irrigation is necessary, check the soil dryness of a field with a shovel or soil probe and look at crop leaves for signs of water stress. Soil moisture sensors installed in fields are also useful for evaluating signs of water stress. The Crop Water Use application provides a model that estimates field soil water conditions and can be a helpful when making decisions about irrigation.
Watching El Nino and La Nina patterns can be helpful for long-term planning but less useful in day-to-day irrigation scheduling. Five-day rainfall predictions for a region are more useful, but isolated storms are common in the summer. Irrigation decisions can be difficult on days when the local weather forecast predicts an 80 percent or greater chance of rain for the region. It is generally best to assume your fields will get little to no rain unless radar shows a large, continuous band of rain moving toward your fields.
The two most common mistakes farmers make are waiting too late to start irrigation applications and not applying enough water to keep up with crop water use. The greatest crop water use occurs on days with bright sun, low humidity and strong winds. Crop growth and leaf canopy cover are also important. It is often difficult to adequately recharge soil water in the profile with later irrigations if a farmer fails to make a needed irrigation application. The economic savings of delaying or skipping an irrigation application are small compared to the potential yield loss from crop stress. Farmers should irrigate when the Crop Water Use application shows the soil water balance is at the trigger point rather than delay watering just because of a high probability of rainfall in the local forecast.
Daily crop water use from ET is mainly affected by sunlight intensity, humidity and wind. Using actual weather to estimate ET is more accurate than long-term historical averages. Sunlight, humidity and wind do not vary significantly across a region, so the nearest University of Missouri weather station is a useful resource. Year-to-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 diff