Chapter 6 - ET_c - Single crop coefficient (K_c)

Length of growth stages
Crop coefficients
Construction of the K_c curve
Calculating ET_c
Alfalfa-based crop coefficients
Transferability of previous K_c values

This chapter deals with the calculation of crop evapotranspiration (ET_c) under standard conditions. No limitations are placed on crop growth or evapotranspiration from soil water and salinity stress, crop density, pests and diseases, weed infestation or low fertility. ET_c is determined by the crop coefficient approach whereby the effect of the various weather conditions are incorporated into ET_o and the crop characteristics into the K_c coefficient:

ET_c = K_c ET_o (58)

The effect of both crop transpiration and soil evaporation are integrated into a single crop coefficient. The K_c coefficient incorporates crop characteristics and averaged effects of evaporation from the soil. For normal irrigation planning and management purposes, for the development of basic irrigation schedules, and for most hydrologic water balance studies, average crop coefficients are relevant and more convenient than the K_c computed on a daily time step using a separate crop and soil coefficient (Chapter 7). Only when values for K_c are needed on a daily basis for specific fields of crops and for specific years, must a separate transpiration and evaporation coefficient (K_cb + K_e) be considered.

The calculation procedure for crop evapotranspiration, ET_c, consists of:

1. identifying the crop growth stages, determining their lengths, and selecting the corresponding K_c coefficients;

2. adjusting the selected K_c coefficients for frequency of wetting or climatic conditions during the stage;

3. constructing the crop coefficient curve (allowing one to determine K_c values for any period during the growing period); and

4. calculating ET_c as the product of ET_o and K_c.

Length of growth stages

FAO Irrigation and Drainage Paper No. 24 provides general lengths for the four distinct growth stages and the total growing period for various types of climates and locations. This information has been supplemented from other sources and is summarized in Table 11.

In some situations, the time of emergence of vegetation and the time of effective full cover can be predicted using cumulative degree-based regression equations or by more sophisticated plant growth models. These types of models should be verified or validated for the local area or for a specific crop variety using local observations.

TABLE 11. Lengths of crop development stages* for various planting periods and climatic regions (days)

