| Crop Coefficients |
|---|
Crop Coefficients and Growing Season Information for Computing Consumptive Water Use by Selected Crops in Western Washington1
| CROP2 | GROWING SEASON BEGINS | GROWING SEASON ENDS5 | JANUARY-APRIL | MAY | JUNE | JULY | AUGUST | SEPTEMBER | OCTOBER | NOVEMBER-DECEMBER |
|---|---|---|---|---|---|---|---|---|---|---|
| Alfalfa | 50° F3 | 32° F | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 | 0.85 |
| Clover | 50° F3 | 32° F | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 |
| Pasture/turf | 50° F3 | 32° F | 0.95 | 0.95 | 0.95 | 0.95 | 0.95 | 0.95 | 0.95 | 0.95 |
| Raspberries | 45° F3 | July 31 | 0.40 | 1.05 | 1.20 | 1.20 | 1.15 | 0.85 | 0.50 | 0.40 |
| Strawberries | 45° F3 | August 31 | 0.40 | 0.40 | 0.50 | 0.30 | 0.40 | 0.40 | 0.40 | 0.40 |
| Apples with cover | 10204 | 32° F | 0.50 | 0.75 | 1.10 | 1.20 | 1.20 | 1.15 | 0.90 | 0.50 |
| Pears with cover | 8504 | 32° F | 0.50 | 0.70 | 1.00 | 1.05 | 1.10 | 1.00 | 0.80 | 0.50 |
| Apples without cover | 10204 | 32° F | 0.45 | 0.55 | 0.80 | 0.90 | 0.90 | 0.85 | 0.65 | 0.45 |
| Pears without cover | 8504 | 32° F | 0.45 | 0.55 | 0.70 | 0.80 | 0.80 | 0.75 | 0.60 | 0.45 |
1. Information in this table is from James and others (1988). The crop coefficients are intended for use with FAO-24 Blaney-Criddle grass consumptive use such as is given in Reference Evapotranspiration. Crop coefficients listed are for use only during the growing season.
2. Crops were selected based on prevalence of crops in Whatcom County as ascertained from the U.S. Department of Agriculture 1997 Census of Agriculture (U.S. Department of Agriculture, 1999). Coefficients for crops not shown in this table but that were represented with 200 or more acres in Whatcom County in the 1997 census, including potatoes, sweet corn, and winter wheat, are computed from crop stages, the timing and appearance of which vary from year-to-year. Monthly crop coefficients for these crops are not amenable to presentation in this tabular format but can be computed using techniques given by James and others (1988).
3. Growing season begins after February 1 and at such time as the 30-day running average of average daily air temperature equals or exceeds the indicated temperature, in degrees Fahrenheit (° F)
4. Growing season begins when cumulative heat units beginning February 1 is equal to or greater than the indicated number. Cumulative heat units (HU) is computed from:
Tmax is daily maximum air temperature on day i, in ° F (i = 1 on February 1); and
where no heat units are accumulated for days with Tmax less than 43 ° F.
5. Growing season is assumed to end on the day of the first frost, which is taken to be the first late-summer or autumn day when the daily minimum air temperature falls to or below 32 ° F, or the indicated date, whichever is given.
References Cited
James, L.G., Erpenbeck, J.M., Bassett, D.L., and Middleton, J.E., 1988, Irrigation requirements for Washington- estimates and methodology: Washington State University Extension Bulletin 1513, 37 p.
U.S. Department of Agriculture, 1999, 1997 Census of Agriculture. Washington-- State and County Data: National Agricultural Statistics Service, digital data, on-line on the World Wide Web from the Universal Resource Locator (URL): http://www.nass.usda.gov/census/census97, accessed November 23, 1999, Adobe Acrobat Portable Document Format (PDF).
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