrigation may be needed, while for rice crop, a large

    amount of water is needed for land soaking. Water for leaching is needed for saline

    soil to lower the salinity level. Water Requirement for Land Soaking

    This is the water required to soak the land prior to the initial breaking of the soil,

    either by plowing or by any other means. The amount of water necessary for land

    9.2 Irrigation Water Requirement 403

  • soaking depends on the residual or existing soil moisture, soil texture, depth of soil

    to be saturated or depth to the hardpan, the type of clay minerals, seepage and

    percolation losses, evaporation from standing water, amount of rainfall occurring at

    the time, and application efficiency.

    This is expressed as:

    WRLS Ws C ET0 P ReEa


    where WRLS is the depth of irrigation water required for land soaking (mm), Wsis the depth of water required to saturate the soil (mm), ET0 is the reference

    evapotranspiration during the time of soil saturation (mm), C is the evaporationcoefficient equating reference evapotranspiration to evaporation rate. The value of

    C is about 0.9.

    P is the deep percolation loss during the soil saturation (mm), Re is the effectiverainfall during the period (mm), and Ea is the application efficiency. Water Requirement for Land Preparation

    This is the irrigation water required to maintain the saturation condition of the

    soil from the first breaking of the soil to seedling or transplanting. This water is

    required to replace evaporation, percolation, and application losses and includes the

    addition of water depth to suppress weeds or soften soil clods. This is expressed by

    the equation:

    WRLP Ds C ET0 P Re (9.3)

    where Dswater depth for submergence (mm).Considering application efficiency,

    WRLP Ds C ET0 P ReEa Water for Leaching

    This is the water required to leach the excess salt out of the root zone. It is the extra

    amount in addition to evapotranspiration. The details regarding leaching and

    salinity management have been described in Vol. 2 (Salinity management). The

    fraction of irrigation water required for leaching (termed as leaching fraction) can

    be calculated using the following formula (Rhoades 1974):

    LR ECw=5ECt ECw (9.4)

    404 9 Crop Water Requirement and Irrigation Scheduling

  • where ECw is salinity of the applied irrigation water, and ECt is the threshold

    salinity (average soil salinity tolerated by the crop).

    The total depth of water for a particular season or period that needs to be applied

    to meet both the crop demand and leaching requirement can be calculated as (Ayers

    and Westcot 1985):

    AW ET=1 LR (9.5)

    where AW is the depth of applied water (cm/season), ET is the total seasonal

    demand (cm/season), and LR is the leaching requirement, expressed as fraction

    (leaching fraction, LF). Net Irrigation Water Requirement for NormalGrowth Period (Wnci)

    For rice, this is the irrigation requirement immediately following transplanting (or a

    week after direct seeding) until the start of terminal drainage which is usually

    referred to as the normal irrigation period. For other crops, this requirement starts as

    soon as planting begins and last as long as water is needed by the crop. It is

    expressed by the equation:

    IWRnc X

    Kc ET0 P Re GWc (9.6)

    where IWRnc is the net irrigation requirement for normal growth period (from

    sowing/transplanting to last watering, i.e., excluding land preparation and pre-

    sowing irrigation) (mm), GWc is the groundwater contribution during the period

    (mm), P is the deep percolation loss during the period concerned (mm), and Kc,ET0, Re are defined earlier.

    The procedure for determining Kc and GWc have been described in Chap. 7.

    9.2.3 Total Irrigation Water Requirement Total Net Irrigation Water Requirement

    It is the water required for normal field crop irrigation plus water required for

    special purposes as discussed above, that is:


    where NIWRis the total net irrigation requirement for successful cultivation of a

    crop (mm), WRLS is the depth of water required for land soaking (mm), WRLP is the

    9.2 Irrigation Water Requirement 405

  • depth of water required for land preparation (mm), WRnc is the depth of water

    required for crop ET demand (mm), and WRL is the depth of water required for

    leaching (mm).

    For all crops and for all conditions, all of the above components may not

    necessary. Only the relevant components should be added together. Gross Irrigation Water Requirement (Wgross)

    During delivery of water in the crop field, some losses occur such as inefficiencies

    in conveyance system, evaporation and wind drift (specially for sprinkler irriga-

    tion). Besides, surface runoff and percolation below root zone may occur from the

    field plot. For these reasons, the water that should be pumped to supply the crop

    water requirement should be higher than the actual (net) crop water requirement.

    The above losses can be minimized through proper management practices, but

    cannot be eliminated completely. These points should be considered during deter-

    mination of total irrigation water requirement.

    Gross water requirement is the water required for irrigation considering field

    application loss and conveyance loss. That is

    Gross irrigation requirement,

    GIWR NIWREa Ec (9.8)

    where Ec is the field conveyance efficiency, Ea is the field application efficiency.

    In summary, the following steps may be followed for the calcu...


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