What Is the Primary Factor in Determining How Much Moisture a Soil Can Hold?

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Chapter 8 - ET_c under soil water stress conditions

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Soil water availability
Water stress coefficient (K_southward)
Soil water balance
Forecasting or allocating irrigations
Furnishings of soil salinity
Yield-salinity human relationship
Yield-moisture stress relationship
Combined salinity-ET reduction relationship
Application

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Forces interim on the soil h2o subtract its potential energy and go far less available for plant root extraction. When the soil is wet, the water has a high potential energy, is relatively free to motion and is easily taken upward by the constitute roots. In dry out soils, the water has a low potential free energy and is strongly bound past capillary and absorptive forces to the soil matrix, and is less easily extracted past the crop.

When the potential energy of the soil water drops below a threshold value, the crop is said to be water stressed. The effects of soil water stress are described by multiplying the basal crop coefficient by the water stress coefficient, Chiliad_s:

ET_{c adj} = (K_s 1000_cb + K_e) ET_o (lxxx)

For soil water limiting conditions, Yard_s < 1. Where at that place is no soil water stress, 1000_southward = one.

One thousand_s describes the effect of h2o stress on crop transpiration. Where the single crop coefficient is used, the result of water stress is incorporated into K_c as:

ET_{c adj} = K_south K_c ET_o (81)

Because the water stress coefficient impacts only ingather transpiration, rather than evaporation from soil, the application of M_south using Equation fourscore is generally more valid than is application using Equation 81. However, in situations where evaporation from soil is not a large component of ET_c, use of Equation 81 will provide reasonable results.

Soil water availability

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Full available water (TAW)
Readily available water (RAW)

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Total available h2o (TAW)

Soil water availability refers to the capacity of a soil to retain h2o bachelor to plants. After heavy rainfall or irrigation, the soil will bleed until field capacity is reached. Field chapters is the amount of water that a well-drained soil should hold against gravitational forces, or the corporeality of water remaining when downward drainage has markedly decreased. In the absence of water supply, the water content in the root zone decreases every bit a result of water uptake by the ingather. As water uptake progresses, the remaining water is held to the soil particles with greater forcefulness, lowering its potential energy and making information technology more than difficult for the institute to extract it. Eventually, a bespeak is reached where the crop tin can no longer extract the remaining water. The h2o uptake becomes zero when wilting betoken is reached. Wilting point is the water content at which plants volition permanently wilt.

As the water content above field capacity cannot be held confronting the forces of gravity and will drain and every bit the water content below wilting indicate cannot be extracted past plant roots, the total available h2o in the root zone is the difference between the water content at field capacity and wilting point:

TAW = g(q _FC - q _WP) Z_r (82)

where

TAW the total available soil h2o in the root zone [mm],
q _FC the water content at field capacity [m³ m^-3],
q _WP the water content at wilting point [m³ m^-3],
Z_r the rooting depth [m].

TAW is the corporeality of h2o that a crop tin excerpt from its root zone, and its magnitude depends on the type of soil and the rooting depth. Typical ranges for field chapters and wilting point are listed in Table xix for diverse soil texture classes. Ranges of the maximum effective rooting depth for various crops are given in Table 22.

Readily available water (RAW)

Although water is theoretically available until wilting point, crop water uptake is reduced well before wilting indicate is reached. Where the soil is sufficiently moisture, the soil supplies water fast enough to meet the atmospheric need of the crop, and water uptake equals ET_c. Every bit the soil h2o content decreases, water becomes more strongly jump to the soil matrix and is more difficult to extract. When the soil h2o content drops below a threshold value, soil water can no longer be transported speedily plenty towards the roots to answer to the transpiration need and the ingather begins to experience stress. The fraction of TAW that a ingather tin can extract from the root zone without suffering h2o stress is the readily available soil water:

RAW = p TAW (83)

where

RAW the readily available soil water in the root zone [mm],
p average fraction of Full Available Soil Water (TAW) that can be depleted from the root zone before moisture stress (reduction in ET) occurs [0-1].

Values for p are listed in Table 22. The factor p differs from one crop to another. The factor p unremarkably varies from 0.30 for shallow rooted plants at loftier rates of ET_c (> 8 mm d^-one) to 0.70 for deep rooted plants at depression rates of ET_c (< 3 mm d^-one). A value of 0.50 for p is commonly used for many crops.

The fraction p is a role of the evaporation power of the atmosphere. At low rates of ET_c, the p values listed in Table 22 are higher than at high rates of ET_c. For hot dry weather conditions, where ET_c is loftier, p is ten-25% less than the values presented in Table 22, and the soil is yet relatively wet when the stress starts to occur. When the crop evapotranspiration is low, p volition be up to 20% more than the listed values. Often, a constant value is used for p for a specific growing period, rather than varying the value each 24-hour interval. A numerical approximation for adjusting p for ET_c rate is p = p_{Table 22} + 0.04 (5 - ET_c) where the adjusted p is limited to 0.ane £ p £ 0.eight and ET_c is in mm/twenty-four hour period. The influence of the numerical adjustment is shown in Figure 41.

TABLE 22. Ranges of maximum constructive rooting depth (Z _r ), and soil water depletion fraction for no stress (p), for common crops

