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6.4: Irrigation Scheduling for Soil Water Maintenance

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    With the soil water maintenance approach, the plant’s needs for water are assumed to be met as long as the soil water is maintained between TAW and MB. As shown earlier, frc and MB are dependent on the plant’s microclimate, specifically the atmospheric demand for water. An important variable in irrigation is the allowed depletion (AD). The interval between irrigations is controlled by the AD and the evapotranspiration. The maximum time interval between irrigations, TMAX, is as follows:

    \(T_{MAX} = \dfrac{AD}{ET}\) (6.8)

    where: TMAX = maximum time interval between irrigations and

    ET = average daily evapotranspiration.

    In Example 6.1, AD was 2.7 inches. What is TMAX if ET = 0.3 of an inch a day? The answer is 9 days. This suggests that if water is not applied until AD is reached, then the appropriate maximum time between irrigation is 9 days. And, if irrigation is withheld for 9 days and limited to 2.7 inches, deep percolation is avoided. Using Equations 6.4 and 6.8 you can determine how the root zone depth, evapotranspiration, and the available water capacity of the soil all influence the frequency and the amount of irrigation. A shallow root zone requires more frequent irrigations but lighter applications.

    A coarse-textured soil with a lower available water capacity requires lighter and more frequent irrigations. Medium-textured soils combined with deep root zones allow for less frequent irrigations and larger water applications. The irrigation interval does not have to equal TMAX; it can be less. It is controlled by ET and the effective depth of water application, i.e.:

    \(T = \dfrac{d_e}{ET}\) (6.9)

    where: T = the time interval between irrigations and

    de = the effective water applied per irrigation.

    Many of the modern irrigation systems are managed to apply light, frequent irrigations even when root zones are deep and the AWC is large. For example, a center pivot irrigation system might be managed to apply an effective depth of 0.9 inches even if AD is much larger. Suppose that SWD = AD = 2.7 inches on the day of irrigation. The effective application of 0.9 inches is okay as long as the irrigation frequency is adjusted accordingly. Using our earlier example where ET = 0.3 inches per day, the appropriate interval between irrigations would be:

    \(T = \dfrac{0.9 \text{ in}}{0.3 \text{ in/d}} = 3 \text{ days}\)

    The basic goals of irrigation management are that the deficit not exceed AD before water is applied and that infiltration not exceed the SWD. To avoid exceeding AD, irrigation should occur on or before the latest date (LD). LD is calculated as:

    \(LD = \dfrac{AD-SWD}{ET_f}\) (6.10)

    or using the balance approach:

    \(LD = \dfrac{AW-MB}{ET_f}\) (6.11)

    where: AW = available water (defined below),

    MB = minimum allowable balance, and

    ETf = forecasted daily ET.

    The LD concept is illustrated in Figure 6.5.

    Figure 6.5. Illustration of latest day (LD) concept.

    illustrationLatestDay.jpeg

    For non-layered soils,

    AW = (θv – θwp)Rd or

    AW = fr(AWC)Rd or

    AW = fr(TAW)

    For layered soils, AW = (θv1 – θwp1)(t1) + (θv2 – θwp2)t2 + ... + (θvn – θwpn) [Rd – (t1 + t2 + ...+ tn-1)]

    where: θv1, θv2, θvn, θwp1, θwp2, θwpn = volumetric water content of soil layer 1, 2, and n,

    respectively, numbered from the surface layer down;

    t1, t2, tn-1 = thickness of soil layers 1, 2, and n – 1, respectively; and

    n = the number of soil layers that contain roots.

    It is convenient to combine Equations 2.12 and 2.14 to obtain:

    SWD = fd (TAW) (6.14)

    Another useful conversion is that

    TAW = AW + SWD (6.15)

    In Example 6.3, the irrigation system should water this location in the irrigated area within 2 days to prevent plant stress. If it will take 3 days to get there, irrigation will be 1 day late. Usually, a beginning or start position and an ending or stop position is designated within the irrigated area. A record should be kept of each position so that irrigation occurs before AD is exceeded at either position. An example of the starting and ending position for a center pivot system is illustrated in Figure 6.6.

