Kc for Initial Stage for Annual Crops

ET during the initial stage for annual crops is predominately in the form of evaporation. Therefore, accurate estimates for Kcini must consider the frequency that the soil surface is wetted during the initial period. Numerical procedures to compute Kcini are given below. Graphical procedures are presented in Doorenbos and Pruitt [55] and in Allen etal. [91].

Evaporation from bare soil, Es, can be characterized as occurring in two distinct stages [96,97]. Stage 1 is termed the "energy-limited" stage. During this stage, moisture is transported to the soil surface at a rate sufficient to supply the potential rate of evaporation (Eso), which is governed by energy availability at the soil surface. Eso (mm day-1) can be estimated from

where ET0 is the the mean ET0 during the initial period (mm day-1).

Stage 2 is termed the "soil-limited" stage, where hydraulic transport of subsurface water to the soil surface is unable to supply water at the potential evaporation rate. During stage 2, the soil surface appears partially dry and a portion of the evaporation occurs from below the soil surface. The energy required for subsurface evaporation is supplied by transport of heat from the soil surface into the soil profile. The evaporation rate during stage-2 drying decreases as soil moisture decreases (Fig. 5.6). It therefore can be expressed as being proportional to the water remaining in the evaporation layer relative to the maximum depth of water that can be evaporated from the same soil layer during stage-2 drying. The maximum total depth of water that can be evaporated from the surface soil layer is termed Wx. If the evaporation rate during stage-2 drying is assumed to be linearly proportional to the equivalent depth of water remaining in the evaporation layer, as shown in Fig. 5.6, then the average soil water evaporation rate can be estimated as proposed by Allen et al. [76].

Table 5.1. Time-averaged crop coefficients Kc, maximum plant heights h, range of maximum root depths Zr, and depletion fraction for no stress F^, well-managed crops in subhumid climates for use with the

FAO-PM ET0

Table 5.1. Time-averaged crop coefficients Kc, maximum plant heights h, range of maximum root depths Zr, and depletion fraction for no stress F^, well-managed crops in subhumid climates for use with the

FAO-PM ET0

Cropa

Kc inib

Kc mid

Kc end

Maximum Plant Height h (m)

Maximum Root Depth Z, (m)

F d ns

a. Small Vegetables

G.7

l.05

0.95

Carrots

1.05

0.95

0.3

0.5-1.0

0.35

Celery

1.05

1.00

0.6

0.3-0.5

0.20

Crucifers: cabbage, cauliflower,

1.05

0.95

0.4

0.4-0.6

0.40

broccoli, Brussels sprouts

Garlic

1.00

0.70

0.3

0.3-0.5

0.30

Lettuce

1.00

0.95

0.3

0.3-0.5

0.30

Onions, dry

1.05

0.75

0.4

0.3-0.6

0.30

Onions, green

1.00

1.00

0.3

0.3-0.6

0.30

Spinach

1.00

0.95

0.3

0.3-0.5

0.20

Radishes

0.90

0.85

0.3

0.3-0.5

0.30

b. Roots and Tubers

G.5

l.l0

0.95

Beets, table

1.05

0.95

0.4

0.6-1.0

0.50

Cassava, year 1

0.3

0.80

0.30

1.0

0.5-0.8

0.60

Cassava, year 2

0.3

1.10

0.50

1.5

0.7-1.0

0.60

Parsnips

0.5

1.05

0.95

0.4

0.5-1.0

0.40

Sugar beets

0.35

1.20

0.70

0.5

0.7-1.2

0.55

Sweet potatoes

1.15

0.65

0.5

1.0-1.5

0.65

Turnips (and rutabagas)