Ingather		Maximum Root Depth 1 (m)	Depletion Fraction 2 (for ET » 5 mm/day) p
a. Minor Vegetables
Broccoli		0.4-0.6	0.45
Brussel Sprouts		0.four-0.6	0.45
Cabbage		0.5-0.8	0.45
Carrots		0.5-1.0	0.35
Cauliflower		0.4-0.7	0.45
Celery		0.three-0.5	0.20
Garlic		0.3-0.v	0.30
Lettuce		0.3-0.5	0.30
Onions
	- dry	0.3-0.6	0.xxx
	- light-green	0.3-0.six	0.30
	- seed	0.iii-0.6	0.35
Spinach		0.iii-0.v	0.twenty
Radishes		0.3-0.5	0.30
b. Vegetables - Solarium Family unit (Solanaceae)
Egg Institute		0.vii-1.2	0.45
Sugariness Peppers (bell)		0.5-1.0	0.30
Love apple		0.7-1.5	0.forty
c. Vegetables - Cucumber Family ( Cucurbitaceae )
Cantaloupe		0.9-1.five	0.45
Cucumber
	- Fresh Market	0.vii-1.2	0.l
	- Car harvest	0.seven-1.two	0.50
Pumpkin, Winter Squash		1.0-1.v	0.35
Squash, Zucchini		0.six-1.0	0.fifty
Sweetness Melons		0.8-1.five	0.40
Watermelon		0.viii-1.5	0.forty
d. Roots and Tubers
Beets, tabular array		0.6-1.0	0.fifty
Cassava
	- year 1	0.5-0.8	0.35
	- twelvemonth two	0.vii-1.0	0.40
Parsnip		0.5-one.0	0.twoscore
Potato		0.4-0.vi	0.35
Sweet Potato		1.0-ane.5	0.65
Turnip (and Rutabaga)		0.5-one.0	0.50
Sugar Beet		0.7-one.2	0.55iii
e. Legumes (Leguminosae)
Beans, green		0.5-0.seven	0.45
Beans, dry and Pulses		0.6-0.nine	0.45
Beans, lima, large vines		0.eight-ane.2	0.45
Chick pea		0.half-dozen-1.0	0.50
Fababean (broad edible bean)
	- Fresh	0.v-0.7	0.45
	- Dry out/Seed	0.v-0.vii	0.45
Grabanzo		0.6-1.0	0.45
Green Gram and Cowpeas		0.6-ane.0	0.45
Groundnut (Peanut)		0.five-i.0	0.fifty
Lentil		0.6-0.8	0.50
Peas
	- Fresh	0.6-1.0	0.35
	- Dry/Seed	0.half dozen-1.0	0.40
Soybeans		0.vi-1.3	0.l
f. Perennial Vegetables (with winter dormancy and initially blank or mulched soil)
Artichokes		0.6-0.9	0.45
Asparagus		1.ii-1.eight	0.45
Mint		0.four-0.8	0.40
Strawberries		0.two-0.3	0.20
g. Fibre Crops
Cotton		ane.0-1.seven	0.65
Flax		1.0-1.v	0.50
Sisal		0.five-1.0	0.80
h. Oil Crops
Castorbean (Ricinus)		1.0-2.0	0.fifty
Rapeseed, Canola		ane.0-1.v	0.lx
Safflower		1.0-2.0	0.lx
Sesame		1.0-1.five	0.60
Sunflower		0.eight-1.5	0.45
i. Cereals
Barley		1.0-one.five	0.55
Oats		ane.0-1.v	0.55
Jump Wheat		ane.0-1.5	0.55
Winter Wheat		ane.5-one.8	0.55
Maize, Field (grain) (field corn)		ane.0-1.vii	0.55
Maize, Sweet (sweet corn)		0.8-i.2	0.50
Millet		one.0-2.0	0.55
Sorghum
	- grain	1.0-two.0	0.55
	- sweet	1.0-2.0	0.50
Rice		0.five-1.0	0.204
j. Forages
Alfalfa
	- for hay	1.0-2.0	0.55
	- for seed	1.0-three.0	0.threescore
Bermuda
	- for hay	i.0-one.5	0.55
	- Spring crop for seed	1.0-1.5	0.60
Clover hay, Berseem		0.6-0.9	0.50
Rye Grass hay		0.six-1.0	0.lx
Sudan Grass hay (almanac)		1.0-1.five	0.55
Grazing Pasture
	- Rotated Grazing	0.5-i.five	0.sixty
	- Extensive Grazing	0.5-ane.5	0.threescore
Turf grass
	- cool season five	0.v-1.0	0.40
	- warm flavour 5	0.5-1.0	0.50
yard. Sugar Cane		1.2-two.0	0.65
l. Tropical Fruits and Trees
Banana
	- 1st year	0.5-0.ix	0.35
	- 2nd yr	0.5-0.9	0.35
Cacao		0.7-one.0	0.xxx
Coffee		0.9-i.five	0.twoscore
Appointment Palms		1.5-2.five	0.50
Palm Copse		0.7-1.1	0.65
Pineapple		0.3-0.6	0.l
Rubber Copse		1.0-1.5	0.xl
Tea
	- non-shaded	0.ix-1.5	0.40
	- shaded	0.9-i.v	0.45
m. Grapes and Berries
Berries (bushes)		0.6-1.two	0.fifty
Grapes
	- Table or Raisin	i.0-2.0	0.35
	- Wine	i.0-2.0	0.45
Hops		one.0-1.two	0.50
n. Fruit Trees
Almonds		1.0-ii.0	0.40
Apples, Cherries, Pears		1.0-2.0	0.50
Apricots, Peaches, Stone Fruit		i.0-2.0	0.fifty
Avocado		0.5-1.0	0.lxx
Citrus
	- seventy% canopy	1.2-1.five	0.l
	- fifty% awning	1.1-1.5	0.50
	- 20% awning	0.8-ane.1	0.50
Conifer Copse		1.0-1.5	0.lxx
Kiwi		0.7-ane.3	0.35
Olives (40 to 60% ground coverage by canopy)		1.2-i.7	0.65
Pistachios		1.0-1.5	0.40
Walnut Orchard		1.7-2.4	0.l

^one The larger values for Z_r are for soils having no significant layering or other characteristics that can restrict rooting depth. The smaller values for Z_r may be used for irrigation scheduling and the larger values for modeling soil h2o stress or for rainfed conditions.

² The values for p apply for ET_c » 5 mm/24-hour interval. The value for p can be adjusted for dissimilar ET_c according to

p = p _{tabular array 22} + 0.04 (five - ET_c)

where p is expressed as a fraction and ET_c as mm/twenty-four hour period.

ⁱⁱⁱ Sugar beets often experience late afternoon wilting in arid climates even at p < 0.55, with usually simply pocket-size impact on saccharide yield.

⁴ The value for p for rice is 0.twenty of saturation.

⁵ Cool season grass varieties include bluegrass, ryegrass and fescue. Warm season varieties include bermuda grass, buffalo grass and St. Augustine grass. Grasses are variable in rooting depth. Some root below 1.2 1000 while others take shallow rooting depths. The deeper rooting depths for grasses represent conditions where careful water management is skillful with higher depletion between irrigations to encourage the deeper root exploration.

