Jerry Wright, Minnesota Extension Engineer
Crop evapo-transpiration (ET) values will be available on the World Wide Web starting May 1. To access daily ET information on color-contoured statewide maps, link to one of the following sites:
Minnesota & Wisconsin (reference ETs only):
http://www.soils.agri.umn.edu/research/climatology/doc/agwx.html, or
http://bob.soils.wisc.edu/wimnext/
North Dakota & Red River Valley (ETs for several crops):
http://www.ext.nodak.edu/weather/cropwater/
The Minnesota and Wisconsin map gives only a reference ET potential. To get daily crop ET value, you need to multiply the reference value by a crop coefficient (Kco) that is dependent on the crop growth stage. Generally, Kco is between 0 and 1; the value increases as the crop develops and is equal to 1.0 by the time the crop canopy closes. For some crops, Kco increases to around 1.1 during critical crop growth stages&emdash;for example, corn between late pollination and early dough stages. The reference ET daily contour maps are generated using data from local, automatic temperature-recording stations across the state and from solar radiation values calculated from satellite-derived insolation maps.
After June 1, irrigators and crop advisors in central Minnesota near Park Rapids, Perham, and Staples can obtain estimated daily crop water use, or ET information, for alfalfa, corn, dry beans, and potatoes via the following telephone hotlines any time of the day:
Park Rapids: (218) 732-1963 - Hubbard SWCD (Soil Water Conservation District) office
Perham: (218) 346-7923 - East Otter Tail SWCD office
Staples: (218) 894-3476 - Central Lakes Ag Center
Daily crop ET values can be very useful to an irrigation manager keeping track of the soil moisture in a field. ET estimates should be combined with regular in-field inspections to determine when to start irrigating. When an irrigation system is operated under an electric load control program, past ET averages can also be very helpful in forecasting to help assure that adequate soil moisture will be available during the power control period. This can help an irrigation operator optimize the crop’s growth while minimizing the potential for leaching of some crop inputs like nitrogen into the groundwater.
calendar, like an irrigation checkbook worksheet, for quick reference when irrigation decisions are made. Consider assigning the task of obtaining and recording ET values to one of the members of your family. Recording forms can be obtained by contacting the ET hotlines, county Extension offices, SWCD offices, or the author.
Messages on the ET hotlines present a reasonable ET estimate of the daily crop water use for alfalfa, corn, dry beans, and potatoes based on each crop’s current stage of growth and the previous days’ climatic conditions as measured by a local weather station. Daily crop ET estimates are determined by multiplying the specific crop’s stage of growth correction factor (Kco) by the daily potential ET value. For example, the daily ET for corn in its sixth-leaf stage is equal to about 50% of the potential ET and at blister stage is equal to about 110% of potential ET.
Potential or reference ET estimates at all hotline sites are calculated using the modified Penman ET; estimates on the World Wide Web are calculated using other methods. ET models take into consideration the days’ high and low temperature and solar radiation (related to percent sunshine); some models also use average humidity and wind speed. The calculated reference daily ET value is very similar to the actual daily crop water use for fully canopied alfalfa 6 to 8 inches in height.
Daily crop ET estimations based on temperature only are also available in the University of Minnesota Extension Service Bulletin Irrigation Scheduling-Checkbook Method, AG-FO-1322. Further information on how to set up an on-farm irrigation water management program is described in Irrigation Water Management Considerations for Sandy Soils in Minnesota, AG-FO-3875. Both bulletins are available from Minnesota county Extension offices.
Daily crop ET information can also be managed with computer software programs like WISDOM or PC-Irrigate. WISDOM was developed by University of Wisconsin Extension specialists and is sold through Gempler’s, telephone (800) 874-4755. PC-Irrigate is available from the University of Nebraska, telephone (402) 472-4259.
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, email jwright@tc.umn.edu, or telephone (320) 589-1711.
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This page is part of the Biosystems and Agricultural Engineering Department web at http://www.bae.umn.edu/