1.10

0.95

0.6

0.5-1.0

0.50

c. Legumes

0.4

l.l5

0.55

Beans, green

0.5

1.05

0.90

0.4

0.5-0.7

0.45

Beans, dry and pulses

0.4

1.15

0.30

0.4

0.6-0.9

0.45

Chick peas

1.00

0.35

0.4

0.6-1.0

0.50

Fava beans (broad beans), fresh

0.5

1.15

1.10

0.8

0.5-0.7

0.45

Fava beans, dry/seed

0.5

1.15

0.30

0.8

0.5-0.7

0.50

Garbanzos

0.4

1.15

0.30

0.8

0.6-1.0

0.50

Green gram and cowpeas

1.05

0.60-0.35

0.4

0.6-1.0

0.50

Groundnuts

1.15

0.60

0.4

0.5-1.0

0.50

Lentils

1.10

0.30

0.5

0.6-0.8

0.50

Peas, fresh

0.5

1.15

1.10

0.5

0.6-1.0

0.35

Peas, dry/seed

1.15

0.30

0.5

0.6-1.0

0.40

Soybeans

1.15

0.50

0.5-1.0

0.6-1.3

0.50

d. Solanum crops

0.6

l.l5

0.S0

Bell peppers, fresh

1.05

0.90

0.7

0.5-1.0

0.30

Eggplant

1.05

0.90

0.8

0.7-1.2

0.45

Potatoes

1.15

0.75-0.40

0.6

0.4-0.6

0.35

Tomatoes

1.15

0.70-0.90

0.6

0.7-1.5

0.40

e. Cucumber crops

G.5G

l.00

0.S0

Cantaloupe

0.50

0.85

0.60

0.3

0.9-1.5

0.45

Cucumber, fresh market

0.60

1.00

0.75

0.3

0.7-1.2

0.50

Cucumber, machine harvest

0.50

1.00

0.90

0.3

0.7-1.2

0.50

Melons

0.50

1.05

0.75

0.4

0.8-1.5

0.40

Pumpkin, winter squash

0.50

1.00

0.80

0.4

1.0-1.5

0.35

Squash (zuchini and crookneck)

0.50

0.95

0.75

0.3

0.6-1.0

0.50

Watermelon

0.40

1.00

0.75

0.4

0.8-1.5

0.40

(Cont.)

Table 5.1. (Continued)

Maximum

Maximum

K J

Plant Height Root Depth

F d ns

Cropa

Kc mid

Kc end

h (m)

£ (m)

f. Tropical fruits

Bananas, year 1

0.50

1.10

1.00

3.0

0.5-0.9

0.35

Bananas, year 2

1.00

1.20

1.10

4.0

0.5-0.9

0.35

Cacao

1.00

1.05

1.05

3.0

0.7-1.0

0.30

Coffee, bare soil

0.90

0.95

0.95

2.0-3.0

0.9-1.5

0.40

Coffee, with weeds

1.05

1.10

1.10

2.0-3.0

0.9-1.5

0.40

Dates

0.90

0.95

0.95

8.0

1.5-2.5

0.50

Palm trees

0.95

1.00

1.00

8.0

0.7-1.0

0.65

Pineapple, bare soil

0.50

0.30

0.30

0.6-1.2

0.3-0.6

0.50

Pineapples, vegetated soil

0.50

0.50

0.50

0.6-1.2

0.3-0.6

0.50

Rubber trees

0.95

1.00

1.00

10

1.0-2.0

0.60

Tea, nonshaded

0.95

1.00

1.00

1.5

0.9-1.5

0.40

Tea, shaded

1.10

1.15

1.15

2.0

0.9-1.5

0.45

g. Multiannual vegetables

(with initially bare soil)

Artichokes

0.50

1.00

0.95

0.7

0.6-0.9

0.45

Asparagus

0.50

0.95

0.30

0.2-0.8

1.2-1.8

0.45

Hops

0.30

1.05

0.85

5.0

1.0-1.2

0.50

h. Fiber crops

0.35

Cotton

1.15-1.20

0.70-0.50

1.2-1.5

1.0-1.7

0.60

Flax

1.10

0.25

1.2

1.0-1.5

0.50

Sisal

0.4-0.7

0.4-0.7

1.5

1.0-2.0

0.80

i. Oil crops

0.35

1.15

0.35

Castor beans (ricinus)

1.15

0.55

0.3

1.0-2.0

0.50

Rapeseed, canola

1.0-1.15

0.35

0.6

1.0-1.5

0.60

Safflower

1.0-1.15

0.25

0.8

1.0-2.0

0.60

Sesame

1.10

0.25

1.0

1.0-1.5

0.60

Sunflower

1.0-1.15

0.35

2.0

0.8-1.5

0.45

j. Cereals

0.30

1.15

0.40

Barley

1.15

0.25

1.0

1.0-1.5

0.55

Oats

1.15

0.25

1.0

1.0-1.5

0.55

Wheat

1.15

0.25-0.40

1.0

1.0-1.5

0.55

Winter wheat

0.4-0.7

1.15

0.25-0.40

1.0

1.0-1.8

0.55

Maize field (grain) (field corn)