FIGURE 41. Depletion factor for different levels of ingather evapotranspiration

Instance 36. Conclusion of readily available soil water for diverse crops and soil types

Approximate RAW for a full-grown onion, lycopersicon esculentum and maize crop. Presume that the crops are cultivated on loamy sand, silt and silty clay soils.
From Table 22	Onion		Zr » 0.four m		p = 0.xxx
	Tomato		Zr » 0.8 yard		p = 0.40
	Maize		Zr » 1.2 thousand		p = 0.55
From Table 19	Loamy sand		q FC » 0.xv g3 g-3		q WP » 0.06 m3 m-iii
	1000 (q FC - q WP) = 90 mm(water)/m(soil depth)
	Silt		q FC » 0.32 mthree m-3		q WP » 0.15 grand3 m-3
	1000 (q FC - q WP) = 170 mm(water)/m(soil depth)
	Silty dirt		q FC » 0.35 k3 yard-3		q WP » 0.23 m3 grand-3
	1000 (q FC - q WP) = 120 mm(h2o)/m(soil depth)
	Loamy sand		Silt		Silty clay
	TAW (Eq. 82) mm	RAW (Eq. 83) mm	TAW (Eq. 82) mm	RAW (Eq. 83) mm	TAW (Eq. 82) mm	RAW (Eq. 83) mm
Onion	36	11	68	20	48	14
Tomato plant	72	29	136	54	96	38
Maize	108	59	204	112	144	79

To express the tolerance of crops to h2o stress as a function of the fraction (p) of TAW is not wholly correct. The rate of root h2o uptake is in fact influenced more directly by the potential energy level of the soil water (soil matric potential and the associated hydraulic conductivity) than by water content. As a certain soil matric potential corresponds in different soil types with different soil h2o contents, the value for p is also a function of the soil type. Generally, information technology tin can be stated that for fine textured soils (clay) the p values listed in Table 22 can be reduced by 5-ten%, while for more coarse textured soils (sand), they can exist increased by v-x%.

RAW is similar to the term Management Allowed Depletion (MAD) introduced by Merriam Even so, values for MAD are influenced by management and economical factors in addition to the physical factors influencing p. Generally, MAD < RAW where there is risk aversion or uncertainty, and MAD > RAW where plant wet stress is an intentional part of soil water management.

Water stress coefficient (Yard_s)

The furnishings of soil water stress on ingather ET are described by reducing the value for the crop coefficient. This is accomplished by multiplying the crop coefficient by the water stress coefficient. One thousand_southward (Equations 80 and 81).

H2o content in me root zone can as well be expressed by root zone depletion, D_r, i.e., h2o shortage relative to field capacity. At field chapters, the root zone depletion is zero (D_r = 0). When soil water is extracted by evapotranspiration, the depletion increases and stress will be induced when D_r becomes equal to RAW. After the root zone depletion exceeds RAW (the water content drops below the threshold q _t), the root zone depletion is high enough to limit evapotranspiration to less than potential values and the crop evapotranspiration begins to decrease in proportion to the amount of h2o remaining in the root zone (Figure 42).

Figure 42. H2o stress coefficient, Grand _south

EXAMPLE 37. Result of water stress on crop evapotranspiration

Estimate the consequence of water stress on the evapotranspiration of a full grown lycopersicon esculentum crop (Zr = 0.eight thou and p = 0.40) cultivated on a silty soil (q FC = 0.32 and q WP = 0.12 miii m-iii) for the coming ten days when the initial root zone depletion is 55 mm and neither rain nor irrigations are either forecasted or planned. The expected ETo for the coming decade is 5 mm/day and Mc = one.2.
From Eq. 82		TAW = 1000 (0.32-0.12) 0.8 = 160 mm
From Eq. 83		RAW = 0.twoscore (160) = 64 mm
(ane)	(2)	(3)	(4)	(5)	(6)	(7)	(viii)
Day	ET o	K c	ET c	D r, i start	K s	ET c adj	D r, i end
	mm/day		mm/day	mm		mm/solar day	mm
offset	-	-	-	-	-	-	55.0
ane	five.0	1.ii	6.0	55.0	1.00	six.0	61.0
2	five.0	1.2	6.0	61.0	i.00	6.0	67.0
three	5.0	i.ii	six.0	67.0	0.97	5.8	72.eight
iv	5.0	1.two	6.0	72.8	0.91	five.4	78.3
5	5.0	one.2	6.0	78.three	0.85	v.one	83.4
6	5.0	1.2	6.0	83.4	0.80	4.8	88.2
7	five.0	1.2	6.0	88.2	0.75	4.5	92.half-dozen
8	5.0	1.ii	vi.0	92.half-dozen	0.70	4.2	96.9
9	5.0	1.two	6.0	96.nine	0.66	3.nine	100.eight
10	5.0	ane.two	6.0	100.8	0.62	3.vii	104.5
(one)	Mean solar day number.
(two)	Reference crop evapotranspiration.
(three)	Crop coefficient.
(iv)	Eq. 58, crop ET if no water stress.
(5)	Root zone depletion at the get-go of the day (cavalcade 8 of previous solar day),
(6)	Eq. 84 where Gdue south = i if Dr, i < RAW.
(7)	Eq. 81, crop ET nether soil water stress conditions.
(eight)	Depletion at end of day.
The case demonstrates that the approximate of Yarddue south requires a daily water residue adding. This is developed further in the side by side section.

Figure 43. Water balance of the root zone

For D_r > RAW, M_s is given by:

(84)

where

1000_southward is a dimensionless transpiration reduction factor dependent on available soil water [0 - 1],
D_r root zone depletion [mm],
TAW total bachelor soil water in the root zone [mm],
p fraction of TAW that a crop tin can excerpt from the root zone without suffering water stress [-].

After the computation of 1000_s, the adjusted evapotranspiration ET_{c adj} is computed by ways of Equation 80 or 81, depending on the coefficients used to describe ingather evapotranspiration. When the root zone depletion is smaller than RAW, K_s = 1.