Crop		Init. (Lini)	Dev. (Ldev)	Mid (Lmid)	Late (Llate)	Total	Plant Date	Region
a. Small Vegetables
Broccoli		35	45	40	15	135	Sept	Calif. Desert, USA
Cabbage		40	60	50	15	165	Sept	Calif. Desert, USA
Carrots		20	30	50/30	20	100	Oct/Jan	Arid climate
		30	40	60	20	150	Feb/Mar	Mediterranean
		30	50	90	30	200	Oct	Calif. Desert, USA
Cauliflower		35	50	40	15	140	Sept	Calif. Desert, USA
Celery		25	40	95	20	180	Oct	(Semi) Arid
		25	40	45	15	125	April	Mediterranean
		30	55	105	20	210	Jan	(Semi) Arid
Crucifers1		20	30	20	10	80	April	Mediterranean
		25	35	25	10	95	February	Mediterranean
		30	35	90	40	195	Oct/Nov	Mediterranean
Lettuce		20	30	15	10	75	April	Mediterranean
		30	40	25	10	105	Nov/Jan	Mediterranean
		25	35	30	10	100	Oct/Nov	Arid Region
		35	50	45	10	140	Feb	Mediterranean
Onion (dry)		15	25	70	40	150	April	Mediterranean
		20	35	110	45	210	Oct; Jan.	Arid Region; Calif.
Onion (green)		25	30	10	5	70	April/May	Mediterranean
		20	45	20	10	95	October	Arid Region
		30	55	55	40	180	March	Calif., USA
Onion (seed)		20	45	165	45	275	Sept	Calif. Desert, USA
Spinach		20	20	15/25	5	60/70	Apr; Sep/Oct	Mediterranean
		20	30	40	10	100	November	Arid Region
Radish		5	10	15	5	35	Mar/Apr	Medit.; Europe
		10	10	15	5	40	Winter	Arid Region
b. Vegetables - Solanum Family (Solanaceae)
Egg plant		30	40	40	20	130\1	October	Arid Region
		30	45	40	25	40	May/June	Mediterranean
Sweet peppers (bell)		25/30	35	40	20	125	April/June	Europe and Medit.
		30	40	110	30	210	October	Arid Region
Tomato		30	40	40	25	135	January	Arid Region
		35	40	50	30	155	Apr/May	Calif., USA
		25	40	60	30	155	Jan	Calif. Desert, USA
		35	45	70	30	180	Oct/Nov	Arid Region
		30	40	45	30	145	April/May	Mediterranean
c. Vegetables - Cucumber Family (Cucurbitaceae)
Cantaloupe		30	45	35	10	120	Jan	Calif., USA
		10	60	25	25	120	Aug	Calif., USA
Cucumber		20	30	40	15	105	June/Aug	Arid Region
		25	35	50	20	130	Nov; Feb	Arid Region
Pumpkin, Winter squash		20	30	30	20	100	Mar, Aug	Mediterranean
		25	35	35	25	120	June	Europe
Squash, Zucchini		25	35	25	15	100	Apr; Dec.	Medit.; Arid Reg.
		20	30	25	15	90	May/June	Medit.; Europe
Sweet melons		25	35	40	20	120	May	Mediterranean
		30	30	50	30	140	March	Calif., USA
		15	40	65	15	135	Aug	Calif. Desert, USA
		30	45	65	20	160	Dec/Jan	Arid Region
Water melons		20	30	30	30	110	April	Italy
		10	20	20	30	80	Mat/Aug	Near East (desert)
d. Roots and Tubers
Beets, table		15	25	20	10	70	Apr/May	Mediterranean
		25	30	25	10	90	Feb/Mar	Mediterranean & Arid
Cassava: year 1		20	40	90	60	210	Rainy	Tropical regions
year 2		150	40	110	60	360	season
Potato		25	30	30/45	30	115/130	Jan/Nov	(Semi) Arid Climate
		25	30	45	30	130	May	Continental Climate
		30	35	50	30	145	April	Europe
		45	30	70	20	165	Apr/May	Idaho, USA
		30	35	50	25	140	Dec	Calif. Desert, USA
Sweet potato		20	30	60	40	150	April	Mediterranean
		15	30	50	30	125	Rainy seas.	Tropical regions
Sugarbeet		30	45	90	15	180	March	Calif., USA
		25	30	90	10	155	June	Calif., USA
		25	65	100	65	255	Sept	Calif. Desert, USA
		50	40	50	40	180	April	Idaho, USA
		25	35	50	50	160	May	Mediterranean
		45	75	80	30	230	November	Mediterranean
		35	60	70	40	205	November	Arid Regions
e. Legumes (Leguminosae)
Beans (green)		20	30	30	10	90	Feb/Mar	Calif., Mediterranean
		15	25	25	10	75	Aug/Sep	Calif., Egypt, Lebanon
Beans (dry)		20	30	40	20	110	May/June	Continental Climates
		15	25	35	20	95	June	Pakistan, Calif.
		25	25	30	20	100	June	Idaho, USA
Faba bean, broad bean		15	25	35	15	90	May	Europe
		20	30	35	15	100	Mar/Apr	Mediterranean
	- dry	90	45	40	60	235	Nov	Europe
	- green	90	45	40	0	175	Nov	Europe
Green gram, cowpeas		20	30	30	20	110	March	Mediterranean
Groundnut		25	35	45	25	130	Dry	West Africa
		35	35	35	35	140	season	High Latitudes
		35	45	35	25	140	May May/June	Mediterranean
Lentil		20	30	60	40	150	April	Europe
		25	35	70	40	170	Oct/Nov	Arid Region
Peas		15	25	35	15	90	May	Europe
		20	30	35	15	100	Mar/Apr	Mediterranean
		35	25	30	20	110	April	Idaho, USA
Soybeans		15	15	40	15	85	Dec	Tropics
