-year differences in ET for a calendar date are usually greater than location-to-location differences within a year. For example, ET differences were usually small in 2014 between the Columbia and Portageville weather stations for a specific summer day. But ET value differences were greater year after year at each location on the same date.
Evapotranspiration is more uniform over a region than rainfall. You can compare data from the University of Missouri weather stations. To access historical and real-time data from the extension weather stations in Missouri, go to http://agebb.missouri.edu/weather/stations. The application accesses the network of University of Missouri weather stations and automatically enters daily evapotranspiration estimates for crops. The application uses latitude and longitude of fields to input rainfall amounts from the National Weather Station rainfall grid. There are not enough University of Missouri weather stations to estimate rainfall in all farm fields, because isolated storms can occur on one field and miss another nearby.
Daily soil water balance is the most important value for managing irrigation timing. When the soil profile is full, also known as the field capacity, the balance is zero. Field capacity is characterized as water that exists in the upper limit of available water. A soil is at field capacity moisture after excess water has drained from a large rain event or irrigation. As water is extracted, the balance becomes negative. Most of the graphics and tables, which will be discussed in greater detail in Navigating the application, are based on the soil water balance.
For each field, rainfall and irrigation are tallied as deposits, and crop water use and soil evaporation are tallied as withdrawals. The goal for farmers is to keep the soil water balance high enough throughout the growing season to prevent significant crop water stress. The soil water balance declines quickly during periods of high crop water use and low rainfall. As soils dry out, crop roots have more difficulty extracting water, which causes crop leaves to wilt and results in yield loss. If soil becomes too dry, plants die.
Soil's available water holding capacity, crop rooting depth and allowable depletion are what you would use to determine a field's irrigation trigger point. Sandy soils hold less available water than loams and clays, so they would have quicker trigger points. To calculate your irrigation trigger, the application multiplies the readily available water in the root zone by the percent allowable depletion.
Rooting depth, a component of the trigger calculation, is one of the inputs in the field setup. Extension and USDA Natural Resource Conservation Service (NRCS) agents in your county can help you decide what rooting depth you should use. Typical rooting depths for many fields in the central United States are 18 to 24 inches. In fields with compacted layers or clay pans that limit root growth in the subsoil, assume shallower rooting depths (6 to 9 inches).
When setting up new fields in the application, you have a choice of entering soil texture or a five-digit mapping unit assigned to soil zones of fields by the NRCS. This information is used to estimate the available water-holding capacity of the soil in the field. The five-digit NRCS mapping unit offers more specific descriptions for different field situations than soil texture alone. Available water represents the upper and lower limits of plant available water is measured between filled capacity and permanent wilting point, or the soil moisture level at which crop plants die from water stress.
Allowable depletion is the final input needed to determine the irrigation trigger for a field. The best setting for allowable depletion is 50 percent for most crops, but use 30 percent for water stress-sensitive crops such as rice. It might help to think of different soils as car fuel tanks of different sizes. Tank size does not affect fuel efficiency, but it does determine how long you can go between fill-ups. As soil dries, the last 50 percent of the wa