Soil h2o balance

The interpretation of Grand_s requires a daily water balance ciphering for the root zone. Schematically (Effigy 43), the root zone can exist presented past means of a container in which the water content may fluctuate. To express the h2o content equally root zone depletion is useful. It makes the adding and subtracting of losses and gains straightforward equally the various parameters of the soil water budget are commonly expressed in terms of h2o depth. Rainfall, irrigation and capillary ascent of groundwater towards the root zone add h2o to the root zone and decrease the root zone depletion. Soil evaporation, crop transpiration and percolation losses remove water from the root zone and increase the depletion. The daily h2o balance, expressed in terms of depletion at the finish of the day is:

D_{r, i} = D_{r, i-1} - (P - RO)_i - I_i - CR_i + ET_{c, i} + DP_i (85)

where

D_{r, i} root zone depletion at the end of day i [mm],
D_{r, i-1} water content in the root zone at the end of the previous day, i-1 [mm],
P_i precipitation on day i [mm],
RO_i runoff from the soil surface on twenty-four hours i [mm],
I_i internet irrigation depth on day i that infiltrates the soil [mm],
CR_i capillary rise from the groundwater table on solar day i [mm],
ET_{c, i} crop evapotranspiration on solar day i [mm],
DP_i water loss out of the root zone by deep percolation on day i [mm].

Limits on D _{r, i}

In Figure 43 it is assumed that water can exist stored in the root zone until field capacity is reached. Although following heavy rain or irrigation the h2o content might temporally exceed field capacity, the total amount of h2o above field chapters is assumed to be lost the same day by deep percolation, following any ET for that solar day. By assuming that the root zone is at field capacity following heavy rain or irrigation, the minimum value for the depletion D_{r, i} is goose egg. As a issue of percolation and evapotranspiration, the water content in the root zone will gradually subtract and the root zone depletion will increase. In the absence of any wetting event, the water content will steadily reach its minimum value q _WP. At that moment no h2o is left for evapotranspiration in the root zone, K_{due south} becomes zero, and the root zone depletion has reached its maximum value TAW. The limits imposed on D_{r, i} are consequently:

0 £ D_{r, i} £ TAW (86)

Initial depletion

To initiate the h2o remainder for the root zone, the initial depletion D_{r, i-1} should be estimated. The initial depletion can exist derived from measured soil h2o content by:

D_{r, i-1} = one thousand(q _FC - q _i-ane) Z_r (87)

where q _i-1 is the average soil h2o content for the effective root zone. Following heavy rain or irrigation, the user can assume that the root zone is most field chapters, i.east., D_{r, i-one} » 0.

Precipitation (P), runoff (RO) and irrigation (I)

P_i is equivalent to daily precipitation. Daily precipitation in amounts less than about 0.two ET_o is ordinarily entirely evaporated and can usually be ignored in the water balance calculations especially when the single crop coefficient arroyo is being used. Ii is equivalent to the mean infiltrated irrigation depth expressed for the entire field surface. Runoff from the surface during precipitation tin exist predicted using standard procedures from hydrological texts.

Capillary ascent (CR)

The amount of h2o transported upward by capillary ascent from the water table to the root zone depends on the soil blazon, the depth of the water tabular array and the wetness of the root zone. CR can normally exist assumed to be cypher when the h2o table is more than than about one thousand below the bottom of the root zone. Some information on CR was presented in FAO Irrigation and Drainage Paper No. 24. CR will be a topic in a hereafter FAO publication.

Evapotranspiration (ET _c )

Where the soil water depletion is smaller than RAW, the crop evapotranspiration equals ET_c = K_c ET_o. As presently every bit D_{r, i} exceeds RAW, the crop evapotranspiration is reduced and ET_c can be computed from Equation lxxx or 81.

Deep percolation (DP)

Following heavy rain or irrigation, the soil h2o content in the root zone might exceed field capacity. In this unproblematic process it is assumed that the soil water content is at q _FC inside the aforementioned twenty-four hour period of the wetting result, and so that the depletion D_{r, i} in Equation 85 becomes zero. Therefore, following heavy rain or irrigation

DP_i = (P_i - RO_i) + I_i - ET_{c, i} - D_{r, i-i} ³ 0 (88)

Equally long equally the soil water content in the root zone is below field capacity (i.east., D_{r, i} > 0), the soil will non bleed and DP_i = 0.

The DP_i term in Equations 85 and 88 is non to be confused with the DP_{e, i} term used in Equations 77 and 79 for the evaporation layer. Both terms can be calculated at the same fourth dimension, but are independent of one some other.

Forecasting or allocating irrigations

Irrigation is required when rainfall is bereft to recoup for the h2o lost by evapotranspiration. The master objective of irrigation is to apply water at the correct period and in the correct corporeality. By calculating the soil water balance of the root zone on a daily ground (Equation 85), the timing and the depth of time to come irrigations tin be planned. To avoid ingather water stress, irrigations should be applied before or at the moment when the readily available soil water is depleted (D_{r, i} £ RAW). To avert deep percolation losses that may leach relevant nutrients out of the root zone, the cyberspace irrigation depth should be smaller than or equal to the root zone depletion (I_i £ D_{r, i}).

Example 38 illustrates the application of a water balance of the root zone to predict irrigation dates to avoid water stress. The example utilizes diverse calculations for K_e from Case 35. A consummate "spreadsheet" that includes all necessary calculations for predicting both irrigation schedules and to predict K_c = Yard_cb + K_e for daily timesteps is presented in Annex eight.