1.20

0.60-0.35

2.2

1.0-1.7

0.55

Maize, sweet (sweet corn)

1.15

1.05

1.5

0.8-1.2

0.50

Millet

1.00

0.30

1.5

1.0-2.0

0.55

Sorghum, grain

1.00-1.10

0.55

1.0-2.0

1.0-2.0

0.55

Sorghum, sweet

1.20

1.05

2.0-4.0

1.0-2.0

0.50

Rice

1.05

1.20

0.90-0.60

1.0

0.5-1.0

0.20e

k. Forages

Alfalfa hayf

0.40

1.20

1.15

0.7

1.0-2.0

0.55

Clover hayf, berseem

0.40

1.15

1.10

0.6

0.6-0.9

0.50

Grazing pasture, rotated grazing

0.40

0.85-1.05

0.85

0.15

0.5-1.5

0.60

Grazing pasture, extensive grazing

0.30

0.75

0.75

0.1

0.5-1.5

0.65

Turf grass, cool season

0.90

0.95

0.95

0.1

0.5-1.0

0.40

Turf grass, warm season

0.80

0.85

0.85

0.1

0.5-1.0

0.50

Bermuda spring crop for seed

0.35

0.90

0.65

0.4

1.0-1.5

0.60

(Cont.)

Table 5.1. (Continued)

Maximum

Maximum

Kc lmb

Plant Height Root Depth

F d ns

Cropa

Kc mid

Kc end

h (m)

£ (m)

Bermuda, summer for hayf

0.55

1.00

0.85

0.35

1.0-1.5

0.55

Rye grass, for hayf

0.50

1.05

1.00

0.35

0.5-1.0

0.60

Sudan grass for hayf

0.50

1.15

1.10

0.8-1.2

1.0-1.5

0.60

l. Sugar Cane

0.40

1.25

0.75

3-4

1.2-2.0

0.65

m. Nonwoody perennials

Mint

0.60

1.15

1.10

0.6

0.4-0.8

0.40

Strawberries

0.40

0.85

0.75

0.2

0.2-0.3

0.20

n. Grapes and berries

Berries (bushes)

0.30

1.05

0.50

1.5

0.6-1.2

0.50

Grapes for table

0.30

0.85

0.45

2.0

1.0-2.0

0.35

Grapes for wine

0.30

0.70

0.45

1.5-2.0

1.0-2.0

0.60

o. Fruit trees

Almonds, no ground coverg

0.40

0.90

0.65

4.0

1.0-2.0

0.60

Avocado, no ground cover

0.60

0.85

0.75

3.0

0.5-1.0

0.70

Citrus, no ground cover h

70% canopy

0.70

0.65

0.70

4.0

1.2-1.5

0.50.

50% canopy

0.65

0.60

0.65

3.0

1.1-1.5

0.50

20% canopy

0.50

0.45

0.55

2.0

0.8-1.1

0.50

Citrus, with active ground cover or weedsg

70% canopy

0.75

0.70

0.75

4.0

1.2-1.5

0.50

50% canopy

0.80

0.80

0.80

3.0

1.1-1.5

0.50

20% canopy

0.85

0.85

0.85

2.0

0.8-1.1

0.50

Conifer trees

1.00

1.00

1.00

10.0

1.0-1.5

0.70

Deciduous orchard, killing frost

Apples, pears, cherries, no ground cover

0.45

0.95

0.70

4.0

1.0-2.0

0.50

Peaches, stone fruit, no ground cover

0.45

0.90

0.65

3.0

1.0-2.0

0.50

Apples, pears, cherries, active ground cover

0.50

1.20

0.95

4.0

1.0-2.0

0.50

Peaches, stone fruit, active ground cover

0.50

1.15

0.90

3.0

1.0-2.0

0.50

Deciduous orchard, no killing frost

Apples, pears, cherries, no ground cover

0.60

0.95

0.75

4.0

1.0-2.0

0.50

Peaches, stone fruit, no ground cover

0.55

0.90

0.65

3.0

1.0-2.0

0.50

Apples, pears, cherries, active ground cover

0.80

1.20

0.85

4.0

1.0-2.0

0.50

Peaches, stone fruit, active ground cover

0.80

1.15

0.85

3.0

1.0-2.0

0.50

Kiwi

0.40

1.05

1.05

3.0

0.7-1.3

0.35

Olives (40%—60% ground coverage by canopy)

