Estimated Crop Coefficients for Strawberry

Several strawberry growers have expressed interest in using evapotranspiration data for scheduling irrigations in strawberries, especially during the production season when  crop water needs are greatest.   Weather-based approaches to scheduling irrigations are used for many cultivated crops.  Windspeed, air temperature, relative humidity, and solar radiation affect plant water-use, or more specifically the water lost by evaporation from the soil and by transpiration from the leaves of the crop.  Using evapotranspiration (ET) data (evaporation + transpiration) from the California Irrigation Management Information System (CIMIS) the consumptive water use of a crop in units of inches or mm per day, can be estimated.  

 CIMIS ET data is available from the Department of Water Resources website (http://wwwcimis.water.ca.gov/cimis/welcome.jsp) for more than 120 locations in California, and is generated by weather stations located on irrigated grass, which serves as a reference crop.   The MyCIMIS feature of the website allows the user to customize the reporting of CIMIS crop ET data, such as specifying type of weather data, stations, time period, and file format to display.  MyCIMIS also allows the user to select for the data to be emailed to their account.  Spatial CIMIS is another feature of the website that produces estimates of reference ET at a 2 km (1.2 mi) resolution using GOES satellite information and by triangulating humidity, temperature and wind speed data from the closest CIMIS stations to the point of interest.  A Google map feature allows the user to locate a field of interest.  Similar to MyCIMIS, a user can select to have updated Spatial CIMIS estimates of reference ET emailed.

ET can be estimated for a specific crop by multiplying reference ET data and the appropriate crop coefficient (Kc):

ET_crop = ET_ref × Kc

The value of Kc can range from almost 0 to greater than 1 and is closely related to the percentage of ground shaded by the canopy.   Irrigation method and physiological stages, such as flowering and senescence are also factored into the crop coefficient.  Crop ET values should be adjusted down by 20% to 30% for crops grown under macro tunnels or greenhouses because of shading.

Because accurate crop coefficients are not available for many crops, estimates of canopy cover serve as a close substitute for the Kc values.  We have taken overhead photos of the UC strawberry variety Albion using an infra-red camera during the last 2 years.  Photos were taken on a monthly schedule for fields with 48-, 52-, and 64- inch wide beds. After analyzing canopy images from 9 fields, we have estimated the Kc values on a weekly schedule during a 12 month period (Table 1).     Because these data represent the average of several fields, values may need to be adjusted for site-specific conditions.  Also, these Kc values for Albion represent Salinas and Pajaro Valley growing conditions and methods.

By irrigating enough to replace water lost by evapotranspiration it is possible to optimize irrigations for production and minimize percolation below the root zone.   Also, it is possible to avoid under-irrigating during periods of high water consumption, which can result in stress and reduced growth.  ET_c estimates can be used to determine day by day soil water depletions from field capacity and thus can be used to also estimate when to irrigate.  For detailed descriptions and examples of this technique, visit http://wwwcimis.water.ca.gov/cimis/infoIrrSchedule.jsp 



Table 1.  Estimated crop coefficient (Kc) for UC strawberry variety Albion.