EXAMPLE 38. Irrigation scheduling to avert crop water stress

Program the irrigation applications for Example 35. It is assumed that: - irrigations are to be applied when RAW is depleted, - the depletion factor (p) is 0.6, - all irrigations and precipitations occur early on in the 24-hour interval, - the depth of the root zone (Zr) on day ane is 0.3 yard and increases to 0.35 grand by day 10, - the root zone depletion at the kickoff of 24-hour interval ane (Dr, i-1) is RAW.
From Eq. 82			TAW = k (0.23 - 0.10) Zr, i = 130 Zr, i [mm]
From Eq. 83			RAW = 0.6 TAW = 78 Zr, i [mm]
On day 1,			when Zr = 0.3 g: Dr, i-1 = RAW = 78 (0.3) = 23 mm
(ane)	(2)	(3)	(4)	(five)	(6)	(vii)	(8)	(9)	(10)	(11)	(12)	(13)	(14)
Mean solar day	ET o	Z r	RAW	D r, i outset	P-RO	I	K s	K cb	M e	K c	ET c	DP	D r, i terminate
	mm/d	m	mm	mm	mm	mm					mm	mm	mm
first	-	-	-	-	-	-	-	-	-	-	-	-	23
i	4.five	0.thirty	23	0	0	40	1	0.30	0.91	i.21	five.5	17	five
2	5.0	0.31	24	5	0	0	1	0.31	0.90	i.21	half-dozen.1	0	12
three	3.9	0.31	24	12	0	0	i	0.32	0.72	i.04	4.0	0	16
4	4.2	0.32	25	16	0	0	ane	0.33	0.37	0.70	2.nine	0	18
v	4.eight	0.32	25	18	0	0	1	0.34	0.18	0.52	2.5	0	21
6	2.seven	0.33	26	fifteen	half-dozen	0	1	0.36	0.64	1.00	two.7	0	xviii
7	5.8	0.33	26	18	0	0	1	0.37	0.45	0.82	4.7	0	22
8	5.1	0.34	26	22	0	0	one	0.38	0.17	0.55	ii.8	0	25
9	4.seven	0.34	27	25	0	0	1	0.39	0.08	0.47	ii.two	0	27
10	v.2	0.35	27	0	0	27	1	0.40	0.81	1.21	6.three	0	6
(1)	Day number.
(ii)	From Case 35.
(3)	Zr is given (interpolated between 0.3 1000 on day 1 and 0.35 m on solar day 10).
(four)	Eq. 83.
(5)	Dr, i first (root zone depletion at the beginning of the day) If precipitation and irrigation occur early in the day so Dr, i starting time = Max(Dr, i-1 end - I - (P-RO), or 0) If precipitation and irrigation occur late in the solar day, and then Dr, i start = Dr, i-ane cease where Dr, i-1 end is taken from cavalcade fourteen of previous day Since the depth of the root zone increases each day, the h2o content of the subsoil (q sub) has to be considered to update Dr, i Dr, i = Dr, i-1 + 1000 (q FC - q sub, i-1) D Zr, l In the case information technology is causeless that q sub is at field capacity (due to prior overirrigation and excessive rainfall on previous days). Therefore, a combination of the equations for Dr, i can be utilized.
(6)	From Instance 35.
(7)	Irrigation is required when Dr, i ³ RAW. On day one, the irrigation depth (infiltrating the soil) is given (from Example 35:ane = twoscore mm) On twenty-four hour period ten, another irrigation is required. An irrigation with a net depth of 27 mm refills the root zone and avoids water loss past deep percolation (DP = 0 mm).
(8)	Eq. 84, where Kdue south = 1 for Dr, i £ RAW.
(9)	From Example 35.
(ten)	Day 1 to 9: From Example 35. Twenty-four hours 10: Post-obit the extra irrigation early in the solar day, the topsoil will be wet and Kr is 1 or from Eq. 71: Kdue east = (1.21 - 0.40) = 0.81.
(xi)	Chiliadc =Thous Kcb + Ke.
(12)	Eq. fourscore.
(13)	Eq. 88, where Dr, i-1 is taken from column 14 of previous day.
(14)	Dr, i (root zone depletion at finish of one twenty-four hour period) = the starting Dr, i at the beginning of the side by side day (see footnote v). From Eq. 85, where Dr, i-ane is taken from column 14 of previous mean solar day.

Effects of soil salinity

Salts in the soil h2o solution can reduce evapotranspiration by making soil water less "bachelor" for plant root extraction. Salts take an affinity for water and hence additional force is required for the crop to extract water from a saline soil. The presence of salts in the soil water solution reduces the total potential energy of the soil water solution. In add-on, some salts cause toxic effects in plants and can reduce found metabolism and growth. A function is presented here that predicts the reduction in evapotranspiration caused by salinity of soil water. The role is derived by combining yield-salinity equations from the FAO Irrigation and Drainage Paper N°29 with yield-ET equations from FAO Irrigation and Drainage Paper N°33. The resulting equation provides a beginning approximation of the reduction in evapotranspiration expected under various salinity conditions.

In that location is evidence that crop yield and transpiration are not as sensitive to depression osmotic potential as they are to low matric potential. Nether saline atmospheric condition, many plants are able to partially compensate for depression osmotic potential of the soil water by edifice up higher internal solute contents. This is done by arresting ions from the soil solution and by synthesizing organic osmolytes. Both of these reactions reduce the touch on of osmotic potential on water availability. However, synthesis of organic osmolytes does require expenditure of metabolic energy. Therefore plant growth is often reduced under saline conditions. The reduced plant growth impacts transpiration by reducing basis cover and is sometimes additionally due to partial stomatal closure.

Other impacts of salts in the soil include direct sodium and chloride toxicities and induced nutrient deficiencies. These deficiencies reduce plant growth past reducing the rate of foliage elongation, the enlargement, and the partition of cells in leaves. The modality depends on the method of irrigation. With sprinkler irrigation, adsorption of sodium and chloride through the leaf can consequence in toxic weather condition for all crop species. With surface or trickle irrigation, directly toxic conditions more often than not occur only in vine and tree crops; however, high levels of sodium tin can induce calcium deficiencies for all ingather species.

Since salt concentration changes as the soil h2o content changes, soil salinity is ordinarily measured and expressed on the ground of the electrical conductivity of the saturation extract of the soil (EC_eastward). The EC_{due east} is defined every bit the electrical conductivity of the soil water solution after the improver of a sufficient quantity of distilled water to bring the soil water content to saturation. EC_e is typically expressed in deciSiemens per meter (dS chiliad^-1). Nether optimum management weather condition, crop yields remain at potential levels until a specific, threshold electrical conductivity of the saturation soil water extract (EC_{e threshold}) is reached. If the boilerplate EC_e of the root zone increases above this critical threshold value, the yield is presumed to begin to decrease linearly in proportion to the increase in salinity. The rate of subtract in yield with increase in salinity is usually expressed as a slope, b, having units of % reduction in yield per dS/m increment in EC_eastward.

