Determining Crop Water Use - Crop Coefficients

Crop Water Use

Actual crop water use is directly related to ET. The crop's water use can be determined by multiplying the calculated ET by a crop coefficient, Kc, which takes into account the difference in ET between the crop and reference evapotranspiration.

ETc = ETo x Kc
Where
ETo = calculated grass reference ET (mm) available on www.farmwest.com
Kc = crop coefficient, Table 2 - Kc most BC crops, Table 3 - Kc for tree fruits and grapes
ETc = crop evapotranspiration or crop water use (mm)

Crop Coefficients and Stages of Growth

The crop coefficients, Kc values, represent the crop type and the development of the crop. There may be several Kc values for a single crop depending on the crop's stage of development.

During the crop's germination and establishment most of the evaporation occurs as evaporation from the soil surface. If the surface is wet frequently due to irrigation or rainfall the evaporation rate is greater. As the foliage develops evaporation from the soil surface decreases and transpiration increases, see Figure 2. For the crop development and late season use Kc values between the two values given in Table 2.

Table 1 Stage of Development

Stage Indicators Crop Coefficient
Initial Planting date (or the start of new leaves for perennials) to 10% ground cover. Kc ini
Crop development 10% ground cover to effective full cover, about 60-70% coverage for tree crops and 70-80% for field and row crops. Kc ini - Kc mid
Mid season Effective full cover to maturity, indicated by yellowing of leave, leaf drop, browning of fruit. This stage is long for perennials but relatively short for vegetables crops that are harvested for their fresh fruit. Kc mid
Late Season Maturity to harvest: the Kc value could be high if the crop is irrigated frequently until fresh harvest or low if the crop is allowed to dry out in the field before harvest. Kc mid - Kc end

The crop coefficients in Table 2 can be used as a general guideline. Soil moisture monitoring devices can be used to adjust the crop coefficients to local conditions.

NOTE: SOIL MOISTURE SHOULD BE MONITORED WHEN USING CLIMATE INFORMATION TO SCHEDULE IRRIGATION. IF THE SOIL MOISTURE IS REMAINING WET THE CROP COEFFICIENTS NEED TO BE LOWERED, IF THE SOIL IS DRYING QUICKER THAN EXPECTED THE CROP COEFFICIENTS NEED TO BE HIGHER. For more information on soil moisture monitoring see Factsheet: Irrigation Scheduling with Tensiometers.

Table 2 Crop Coefficients

Crop Kc ini Kc mid Kc end Crop Kc ini Kc mid Kc end
alfalfa 0.4 1.2 1.15 onions 0.7 1.05 0.95
apples 0.5 1.2 0.85 pasture 0.4 1 0.85
apricots 0.45 1.15 0.85 peaches 0.45 1.15 0.85
beans, green 0.5 1.05 0.9 pears 0.5 1.2 0.85
beets 0.5 1.05 0.95 peas 0.5 1.15 1.1
berries, bushes 0.3 1.05 0.5 potato 0.5 1.15 0.75
broccoli 0.7 1.05 0.95 pumpkin 0.5 1 0.8
cabbage 0.7 1.05 0.95 radish 0.7 0.9 0.85
cabbage-local 0.7 1.05 0.95 small vegetables 0.7 1.05 0.95
carrots 0.7 1.05 0.95 spinach 0.7 1.05 0.95
cauliflower 0.7 1.05 0.95 squash 0.5 0.95 0.75
celery 0.7 1.05 0.95 stone fruits 0.45 1.15 0.85
cereal 0.3 1.15 0.25 sweet corn 0.3 1.15 0.4
cherries 0.5 1.2 0.85 sweet peppers 0.7 1.05 0.85
cucumber 0.6 1 0.75 tomato 0.7 1.05 0.8
grapes 0.3 0.8 0.5 tubers 0.5 1.05 0.95
green onions 0.7 1.05 0.95 watermelon 0.4 1 0.75
lettuce 0.7 1 0.95        

Table 2 Crop Coefficients Tree Fruits And Grapes

Crop April May June July Aug Sept Oct
Apples, Cherries, and Pears with a cover crop*
Lower Mainland / Vancouver Island 0.5 0.7 0.9 1 1 0.95 0.75
Okanagan / SW Interior / Cariboo 0.45 0.85 1.15 1.25 1.25 1.2 0.95
Kootenay 0.45 0.8 1.1 1.2 1.2 1.15 0.7
Apricots, Peaches and other Stone Fruit with cover crop*
Lower Mainland / Vancouver Island 0.85 0.9 1 1 1 0.95 0.8
Okanagan / SW Interior / Cariboo 0.45 0.8 1.1 1.2 1.2 1.15 0.9
Kootenay 0.5 0.7 1 1.05 1.1 1 0.8
Grapes
Lower Mainland / Vancouver Island 0.5 0.55 0.65 0.65 0.65 0.65 0.5
Okanagan / SW Interior / Cariboo - 0.5 0.7 0.8 0.85 0.8 0.7
Kootenay - 0.45 0.7 0.85 0.9 0.8 0.7

*If there is no cover crop, grass, between the trees the crop coefficients will be about 10% lower in April May, September and October and 20% lower in June, July and August.

For more information on crop coeffficients see Factsheet: Crop Coefficients for use in Irrigation Scheduling