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    CIVL 404 Irr N DRG 5 - Irrigation Water Requirement Part2 (2021-22)

    Uploaded by

    Akolbila Emmanuel
    This document discusses the calculation of irrigation water requirement (IWR). It defines IWR as the difference between crop water requirement and effective rainfall. It describes how to calculate effective rainfall and then walks through an example of determining IWR for both field crops and paddy rice, accounting for factors like soil saturation, percolation losses, and establishing a water layer for rice. The document provides the key steps and formulas for estimating IWR for different crop types.

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    CIVL 404 Irr N DRG 5 - Irrigation Water Requirement Part2 (2021-22)

    Uploaded by

    Akolbila Emmanuel
    25 views23 pages
    This document discusses the calculation of irrigation water requirement (IWR). It defines IWR as the difference between crop water requirement and effective rainfall. It describes how to calculate effective rainfall and then walks through an example of determining IWR for both field crops and paddy rice, accounting for factors like soil saturation, percolation losses, and establishing a water layer for rice. The document provides the key steps and formulas for estimating IWR for different crop types.

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    CIVL 404 Irr n Drg 5 - Irrigation Water Requirement Part2 (2021-22)

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    This document discusses the calculation of irrigation water requirement (IWR). It defines IWR as the difference between crop water requirement and effective rainfall. It describes how to calculate effective rainfall and then walks through an example of determining IWR for both field crops and paddy rice, accounting for factors like soil saturation, percolation losses, and establishing a water layer for rice. The document provides the key steps and formulas for estimating IWR for different crop types.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    25 views23 pages

    CIVL 404 Irr N DRG 5 - Irrigation Water Requirement Part2 (2021-22)

    Uploaded by

    Akolbila Emmanuel
    This document discusses the calculation of irrigation water requirement (IWR). It defines IWR as the difference between crop water requirement and effective rainfall. It describes how to calculate effective rainfall and then walks through an example of determining IWR for both field crops and paddy rice, accounting for factors like soil saturation, percolation losses, and establishing a water layer for rice. The document provides the key steps and formulas for estimating IWR for different crop types.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    of 23

    Irrigation Water Requirement

    Determination of Effective Rainfall


    Calculation of IWR
    Example
    Irrigation Water Requirement - Recap
    • Crop water requirement can be supplied by rainfall,
    irrigation, or a combination of both.

    • Irrigation water requirement (IWR) is the difference between


    the crop water requirement (CWR) and the effective rainfall
    (Pe).
    IWR = CWR – Pe
    Irrigation Water Requirement - Recap
    • Where all the water needed for optimal growth of the crop is
    provided by rainfall, irrigation is not required and IWR = 0.

    • Where there is no rainfall at all during the growing season,


    all water has to be supplied by irrigation.

    • Consequently, IWR = CWR (ie, ET crop)


    Determination of Effective Rainfall
    Estimating Effective Rainfall
    • When it rains (Precipitation, P), some of it infiltrates into the soil
    (Infiltration, I); some stagnates on the surface (Depression
    storage, D): some flows over the surface as runoff (R).

    • When the rainfall stops, some of the water stagnating on the


    surface (D) evaporates to the atmosphere (Evaporation, E): the
    rest slowly infiltrates into the soil (I).

    • From all the water that infiltrates into the soil (I), some
    percolates below the root zone (D.P), while the rest remains
    stored in the root zone (effective rainfall, Pe)
    Estimating Effective Rainfall
    • Effective rainfall, Pe = P – R – E – D.P.

    • Effective rainfall is the total rainfall - runoff - evaporation -


    deep percolation;

    • Only the water retained in the root zone can be used by the
    plants
    Estimating Effective Rainfall
    • This represents what is called the effective part of the
    rainwater.

    • It is used to define the fraction of the total amount of


    rainwater useful for meeting the water need of the crops.
    Estimating Effective Rainfall
    • Formulae for estimating the fraction of the total rainfall
    which is used effectively
    • Applicable in areas with a maximum slope of 4-5%

    Pe = 0.8 P 25 if P > 75 mm/month


    Pe = 0.6 P 10 if P < 75 mm/month
    Where:
    P = rainfall or precipitation (mm/month)
    Pe = effective rainfall or effective precipitation (mm/month)
    Estimating Effective Rainfall
    QUESTION
    • Calculate the effective rainfall for the following monthly
    rainfall figures: P = 35, 90,116, 5, 260, 75 mm
    • ANSWER:
    CALCULATION OF IRRIGATION WATER
    REQUIREMENT (IWR)
    Calculating IWR – Example (cont. from previous
    example)

    • The following rainfall figures are given: P (mm/month): 20, 38, 40, 80, 16
    • Step 1: Calculate for each month the effective rainfall using the formulae
    Calculating IWR – Example (cont. from previous
    example)

    • Step 2: Calculate the irrigation water need, both in mm/month and


    mm/day, using the formula: IWR = ET crop - Pe; e.g. Feb: IWR = 69 - 2 =
    67 mm, etc
    IRRIGATION WATER REQUIREMENT FOR PADDY
    RICE
    IWR For Paddy Rice (cont.)