		20	30/35	60	25	140	May	Central USA
		20	25	75	30	150	June	Japan
f. Perennial Vegetables (with winter dormancy and initially bare or mulched soil)
Artichoke		40	40	250	30	360	Apr (1st yr)	California
		20	25	250	30	325	May (2nd yr)	(cut in May)
Asparagus		50	30	100	50	230	Feb	Warm Winter
		90	30	200	45	365	Feb	Mediterranean
g. Fibre Crops
Cotton		30	50	60	55	195	Mar-May	Egypt; Pakistan; Calif.
		45	90	45	45	225	Mar	Calif. Desert, USA
		30	50	60	55	195	Sept	Yemen
		30	50	55	45	180	April	Texas
Flax		25	35	50	40	150	April	Europe
		30	40	100	50	220	October	Arizona
h. Oil Crops
Castor beans		25	40	65	50	180	March	(Semi) Arid Climates
		20	40	50	25	135	Nov.	Indonesia
Safflower		20	35	45	25	125	April	California, USA
		25	35	55	30	145	Mar	High Latitudes
		35	55	60	40	190	Oct/Nov	Arid Region
Sesame		20	30	40	20	100	June	China
Sunflower		25	35	45	25	130	April/May	Medit.; California
i. Cereals
Barley/Oats/Wheat		15	25	50	30	120	November	Central India
		20	25	60	30	135	March/Apr	35-45 °L
		15	30	65	40	150	July	East Africa
		40	30	40	20	130	Apr
		40	60	60	40	200	Nov
		20	50	60	30	160	Dec	Calif. Desert, USA
Winter Wheat		202	602	70	30	180	December	Calif., USA
		30	140	40	30	240	November	Mediterranean
		160	75	75	25	335	October	Idaho, USA
Grains (small)		20	30	60	40	150	April	Mediterranean
		25	35	65	40	165	Oct/Nov	Pakistan; Arid Reg.
Maize (grain)		30	50	60	40	180	April	East Africa (alt.)
		25	40	45	30	140	Dec/Jan	Arid Climate
		20	35	40	30	125	June	Nigeria (humid)
		20	35	40	30	125	October	India (dry, cool)
		30	40	50	30	150	April	Spain (spr, sum.); Calif.
		30	40	50	50	170	April	Idaho, USA
Maize (sweet)		20	20	30	10	80	March	Philippines
		20	25	25	10	80	May/June	Mediterranean
		20	30	50/30	10	90	Oct/Dec	Arid Climate
		30	30	30	103	110	April	Idaho, USA
		20	40	70	10	140	Jan	Calif. Desert, USA
Millet		15	25	40	25	105	June	Pakistan
		20	30	55	35	140	April	Central USA
Sorghum		20	35	40	30	130	May/June	USA, Pakis., Med.
		20	35	45	30	140	Mar/April	Arid Region
Rice		30	30	60	30	150	Dec; May	Tropics; Mediterranean
		30	30.	80	40	180	May	Tropics
j. Forages
Alfalfa, total season 4		10	30	var.	var.	var.		last -4°C in spring until first -4°C in fall
Alfalfa 4 1st cutting cycle		10	20	20	10	60	Jan Apr (last - 4°C)	Calif., USA.
		10	30	25	10	75		Idaho, USA.
Alfalfa 4, other cutting cycles		5	10	10	5	30	Mar	Calif., USA.
		5	20	10	10	45	Jun	Idaho, USA.
Bermuda for seed		10	25	35	35	105	March	Calif. Desert, USA
Bermuda for hay (several cuttings)		10	15	75	35	135	---	Calif. Desert, USA
Grass Pasture 4		10	20	--	--	--		7 days before last -4°C in spring until 7 days after first -4°C in fall
Sudan, 1st cutting cycle		25	25	15	10	75	Apr	Calif. Desert, USA
Sudan, other cutting cycles		3	15	12	7	37	June	Calif. Desert, USA
k. Sugar Cane
Sugarcane, virgin		35	60	190	120	405		Low Latitudes
		50	70	220	140	480		Tropics
		75	105	330	210	720		Hawaii, USA
Sugarcane, ratoon		25	70	135	50	280		Low Latitudes
		30	50	180	60	320		Tropics
		35	105	210	70	420		Hawaii, USA
l. Tropical Fruits and Trees
Banana, 1st yr		120	90	120	60	390	Mar	Mediterranean
Banana, 2nd yr		120	60	180	5	365	Feb	Mediterranean
Pineapple		60	120	600	10	790		Hawaii, USA
m. Grapes and Berries
Grapes		20	40	120	60	240	April	Low Latitudes
		20	50	75	60	205	Mar	Calif., USA
		20	50	90	20	180	May	High Latitudes
		30	60	40	80	210	April	Mid Latitudes (wine)
Hops		25	40	80	10	155	April	Idaho, USA
n. Fruit Trees
Citrus		60	90	120	95	365	Jan	Mediterranean
Deciduous Orchard		20	70	90	30	210	March	High Latitudes
		20	70	120	60	270	March	Low Latitudes
		30	50	130	30	240	March	Calif., USA
Olives		30	90	60	90	2705	March	Mediterranean
Pistachios		20	60	30	40	150	Feb	Mediterranean
Walnuts		20	10	130	30	190	April	Utah, USA
o. Wetlands - Temperate Climate
Wetlands (Cattails, Bulrush)		10	30	80	20	140	May	Utah, USA; killing frost
		180	60	90	35	365	November	Florida, USA
Wetlands (short veg.)		180	60	90	35	365	November	frost-free climate

