CIVE 633 - ENVIRONMENTAL HYDROLOGY

MANUAL 71 LECTURE 02, CHAPTERS 12 AND 13

LEACHING REQUIREMENT [243]


  • The leaching fraction is the ratio of depth of drainage to depth of applied water (irrigation plus rainfall).

  • The leaching fraction is the ratio of salt content of applied water to salt content of drain water.

  • The leaching fraction is the ratio of electrical conductivity of applied water to electrical conductivity of drain water.

  • Fig. 1 shows that relationship between the salinity of applied water (Ca), in dS/m, and the crop salt tolerance threshold value (Ct), in dS/m.

  • For example, assume tomatoes are to be grown in an arid region, and irrigated with water of ECi = 3 dS/m. Assume rainfall to be insignificant.

  • The salt tolerance threshold value of tomatoes is Ct = 2.5 dS/m.

  • From Fig. 1, with ECi = 3 dS/m, and Ct = 2.5 dS/m, the leaching requirement Lr = 0.2.

  • This means that 20% of the applied water will have to drain.

  • That means that if PET = 100 units, the amount applied will have to be 125 units, 25 units will drain, and 25/125 will be Lr = 0.2.



YIELD RESPONSE FUNCTIONS [268]


  • The yield response function is expressed as follows:

    Yr = [100 - b(EC - a)]/100    [%]

    or

    Yr = 1 - (b/100)(EC - a)    [ratio]

    where

    Yr = relative yield, in % or ratio;

    a = salt tolerance threshold value, applicable to a given crop, in dS/m;

    b = slope of yield reduction line, expressed in % reduction per dS/m, applicable to a given crop; and

    EC = electrical conductivity of a saturated soil extract taken from the root zone.


  • For instance, if a crop has a = 4 and b = 20, the relative yield would be:

    At EC ≤ 4 dS/m:   Yr = 1.0

    At EC = 5 dS/m:   Yr = 0.8

    At EC = 6 dS/m:   Yr = 0.6

    At EC = 7 dS/m:   Yr = 0.4

    At EC = 8 dS/m:   Yr = 0.2

    At EC = 9 dS/m:   Yr = 0.0

  • For instance, if a crop has a = 1.7 and b = 12, the relative yield would be:

    At EC ≤ 1.7 dS/m:   Yr = 1.0

    At EC = 2 dS/m:   Yr = 1.0 - (12 × 0.3)/100 = 0.964

    At EC = 3 dS/m:   Yr = 0.844

    At EC = 4 dS/m:   Yr = 0.724

    At EC = 5 dS/m:   Yr = 0.604

    At EC = 6 dS/m:   Yr = 0.484

    At EC = 7 dS/m:   Yr = 0.364

    At EC = 8 dS/m:   Yr = 0.244

    At EC = 9 dS/m:   Yr = 0.124

    At EC = 10 dS/m:   Yr = 0.004





    Table 1.   Salt tolerance parameters of herbaceous crops:
    Fiber and grains
    Crop a bRating
    Barley 8.05.0T
    Bean 1.019.0S
    Broad bean1.69.6MS
    Corn 1.712.0MS
    Cotton 7.75.2T
    Cowpea 4.912.0MT
    Flax 1.712.0MS
    Peanut 3.229.0MS
    Rice 3.012.0S
    Rye 11.410.8T
    Sorghum 6.816.0MT
    Soybean 5.020.0MT
    Sugar beet7.05.9T
    Sugarcane 1.75.9MS
    Wheat 6.07.1MT
    Wheat (semidwarf) 8.63.0T
    Wheat (durum) 5.93.8T


    Table 2.   Salt tolerance parameters of herbaceous crops:
    Vegetables and fruits
    Crop a bRating
    Asparragus 4.1 2.0T
    Bean 1.0 19.0S
    Beet, red 4.0 9.0MT
    Broccoli 2.8 9.2MS
    Cabbage 1.8 9.7MS
    Carrot 1.0 14.0S
    Celery 1.8 6.2MS
    Cucumber 2.5 13.0MS
    Eggplant 1.1 6.9MS
    Lettuce 1.3 13.0MS
    Onion 1.6 16.0S
    Pepper 1.5 4.0MS
    Potato 1.7 12.0MS
    Radish 1.2 13.0MS
    Spinach 2.0 7.6MS
    Squash, scallop 3.2 16.0MS
    Squash, zucchini 4.7 9.4MT
    Strawberry 1.0 33.0S
    Sweet potato 1.5 11.0MS
    Tomato 2.5 9.9MS
    Turnip 0.9 9.0MS


    Table 3.   Salt tolerance parameters of woody crops
    Crop a bRating
    Almond 1.5 19.0S
    Apricot 1.6 24.0S
    Blackberry 1.5 22.0S
    Boisenberry 1.5 22.0S
    Date palm 4.0 3.6T
    Grape 1.5 9.6MS
    Grapefruit 1.8 16.0S
    Orange 1.7 16.0S
    Peach 1.7 21.0S
    Plum, prune 1.5 18.0S

 
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