The Pacific Northwest Cooperative
About AgriMet Crop CoefficientsAgriMet estimates crop evapotranspiration (ETc) by first modeling reference evapotranspiration (ETr) from observed weather conditions at each weather station. AgriMet uses alfalfa as its reference crop. This reference ET is then multiplied by a crop coefficient (Kc) that varies through the season depending on the growth stage of the crop. Crop coefficients, from plant emergence to termination, define a growth curve know as a crop curve . The product of reference ET times the crop coefficient is the estimated crop specific consumptive use:
ETc = ETr * Kc
AgriMet divides the crop curve into 21 points, ranging from 0 to 200 "percent" of the growth stage. The zero value typically represents plant emergence (or break dormancy in the case of perennials); 100% typically represents full cover; and 200% typically represents the termination date (harvest, dormancy, or first frost).
AgriMet uses three dates to tie into the crop curve: the start date, cover date, and terminate date. The start dates are supplied by local field contacts (cooperative extension agents, producers, experiment station specialists, etc.) for each crop at each station. Cover and terminate dates are then determined based on historical relationships with the start date. Cover and terminate dates are occasionally adjusted during the growing season based on local guidance.
During the growing season, the AgriMet computer model interpolates its "position" on the crop curve based on the current date. It first determines if the current date is between the start and cover date, or cover and terminate date. Then it interpolates between these two points. This procedure is used to select the correct crop coefficient for the current day, specific crop, and specific station.
AgriMet uses the 1982 Kimberly-Penman Evapotranspiration model to compute reference ET (ETr) at each of the weather stations in the AgriMet network (see the AgriMet system map or the station list for locations of the weather stations). AgriMet uses alfalfa as the reference crop for computation of ETr. During each day of the growing season, the reference ET at each station is multiplied by the daily crop coefficient to compute the estimated consumptive use of each crop for that day. This information is then compiled into a 4 day crop water use chart as well as an ET Summary that shows the crop consumptive use for the entire growing season.
For information on specific crop coefficients, see list of Crop Coefficients . Note that these crop coefficients are for use with Alfalfa Reference (Tall Crop) ET.