All plants do not respond to salinity in a like manner; some crops can produce adequate yields at much college soil salinity levels than others. This is because some crops are amend able to make the needed osmotic adjustments that enable them to extract more water from a saline soil, or they may be more tolerant of some of the toxic effects of salinity. Salt tolerance for many agricultural crops are provided in the FAO Irrigation and Drainage Papers No. 33 and 48. The EC_{eastward threshold} and slope b from these sources are listed in Table 23.

Equally tin can exist observed from the data in Table 23, at that place is an viii to 10-fold range in salt tolerance of agricultural crops. The effect of soil salinity on yield and crop evapotranspiration is hence crop specific.

The EC_{eastward threshold} and b parameters in Table 23 were determined primarily in research experiments using about steady-country irrigation where soil water contents were maintained at levels close to field capacity. However, under most types of irrigation scheduling for sprinkler and surface irrigation, the soil water content is typically depleted to well below field capacity, so that the EC of the soil water solution, EC_SW, increases prior to irrigation, even though the EC of the saturation extract does not alter. The increased salt concentration in the soil water solution reduces the osmotic potential of the soil water solution (it becomes more negative), and so that the plant must expend more metabolic energy and may exert more than mechanical force to absorb water. In addition, metabolic and toxic effects of salts on plants may become more pronounced as the soil dries and concentrations increase. However, the variation in soil water content during an irrigation interval has non been plant to strongly influence ingather evapotranspiration. This is because of the ascension of soil water content to levels that are to a higher place that experienced under steady country irrigation early in a long irrigation interval. At that place is a similar, counteractive decrease in soil water content subsequently in a long irrigation interval. In addition, the distribution of salts in the root zone under low frequency irrigation tin reduce salinity impacts during the starting time portion of the irrigation interval. Also, under loftier frequency irrigation of the soil surface, soil evaporation losses are college. Consequently, given the aforementioned application depth, the leaching fraction is reduced. For these reasons, the length of irrigation interval and the change in EC of soil h2o during the interval have commonly not been found to be factors in the reduction of ET, given that the same depths of water are infiltrated into the root zone over fourth dimension.

In some cases, increased evaporation under high frequency irrigation of the soil surface tin counteract reductions in One thousand_c caused past loftier EC_{due east} of the root zone. Under these conditions, the total Chiliad_c and ET_c are non very different from the non-saline, standard conditions under less frequent irrigation, even though ingather yields and crop transpiration are reduced. Because of this, under saline conditions, the K_s reducing factor should only exist applied with the dual 1000_c arroyo.

In review articles on impacts of salinity on ingather production, Letey et al. (1985) and Shalhevet (1994) concluded that effects of soil salinity and water stress are generally additive in their impacts on crop evapotranspiration. Therefore, the same yield-ET functions may hold for both h2o shortage induced stress and for salinity induced stress.

Yield-salinity relationship

A widely adept approach for predicting the reduction in crop yield due to salinity has been described in the FAO Irrigation and Drainage Paper N°29. The approach presumes that, under optimum management conditions, ingather yields remain at potential levels until a specific, threshold conductivity of the soil water solution is reached. When salinity increases across this threshold, crop yields are presumed to decrease linearly in proportion to the increase in salinity. The soil h2o salinity is expressed every bit the conductivity of the saturation extract, EC_e. In equation form, the procedure followed in FAO Irrigation and Drainage Paper N°29 is:

(89)

for weather where EC_e > EC_{e threshold} where:

Y_a actual crop yield

Y₁₀₀₀ maximum expected crop yield when EC_e < EC_{e threshold}

EC_e mean electrical conductivity of the saturation extract for the root zone [dS grand^-one]

EC_{due east threshold} electric conductivity of the saturation extract at the threshold of EC_e when crop yield first reduces below Y₁₀₀₀ [dS thousand^-1]

b reduction in yield per increment in EC_e [%/(dS thousand^-1)]

Values for EC_{e threshold} and b have been provided in the FAO Irrigation and Drainage Newspaper N°29 and 48 and are listed in Table 23 for many agronomical crops.

Salinity-yield data from the FAO Irrigation and Drainage papers Nos. 29 and 48 were generally from studies where soil water content was held at nigh-3 m potential (-30 kPa) or college at the 0.3 to 0.half-dozen m depth, depending on the crop. However, these papers indicate that for most crops, the data are transferable to typical field situations where the readily available soil water (RAW) is depleted between irrigations.

Yield-moisture stress relationship

A simple, linear crop-water production function was introduced in the FAO Irrigation and Drainage Paper N°33 to predict the reduction in ingather yield when crop stress was caused by a shortage of soil water:

(ninety)

where:

Grand_y a yield response cistron [-]
ET_{c adj} adjusted (bodily) ingather evapotranspiration [mm d^-1]
ET_c ingather evapotranspiration for standard weather condition (no water stress) [mm d^-1]

Yard_y is a factor that describes the reduction in relative yield co-ordinate to the reduction in ET_c caused past soil water shortage. In FAO N°33, One thousand_y values are crop specific and may vary over the growing season. In general, the decrease in yield due to water deficit during the vegetative and ripening period is relatively small, while during the flowering and yield germination periods information technology volition be large. Values for Grand_y for individual growth periods and for the complete growing flavour take been included in the FAO Irrigation and Drainage Paper North°33. Seasonal values for K_y are summarized in Table 24.

Combined salinity-ET reduction human relationship

---

No water stress (D_r < RAW)
With water stress (D_r > RAW)

---

No water stress (D_r < RAW)

When salinity stress occurs without water stress, Equations 89 and 90 tin be combined and solved for an equivalent One thousand_s, where G_south = ET_{c adj}/ET_c:

(91)

for conditions when EC_{due east} > EC_{e threshold} and soil water depletion is less than the readily available soil water depth (D_r < RAW). D_r and RAW are defined in the previous section.

With water stress (D_r > RAW)

When soil water stress occurs in improver to salinity stress, Equation 84 in Chapter eight and Equations 89 and ninety are combined to yield:

(92)

for weather condition when EC_eastward > EC_{e threshold} and D_r > RAW. Effigy 44 shows the touch on of salinity reduction on 1000_southward as salinity increases. Notation that the approach presumes that RAW (and p) exercise non change with increasing salinity. This may or may not be a good assumption for some crops.

