Kc ini EsoETo557

Where furrow or trickle irrigation is practiced, and only a portion of the soil surface is wetted, the value calculated for Kcini in Eqs. (5.56) and (5.57) should be reduced in proportion to the average fraction of surface wetted, fw (0.2-1.0). Indicative values for fw are given in [91] and [98]: fw = 1.0 for rain, sprinkling, and basin and border irrigation; fw = 0.4 to 0.6 for furrows; fw = 0.3 to 0.4 for irrigation with alternate furrows; fw = 0.2 to 0.5 for trickle irrigation.

The value for the infiltration depth from irrigation (In) used in the following procedure also should be adjusted accordingly:

where In is the equivalent depth (mm) of infiltration from irrigation over the fw fraction of the surface wetted, and Ins is the depth (mm) of infiltration from irrigation, expressed as one-dimensional depth over the entire surface area.

W1 (Fig. 5.6), is generally highest for medium-textured soils with high moisture retention and moderate hydraulic conductivity, and is lower for coarse soils having low moisture-retention capacity and for clays that have low hydraulic conductivity. Maximum values for W1 (termed W1 max) are listed by Allen etal. [91]. Ritchie etal. [99] predicted potential values of W1 (mm) according to soil texture:

W1 max = 8 + 0.08 (Cl) for Sa < 80 and Cl < 50%, where Sa and Cl are the percentages of sand and clay in the soil.

The value for Wx is the maximum depth of water that can be evaporated from the soil following wetting. The value for Wx is governed by the depth of the soil profile contributing to soil water evaporation, by the soil water-holding properties within the evaporating layer, by the unsaturated hydraulic conductivity, by the presence of a hy-draulically limiting layer beneath the evaporating layer, by the conduction of sensible heat into the soil to supply energy for subsurface evaporation, and by any root extraction of water from the soil layer. During winter and other cool season months, less radiation energy is available to penetrate the soil surface and to evaporate water from within a drying soil. An approximation for the maximum value of Wx for initial periods having ET0 > 5 mm day-1 is

Table 5.2. Typical values for #ul, $LL, and Oam and sand, silt, and clay percentages for general soil classifications

MeanValuesof SoilWater Soil Texturesb

Table 5.2. Typical values for #ul, $LL, and Oam and sand, silt, and clay percentages for general soil classifications

MeanValuesof SoilWater Soil Texturesb

Soil Classification

$UL

0LL

Sand

Silt

Clay

Sand

0.12

0.04

0.08

92

4

4

Loamy sand

0.14

0.06

0.08

84

6

10

Sandy loam

0.23

0.10

0.13

65

25

10

Loam

0.26

0.12

0.15

40

40

20

Silt loam

0.30

0.15

0.15

20

65

15

Silt

0.32

0.15

0.17

7

88

5

Silty clay

0.34

0.19

0.15

10

55

35

Loam

Silty clay

0.36

0.21

0.15

8

47

45

Clay

0.36

0.21

0.15

22

20

58

a Available moisture, #aM = $Ul — $LL. b From the SCS soil texture triangle. Source: [2].

a Available moisture, #aM = $Ul — $LL. b From the SCS soil texture triangle. Source: [2].

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