* Lengths of crop development stages provided in this table are indicative of general conditions, but may vary substantially from region to region, with climate and cropping conditions, and with crop variety. The user is strongly encouraged to obtain appropriate local information.

¹ Crucifers include cabbage, cauliflower, broccoli, and Brussel sprouts. The wide range in lengths of seasons is due to varietal and species differences.

² These periods for winter wheat will lengthen in frozen climates according to days having zero growth potential and wheat dormancy. Under general conditions and in the absence of local data, fall planting of winter wheat can be presumed to occur in northern temperate climates when the 10-day running average of mean daily air temperature decreases to 17° C or December 1, whichever comes first. Planting of spring wheat can be presumed to occur when the 10-day running average of mean daily air temperature increases to 5° C. Spring planting of maize-grain can be presumed to occur when the 10-day running average of mean daily air temperature increases to 13° C.

³ The late season for sweet maize will be about 35 days if the grain is allowed to mature and dry.

⁴ In climates having killing frosts, growing seasons can be estimated for alfalfa and grass as:

alfalfa: last -4° C in spring until first -4° C in fall (Everson, D. O., M. Faubion and D. E. Amos 1978. "Freezing temperatures and growing seasons in Idaho." Univ. Idaho Agric. Exp. station bulletin 494. 18 p.)

grass: 7 days before last -4° C in spring and 7 days after last -4° C in fall (Kruse E. G. and Haise, H. R. 1974. "Water use by native grasses in high altitude Colorado meadows." USDA Agric. Res. Service, Western Region report ARS-W-6-1974. 60 pages)

⁵ Olive trees gain new leaves in March. See footnote 24 of Table 12 for additional information, where the K_c continues outside of the "growing period".

Primary source: FAO Irrigation and Drainage Paper 24 (Doorenbos and Pruitt, 1977), Table 22.

The lengths of the initial and development periods may be relatively short for deciduous trees and shrubs that can develop new leaves in the spring at relatively fast rates (Figure 23).

The rate at which vegetation cover develops and the time at which it attains effective full cover are affected by weather conditions in general and by mean daily air temperature in particular. Therefore, the length of time between planting and effective full cover will vary with climate, latitude, elevation and planting date. It will also vary with cultivar (crop variety). Generally, once the effective full cover for a plant canopy has been reached, the rate of further phenological development (flowering, seed development, ripening, and senescence) is more dependent on plant genotype and less dependent on weather. As an example, Figure 28 presents the variation in length of the growing period for one cultivar of rice for one region and for various planting dates.

The end of the mid-season and beginning of the late season is usually marked by senescence of leaves, often beginning with the lower leaves of plants. The length of the late season period may be relatively short (less than 10 days) for vegetation killed by frost (for example, maize at high elevations in latitudes > 40°N) or for agricultural crops that are harvested fresh (for example, table beets and small vegetables).

High temperatures may accelerate the ripening and senescence of crops. Long duration of high air temperature (> 35°C) can cause some crops such as turf grass to go into dormancy. If severely high air temperatures are coupled with moisture stress, the dormancy of grass can be permanent for the remainder of the growing season. Moisture stress or other environmental stresses will usually accelerate the rate of crop maturation and can shorten the mid and late season growing periods.

The values in Table 11 are useful only as a general guide and for comparison purposes. The listed lengths of growth stages are average lengths for the regions and periods specified and are intended to serve only as examples. Local observations of the specific plant stage development should be used, wherever possible, to incorporate effects of plant variety, climate and cultural practices. Local information can be obtained by interviewing farmers, ranchers, agricultural extension agents and local researchers, by conducting local surveys, or by remote sensing. When determining stage dates from local observations, the guidelines and visual descriptions may be helpful.

FIGURE 28. Variation in the length of the growing period of rice (cultivar: Jaya) sown during various months of the year at different locations along the Senegal River (Africa)

Crop coefficients

Tabulated K_c values
Crop coefficient for the initial stage (K_{c ini})
Crop coefficient for the mid-season stage (K_{c mid})
Crop coefficient for the end of the late season stage (K_{c end})

Changes in vegetation and ground cover mean that the crop coefficient K_c varies during the growing period. The trends in K_c during the growing period are represented in the crop coefficient curve. Only three values for K_c are required to describe and construct the crop coefficient curve: those during the initial stage (K_{c ini}), the mid-season stage (K_{c mid}) and at the end of the late season stage (K_{c end}).

Tabulated K_c values

Table 12 lists typical values for K_{c ini}, Kc _mid and K_{c end} for various agricultural crops. The coefficients presented are organized by group type (i.e., small vegetables, legumes, cereals, etc.) to assist in locating the crop in the table and to aid in comparing crops within the same group. There is usually close similarity in the coefficients among the members of the same crop group, as the plant height, leaf area, ground coverage and water management are normally similar.