Limitations

Because the bear on of salinity on plant growth and yield and on crop evapotranspiration is a time-integrated process, generally only the seasonal value for 1000_y is used to predict the reduction in evapotranspiration. At that place are Grand_y values in FAO Irrigation and Drainage paper N°33 for merely almost 23 crops. The seasonal values for Chiliad_y from paper N°33 are summarized in Tabular array 24. For many crops, the seasonal K_y is near 1. For crops where 1000_y is unknown, the user may use Thou_y = 1 in Equations 91 and 92 or may select the Thousand_y for a crop type that has similar behaviour.

Equations 91 and 92 are suggested as only approximate estimates of salinity impacts on ET, and stand for general effects of salinity on evapotranspiration as occurring over an extended menstruum of fourth dimension (as measured in weeks or months). These equations are non expected to be accurate for predicting ET_c for specific days. Nor exercise they include other complicating effects such as specific ion toxicity. Application of equations 91 and 92 presumes that the EC_e represents the average EC_e for the root zone.

The equations presented may non exist valid at high salinity, where the linear relationships between EC_e, ingather yield and K_south may not concur. The use of Equations 91 and 92 should ordinarily exist restricted to EC_{due east} < EC_threshold + 50/b. In addition, the equations predict Y_a = 0 earlier K_{due south} = 0 when K_y > 1 and vice versa.

As indicated earlier, reduction in ET_c in the presence of soil salinity is often partially caused by reduced plant size and fraction of footing cover. These furnishings are largely included in the coefficient values in Table 23. Therefore, where plant growth is affected by salinity and Equations 91 and 92 are applied, no other reductions in K_c are required, for example using LAI or fraction of footing cover, as described in Chapter nine.

Tabular array 23. Common salt tolerance of mutual agricultural crops expressed equally electrical electrical conductivity of the soil saturation extract at the threshold when crop yield first reduces below the full yield potential (EC _{eastward, threshold} ) and as the slope (b) of reduction in crop yield with increasing salinity beyond EC _{e, threshold} .

Crop 1		EC e treshold ii (dS m -ane ) 3	b iv (%/dS m -1 )	Rating 5
a. Pocket-sized vegetables
Broccoli		2.eight	9.2	MS
Brussels sprouts		ane.8	nine.7	MS
Cabbage		i.0-1.8	9.viii-14.0	MS
Carrots		1.0	14.0	S
Cauliflower		1.viii	6.2	MS
Celery		1.eight-2.5	six.ii-13.0	MS
Lettuce		1.3-1.seven	12.0	MS
Onions		1.2	xvi.0	South
Spinach		two.0-3.2	7.vii-16.0	MS
Radishes		one.ii-two.0	7.6-13.0	MS
b. Vegetables - Solanum Family (Solanaceae)
Egg Institute		-	-	MS
Peppers		1.5-1.7	12.0-14.0	MS
Tomato		0.nine-ii.v	nine.0	MS
c. Vegetables Cucumber Family (Cucurbitaceae)
Cucumber		1.1-two.5	7.0-thirteen.0	MS
Melons			-	MS
Pumpkin, winter squash		1:two	13.0	MS
Squash, Zucchini		4.7	10.0	MT
Squash (scallop)		three.2	16.0	MS
Watermelon		-	-	MS
d. Roots and Tubers
Beets, red		iv.0	9.0	MT
Parsnip		-	-	Due south
Potato		1.7	12.0	MS
Sweetness tater		i.v-2.5	10.0	MS
Turnip		0.9	nine.0	MS
Sugar beet		7.0	five.9	T
eastward. Legumes (Leguminosae)
Beans		1.0	xix.0	Southward
Broadbean (faba bean)		1.five-i.6	9.vi	MS
Cowpea		4.9	12.0	MT
Groundnut (Peanut)		three.two	29.0	MS
Peas		one.v	14.0	S
Soybeans		v.0	twenty.0	MT
f. Perennial Vegetables (with wintertime dormancy and initially bare or mulched soil)
Artichokes		-	-	MT
Asparagus		iv.1	two.0	T
Mint		-	-	-
Strawberries		1.0-i.5	eleven.0-33.0	S
yard. Fibre crops
Cotton wool		vii.7	v.ii	T
Flax		1.7	12.0	MS
h. Oil crops
Casterbean		-	-	MS
Safflower		-	-	MT
Sunflower		-	-	MS
i. Cereals
Barley		8.0	5.0	T
Oats		-	-	MT
Maize		ane.7	12.0	MS
Maize, sweet (sweet corn)		one.vii	12.0	MS
Millet		-	-	MS
Sorghum		half dozen.8	16.0	MT
Rice half dozen		3.0	12.0	S
Wheat (Triticum aestivum)		6.0	7.1	MT
Wheat, semidwarf (T. aestivum)		8.6	iii.0	T
Wheat, durum (Triticum turgidum)		5.seven-five.9	3.8-5.5	T
j. Forages
Alfalfa		two.0	7.3	MS
Barley (forage)		6.0	7.1	MT
Bermuda		6.9	vi.iv	T
Clover, Berseem		1.five	5.vii	MS
Clover (alsike, ladino, ruby-red, strawberry)		one.5	12.0	MS
Cowpea (fodder)		2.five	11.0	MS
Fescue		3.9	5.iii-6.2	MT
Foxtail		1.5	nine.half-dozen	MS
Hardinggrass		four.6	7.6	MT
Lovegrass		2.0	8.4	MS
Maize (forage)		ane.8	7.4	MS
Orchardgrass		1.five	six.2	MS
Rye-grass (perennial)		5.6	vii.6	MT
Sesbania		2.iii	7.0	MS
Sphaerophysa		ii.2	vii.0	MS
Sudangrass		ii.8	four.three	MT
Trefoil, narrowleaf birdsfoot		v.0	x.0	MT
Trefoil, big		2.three	19.0	MS
Vetch, common		3.0	11.0	MS
Wheatgrass, tall		vii.5	four.2	T
Wheatgrass, fairway crested		vii.5	vi.nine	T
Wheatgrass, standard crested		3.5	four.0	MT
Wildrye, beardless		2.7	half-dozen.0	MT
thousand. Sugar pikestaff		ane.seven	five.9	MS
50. Tropical Fruits and Trees
Banana		-	-	MS
Coffee		-	-	-
Date Palms		4.0	iii.6	T
Palm trees		-	-	T
Pineapple (multi-year ingather)		-	-	MT
Tea		-	-	-
grand. Grapes and berries
Blackberry		1.5	22.0	S
Boysenberry		1.5	22.0	S
Grapes		1.5	9.vi	MS
Hops		-	-	-
n. Fruit copse
Almonds		1.five	19.0	S
Avocado		-	-	S
Citrus (Grapefruit)		1.8	sixteen.0	S
Citrus (Orangish)		1.7	16.0	S
Citrus (Lemon)		-	-	S
Citrus (Lime)		-	-	Southward
Citrus (Pummelo)		-	-	S
Citrus (Tangerine)		-	'	South
Conifer trees		-	-	MS/MT
Deciduous orchard
	- Apples	-	-	Southward
	- Peaches	1.seven	21.0	South
	- Cherries	-	-	South
	- Pear	-	. -	S
	- Apricot	1.6	24.0	Due south
	- Plum, prune	one.5	18.0	Southward
	- Pomegranate	-	-	MT
Olives		-	-	MT

