The Berry DisPatch

Newsletter of the Wisconsin Berry Growers Association

Daily Crop “ET” Available over the Internet

April, 2003

By Jerry Wright

Extension Engineer, Univ. of Minnesota

P.O. Box 471 Morris, MN 56267

(320) 589-1711 or jwright@umn.edu

Daily crop ET information for irrigation scheduling by Minnesota and Wisconsin growers can be easily accessed over the internet by linking to one of the following WEB pages. This information can be most helpful in deciding when one needs to start irrigating.

Minnesota & Wisconsin (reference) Crop ETs: http://www.soils.wisc.edu/wimnext/

Or go to the Links page at www.wiberries.org

The Minnesota and Wisconsin ET daily statewide colored coded maps are available around the middle of April. Each daily ET map gives a reasonable estimation of the potential (reference) ET value across the area based on the weather conditions of that day. Potential ET values are very similar to the daily crop ET from a full canopy crop.

The daily values from Wisconsin site can also be sent directly to a user by e-mail each day for a given field site within the map area if requested over the WEB site.

These reference ET daily contour maps are generated using data from local airport automatic temperature recording stations across the state and solar radiation estimations calculated from a GOES satellite that takes a picture about every 10 minutes to estimate the daily solar radiation for a given location. A comparison of the last few years' values with ET estimations from local weather stations at Staples and Morris has found them to be very similar with the Web site values generally being around 10 percent greater in value than reported by local ET hotline services using site weather stations.

The daily WI-MN ET maps are created and managed by University of Wisconsin Extension Soils Scientist, Bill Bland. To estimate the daily ET value for a crop before full canopy and near maturity the ET value must be taken times a crop coefficient (Kco) constant that may range from .25 to 1.0 and is dependent on the growth stage of the specific crop. Generally, for annual crops, the correction factor starts out around .25 and increases to 1.0 or greater as the plant nears full canopy and holds there until produce starts to mature. As a crop matures, some crop types will have their actual ET decrease slightly each day as they come to maturity, hence the crop correction factor will go from 1.0 to maybe as low as .40 as the leaves senesce and reach maturity. The graph (pg. 3) presents a possible crop correction factor for different stages of strawberry growth (curve developed by USBR, Mid-Pacific Region 1975).

Keeping track of the crop’s daily ET use and regular in-field soil moisture checking can go a long ways in helping an operator optimize the crop’s growth while reducing the potential for leaching of some crop inputs like nitrogen into the groundwater.

These daily ET values best serve the user if recorded down on a calendar log like an irrigation checkbook worksheet or computer spreadsheet for quick reference when making your irrigation decisions. Consider assigning the updating task to one of the younger members of your family. Worksheets from the revised Irrigation Scheduling bulletin FO-01322 can be printed off the WEB at http://www.extension.umn.edu/distribution/cropsystems/DC1322.html or be purchased with a credit card at the University of Minnesota Extension Service - catalog web page at http://www.extension.umn.edu or by calling 1-800-876-8636 or 612-624-4900.

For more information on how to use daily crop ET information contact Jerry Wright, Extension Engineer at the West Central Research and Outreach Center in Morris, MN (320) 589 -1711 or jwright@umn.edu or your local Extension or SWCD office.