    • Determination of IWR for Paddy rice is an exception.

    • In addition to CWR, water supplied by rainfall or irrigation


    should include water for:

    • saturation of the soil before planting


    • percolation and seepage losses
    • establishment of a water layer
    IWR For Field Crops - Procedure

    • For all field crops, the irrigation water requirement (IWR) is


    determined as follows:

    • Step 1: Determine the reference crop evapotranspiration: ETo


    • Step 2: Determine the crop factors: Kc
    • Step 3: Calculate the crop water need: ET crop = ETo × Kc
    • Step 4: Determine the effective rainfall: Pe
    • Step 5: Calculate the irrigation water need: IWR = ET crop - Pe
    IWR For Paddy Rice - Procedure

    • Determination of the irrigation water need for paddy rice


    requires the following steps:

    • Step 1: Determine the reference crop evapotranspiration: ETo


    • Step 2: Determine the crop factors: Kc
    • Step 3: Calculate the crop water need: ET crop = ETo × Kc
    • Step 4: Determine the amount of water needed to saturate the
    soil for land preparation by puddling: SAT
    IWR For Paddy Rice – Procedure (cont.)
    • Determination of the irrigation water need for paddy rice requires the
    following steps:

    • Step 5: Determine the amount of percolation and seepage losses:


    PERC
    • Step 6: Determine the amount of water needed to establish a water
    layer: WL
    • Step 7: Determine the effective rainfall: Pe
    • Step 8: Calculate the irrigation water need: IWR = ET crop + SAT +
    PERC + WL - Pe
    IWR For Paddy Rice – Procedure (cont.)
    • Step 1, 2 and 3 - Determining ETo, Kc and ET crop – similar to other
    field crops as seen earlier
    • Step 4: Determine the amount of water needed to saturate the soil
    for land preparation by puddling: SAT
    • In the month before sowing or transplanting, water is needed to
    saturate the root zone.
    • The amount of water needed depends on the soil type and rooting
    depth.
    • Assume amount of water needed to saturate the root zone is 200 mm
    IWR For Paddy Rice – Procedure (cont.)
    • Step 5: Determine the amount of percolation and seepage losses:
    PERC

    • Percolation and seepage losses depend on type of soil.


    • Will be low in very heavy, well-puddled clay soils and high in the
    case of sandy soils.

    • The percolation and seepage losses vary between 4 and 8


    mm/day; Average PERC = 6mm/day
    IWR For Paddy Rice – Procedure (cont.)
    • Step 6: Determine the amount of water needed to establish a water
    layer: WL

    • A water layer is established during transplanting or sowing and


    maintained throughout the growing season.
    • Amount of water needed for maintaining the water layer has already
    been taken into account with the determination of the percolation
    and seepage losses.
    • The amount of water needed to establish the water layer, however,
    still has to be considered.
    • Assume a water layer of 100 mm
    IWR For Paddy Rice – Procedure (cont.)

    • Step 7: Determine the effective rainfall: Pe

    • The effective rainfall is calculated using the same formulae as


    seen previously

    • Step 8: Calculate the irrigation water requirement:


    IWR = ET crop + SAT + PERC + WL - Pe
    IWR For Paddy Rice – Example
    QUESTION
    • Calculate the irrigation water requirement (IWR) of paddy rice for the
    month of April when given:

    • · ETo = 6 mm/day
    • · Kc = 1.1
    • · the root zone has already been saturated in the previous month
    • · PERC = 5 mm/day
    • · the water layer (100 mm) needs to be established during April
    • · Pe = 135 mm/month
    IWR For Paddy Rice – Example
    ANSWER

    • IWR = ET crop + SAT + PERC + WL - Pe


    • ET crop = ETo × Kc = 6 × 1.1 = 6.6 mm/day = 6.6 × 30 = 198 mm/month
    • SAT = 0 mm
    • PERC = 5 mm/day = 5 × 30 = 150 mm/month
    • WL = 100 mm
    • Pe = 135 mm/month
    • IWR = 198 + 0 + 150 + 100 - 135 = 313 mm/month = 10.4 mm/day
    • Thus the irrigation water need during April is 313 mm or 10.4 mm/day.

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