^one The data serve just as a guideline - Tolerance vary depending upon climate, soil conditions and cultural practices. Crops are often less tolerant during germination and seedling stage.

ⁱⁱ EC_{eastward, threshold} means average root zone salinity at which yield starts to decline

^three Root zone salinity is measured by electrical conductivity of the saturation extract of the soil, reported in deciSiemens per metre (dS m^-1) at 25 °C

⁴ b is the percentage reduction in ingather yield per 1 dS/thou increase in EC_e beyond EC_{east threshold}

⁵ Ratings are: T = Tolerant, MT = Moderately Tolerant, MS = Moderately Sensitive and S = Sensitive

⁶ Because paddy rice is grown under flooded conditions, values refer to the electrical conductivity of the soil water while the plants are submerged

Chief sources:

Ayers and Westcot, 1985. FAO Irrigation and Drainage Paper North° 29. Water quality for agronomics; Rhoades, Kandiah and Mashali, 1992. FAO Irrigation and Drainage Paper N° 48. The apply of saline waters for crop productions.

Application

Under steady land atmospheric condition, the value for EC_e can be predicted every bit a function of EC of the irrigation water (EC_iw) and the leaching fraction, using a standard leaching formula. For example, the FAO-29 leaching formula LR = EC_iw/(5 EC_e - EC_iw predicts the leaching requirement when approximately a 40-xxx-20-10 percent h2o extraction design occurs from the upper to lower quarters of the root zone prior to irrigation. EC_iw is the electric conductivity of the irrigation water. From this equation, EC_e is estimated as:

(93)

Tabular array 24. Seasonal yield response functions from FAO Irrigation and Drainage Paper No. 33.

Crop	K y
Alfalfa	ane.1
Banana	1.2-1.35
Beans	1.xv
Cabbage	0.95
Citrus	1.1-1.3
Cotton	0.85
Grape	0.85
Groundnet	0.70
Maize	1.25
Onion	1.1
Peas	1,15
Pepper	1.1
Tater	one.1
Safflower	0.8
Sorghum	0.9
Soybean	0.85
Spring Wheat	1.15
Sugarbeet	1.0
Sugarcane	i.two
Sunflower	0.95
Tomato	1.05
Watermelon	ane.1
Winter wheat	one.05

where LF, the actual leaching fraction, is used in identify of LR, the leaching requirement. Equation 93 predicts EC_e = 1.5 EC_iw nether conditions where a xv-20 percent leaching fraction is employed. Other leaching fraction equations tin be used in place of the FAO-29 equation to fit local characteristics. Equation 93 is only truthful if the irrigation water quality and the leaching fraction are constant over the growing season. Time is required to attain a salt equilibrium in the soil. If in that location are important winter rains of high quality water and often first-class leaching, the table salt balance in the soil will be quite unlike at the beginning of the flavor and with a lower boilerplate EC_e of the root zone than would be predicted from Equation 93. An advisable local calibration of Equation 93 is desirable nether these particular weather.

Figure 44. The effect of soil salinity on the water stress coefficient K _s

Instance 39. Effect of soil salinity on crop evapotranspiration

A field of beans is cultivated on a silt loam soil and is irrigated during the midseason period using water having salinity ECiw = 1 dS one thousand-1. A 15 pct leaching fraction is employed. The ECe, threshold and slope from Table 23 are 1.0 dS 1000-one and 19 %/(dS 1000-ane) respectively. The seasonal Thouy from FAO Irrigation and Drainage Paper No 33 and Table 24 for beans is Ky = i.15. Compare the upshot on crop evapotranspiration for various levels of soil water depletion in the root zone under saline and nonsaline conditions. The TAW and RAW for the bean crop are 110 and 44 mm (for p = 0.4).
Since the leaching fraction is 0.15, ECeast is estimated from Equation 93 as ECeastward = 1.v ECdue west = i.v (1) = 1.5 dS k-1. The Msouth in the presence of salinity stress and absence of wet stress is: The Msouth in the presence of wet stress, only in the absenteeism of salinity stress is: The Thousands in the presence of both wet stress and salinity stress is:
The effect on crop evapotranspiration for various soil water depletions in the root zone (Dr) are:
D r (mm)	K s no soil salinity		Thousand south with soil salinity (EC e = one.5 dS m -ane ) (Eq. 92)		Additional reduction in potential ET c due to salinity
0	1.00	no reduction in ETc	0.92	8% reduction in ETc	8%
35	1.00	no reduction in ETc	0.92	eight% reduction in ETc	8%
40	1.00	no reduction in ETc	0.92	8% reduction	eight%
44	i.00	no reduction in ETc	0.92	8% reduction	8%
50	0.91	9% reduction	0.83	17% reduction	8%
60	0.76	24% reduction	0.69	31 % reduction	7%
70	0.61	39% reduction	0.56	44% reduction	five%
80	0.45	55% reduction	0.42	58% reduction	three%
xc	0.30	70% reduction	0.28	72% reduction	ii%
100	0.fifteen	85% reduction	0.14	86% reduction	1%
110	0.00	ETc = 0	0.00	ETc = 0	--

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