0.65

0.70

0.70

3.0-5.0

1.2-1.7

0.70

Pistachios, no ground cover

0.40

1.10

0.45

3.0-5.0

1.0-1.5

0.60

Walnut orchard

0.50

1.10

0.65

4.0-5.0

1.7-2.4

0.50

p. Wetlands, temperate climate

Cattails, bulrushes, killing frost

0.30

1.20

0.30

2.0

Cattails, bulrushes, no frost

0.60

1.20

0.60

2.0

Short vegetation, no frost

1.05

1.10

1.10

0.3

Reed swamp, standing water

1.00

1.20

1.00

1-3

Reed swamp, moist soil

0.90

1.20

0.70

1-3

a When stand density, height, or leaf area are less than that attained under pristine conditions, the value for Kcmid and, for most crops, for Kcend should be reduced from 0.1 up to 0.5 according to the amount of effective, active leaf area relative to that for healthy vegetation under potential, pristine growing conditions.

Maximum plant heights are indicative and should be adapted for local conditions. Smaller values for maximum root depths refer to irrigated crops and the large ones to rainfed conditions, when the soil is nonrestrictive to root development. b These are general values under typical irrigation management and soil wetting. For frequent wettings, such as with high-frequency sprinkle irrigation or rainfall, these values may increase substantially. Appropriate computation procedures are given in Section " Kc for Initial Stage for Annual Crops". c Values represent those for a semihumid climate (^^min ^ 45%) with moderate wind speed (averaging 2 ms-1). For more humid or arid conditions or for more or less windy conditions, values should be modified as described in Section " Kc for Midseason and End of Season". d Fraction of available water that can be depleted before physiological, soil, and climatic influences reduce plant transpiration because of moisture stress. Fns is similar to the term "management-allowed depletion" (MAD). However, the values for MAD are influenced by management and economic factors in addition to physical factors. Generally, MAD < Fns when there is risk aversion or uncertainty, and MAD > Fns when plant moisture stress is an intentional part of soil water management. Values listed for Fns are for ETC in the range of 5 to 6 mm day-1 and for nonsaline conditions. For ETc outside this range, FAO [3, 90] suggests increasing Fns for ETc < 5 mm day-1 and decreasing Fns for ETc > 6 mm day-1. The modifications to Fns can be characterized by the equation Fns = Fns tab + 0.04 (5 - ETc), where Fns tab is the Fns value listed in the table and ETc is the estimated crop ET (mm day-1), from Eqs. (5.52) and (5.53) (both calculated using Ks = 1) and Eqs. (5.54). e Fns for rice is 0.20 of saturation [90].

f The three coefficients for hay crops represent the periods immediately following cutting, during full cover, and immediately before cutting, respectively. g Kc end values are Kc prior to leaf drop. After leaf drop Kc end = 0.20 for bare or dead ground cover, and

Kc end = 0.50 to 0.80 for actively growing ground cover. h For humid and subhumid climates, where there is less stomatal control, the Kc should be increased by 0.1-0.2.

Figure 5.6. Two-stage model for soil evaporation. Source: Adapted from [96].

The basic equation for Kc^ is

tw ETr:

for tw > t1, where W\ is the cumulative depth (mm) of soil water evaporation when stage-1 drying is complete, Wx is the cumulative depth (mm) of soil water evaporation when soil evaporation effectively ceases (i.e., Wx = maximum potential soil water that can be evaporated from the soil surface layer between two wetting events), Eso is the potential evaporation rate during stage 1 (mmday-1), tw is the mean interval between wetting events (days), and t1 is the time when stage-1 drying is completed (t1 = W1 /Eso) (days). When tw < t1, that is, the entire process resides within stage 1, then

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