The coefficients in Table 12 integrate the effects of both transpiration and evaporation over time. The effects of the integration over time represent an average wetting frequency for a 'standard' crop under typical growing conditions in an irrigated setting. The values for K_c during the initial and crop development stages are subject to the effects of large variations in wetting frequencies and therefore refinements to the value used for K_{c ini} should always be made. For frequent wettings such as with high frequency sprinkler irrigation or rainfall, the values for K_{c ini} may increase substantially.

TABLE 12. Single (time-averaged) crop coefficients, K_c, and mean maximum plant heights for non stressed, well-managed crops in subhumid climates (RH_min » 45%, u₂ » 2 m/s) for use with the FAO Penman-Monteith ET_o.

Crop			Kc mid	Kc end	Maximum Crop Height (h) (m)
a. Small Vegetables		0.7	1.05	0.95
Broccoli			1.05	0.95	0.3
Brussel Sprouts			1.05	0.95	0.4
Cabbage			1.05	0.95	0.4
Carrots			1.05	0.95	0.3
Cauliflower			1.05	0.95	0.4
Celery			1.05	1.00	0.6
Garlic			1.00	0.70	0.3
Lettuce			1.00	0.95	0.3
Onions
	- dry		1.05	0.75	0.4
	- green		1.00	1.00	0.3
	- seed		1.05	0.80	0.5
Spinach			1.00	0.95	0.3
Radish			0.90	0.85	0.3
b. Vegetables - Solanum Family (Solanaceae)		0.6	1.15	0.80
Egg Plant			1.05	0.90	0.8
Sweet Peppers (bell)			1.052	0.90	0.7
Tomato			1.152	0.70-0.90	0.6
c. Vegetables - Cucumber Family (Cucurbitaceae)		0.5	1.00	0.80
Cantaloupe		0.5	0.85	0.60	0.3
Cucumber
	- Fresh Market	0.6	1.002	0.75	0.3
	- Machine harvest	0.5	1.00	0.90	0.3
Pumpkin, Winter Squash			1.00	0.80	0.4
Squash, Zucchini			0.95	0.75	0.3
Sweet Melons			1.05	0.75	0.4
Watermelon		0.4	1.00	0.75	0.4
d. Roots and Tubers		0.5	1.10	0.95
Beets, table			1.05	0.95	0.4
Cassava
	- year 1	0.3	0.803	0.30	1.0
	- year 2	0.3	1.10	0.50	1.5
Parsnip		0.5	1.05	0.95	0.4
Potato			1.15	0.754	0.6
Sweet Potato			1.15	0.65	0.4
Turnip (and Rutabaga)			1.10	0.95	0.6
Sugar Beet		0.35	1.20	0.705	0.5
e. Legumes (Leguminosae)		0.4	1.15	0.55
Beans, green		0.5	1.052	0.90	0.4
Beans, dry and Pulses		0.4	1.152	0.35	0.4
Chick pea			1.00	0.35	0.4
Fababean (broad bean)
	- Fresh	0.5	1.152	1.10	0.8
	- Dry/Seed	0.5	1.152	0.30	0.8
Grabanzo		0.4	1.15	0.35	0.8
Green Gram and Cowpeas			1.05	0.60-0.356	0.4
Groundnut (Peanut)			1.15	0.60	0.4
Lentil			1.10	0.30	0.5
Peas
	- Fresh	0.5	1.152	1.10	0.5
	- Dry/Seed		1.15	0.30	0.5
Soybeans			1.15	0.50	0.5-1.0
f. Perennial Vegetables (with winter dormancy and initially bare or mulched soil)		0.5	1.00	0.80
Artichokes		0.5	1.00	0.95	0.7
Asparagus		0.5	0.957	0.30	0.2-0.8
Mint		0.60	1.15	1.10	0.6-0.8
Strawberries		0.40	0.85	0.75	0.2
g. Fibre Crops		0.35
Cotton			1.15-1.20	0.70-0.50	1.2-1.5
Flax			1.10	0.25	1.2
Sisal 8			0.4-0.7	0.4-0.7	1.5
h. Oil Crops		0.35	1.15	0.35
Castorbean (Ricinus)			1.15	0.55	0.3
Rapeseed, Canola			1.0-1.159	0.35	0.6
Safflower			1.0-1.159	0.25	0.8
Sesame			1.10	0.25	1.0
Sunflower			1.0-1.159	0.35	2.0
i. Cereals		0.3	1.15	0.4
Barley			1.15	0.25	1
Oats			1.15	0.25	1
Spring Wheat			1.15	0.25-0.410	1
Winter Wheat
	- with frozen soils	0.4	1.15