    Example 6.3

    Field beans (Crop Group 3) are being grown in a fine sandy loam soil (AWC = 0.13 in/in). The feel and appearance method for determining soil water revealed that the average fr = 0.80 in the root zone. Determine the latest date for irrigatin. Assume that the root zone depth is 24 in, and ET of the unstressed crop is 0.3 in/d.

    Given: AWC = 0.13 in/in Rd = 2 ft = 24 in

    Current fd = 0.20 ETf = 0.3 in/d

    Find: LD

    Solution

    fdc = 0.40 (Table 6.1)

    TAW = (Rd)(AWC) (Eq. 6.2)

    TAW = (24 in)(0.13 in/in) = 3.1 in

    AD = fdc (TAW) (Eq. 6.4)

    AD = (0.40)(3.1 in) = 1.2 in

    SWD = fd (TAW) (Eq. 6.14)

    SWD = (0.2)(3.1 in) = 1.2 in

    \(LD= \dfrac{AD-SWD}{ET_f}\) (Eq. 6.10)

    \(LD = \dfrac {1.2\text{ in}-0.6\text{ in}}{0.3\text{ in/d}}=2\text{ d}\)

    Alternate solution:

    Since fdc = 0.40, frc = 0.60

    MB = frc (TAW) (Eq. 6.4)

    MB = (0.60)(24 in)(0.13 in/in) = 1.9 in

    AW = fr (AWC)Rd (Eq. 6.12b)

    AW = (0.80)(0.13 in/in)(24 in) = 2.5 in

    \(LD=\dfrac{AW-MB}{ET_f}\) (Eq. 6.11)

    \(LD=\dfrac {2.5\text{ in}-1.9\text{ in}}{0.3\text{ in/d}}=2\text{ d}\)

    Figure 6.6. Location of beginning and ending positions for a center pivot irrigation system.

    locationBeginning.jpeg

    Example 6.4

    Suppose in Example 6.3 that de = 0.5 in, ra = 0.4 in, and SWD = 0.6 in. Find the earliest date you should irrigate

    Given: de = 0.5 in

    ra = 0.4 in

    SWD = 0.6 in

    Find: ED

    Solution

    \(ED = \dfrac {r_a+d_{ep}-SWD}{ET_f}\) (Eq. 6.16)

    \(ED = \dfrac{0.4\text{ in} +0.5\text{ in} -0.6\text{ in}}{0.3\text{ in/d}} = 1\text{ d}\)

    Since the LD date was 2 d, irrigation should occur either 1 or 2 days from now.

    Figure 6.7. Illustration of the earliest date (ED) for irrigation concept.

    illustrationEarliestDate.png

    Another goal is not to irrigate too soon, i.e., not before the earliest date (ED). To avoid deep percolation there must be room in the root zone to store the planned effective depth of water, dep. In addition, in humid and semihumid regions, it is good management to allow room in the soil profile for storing rainfall that might occur immediately following the irrigation. This is called the rainfall allowance, ra. ED is calculated as:

    \(ED = \dfrac{r_a+d_{ep}-SWD}{ET_f}\) (6.16)

    \(ED = \dfrac{r_a+d_{ep}-(TAW-AW)}{ET_f}\) (6.17)

    The ED concept is illustrated in Figure 6.7. The concepts of TAW, MB, AW, AD, and SWD and how they change with time for an annual crop are shown in Figure 6.8. Note that one goal of irrigation scheduling is to keep the AW between TAW and MB.

    Figure 6.8. Illustration of key irrigation scheduling terms and their changes with time for annual crops.

    illustrationKeyIrrigation.jpeg

    6.4: Irrigation Scheduling for Soil Water Maintenance is shared under a CC BY-NC-ND 4.0 license and was authored, remixed, and/or curated by LibreTexts.