When using ET information from farmwest.com, the data provided may not always match what is actually happening in the field. This is often due to the data not being properly corrected for a particular field situation. Following is a list of possible problems and remedial actions that can help ensure successful use of the ET information for irrigation scheduling.
The Factsheets referenced are available from the BC Ministry of Agriculture, Food and Fisheries.
1. The weather station site has improper exposure. ET estimates can be higher or lower than ET of the irrigated field. Watch ET from surrounding weather stations - the closest station is not always the most representative. Variations in temperature, humidity, and wind occur over short distances as the topography changes. If the irrigated field has an unusual exposure, a fixed percentage adjustment to the data may be required.
2. The estimate of crop growth stage does not match the actual growth stage for the irrigated field. It is difficult to specify specific growth stage lengths for every hybrid variety. Check the growth stage in the field and choose a crop coefficient that reflects the appropriate crop maturity.
3. Prior to full canopy, rain or irrigation has made the soil surface wet. Actual crop ET rates may be slightly higher than the estimates. If there is a higher than usual frequency of days with precipitation or irrigation make a check of the actual soil moisture conditions.
4. Prior to irrigation, there is a dry root zone in the field. Actual crop ET rates may be lower than the estimated ET. Check the root zone for soil moisture and irrigate if conditions warrant.
5. There are differences in emergence dates on irrigated fields. Obtain a separate ET estimate for each irrigated field by selecting the crop and emergence date for all scheduled fields.
6. Field observations of soil moisture are consistently higher than estimates from scheduling programs. The scheduling programs may not properly represent drainage in the soils. For slow draining soils, the water (above field capacity) is actually available to the crop for a longer time than in a faster draining soil. The 'effective' field capacity is higher after rainfall or irrigation.
7. The plant population in the irrigated field is higher or lower than average. Recognize that a higher population (density) will have higher ET and a lower population will have lower ET in the early and late season. Differences during mid-season disappear as both population densities have sufficient leaf area.
Adapted From the University of Nebraska-Lincoln cooperative extension http://cropwatch.unl.edu/irrigate.htm