 called evapotranspiration, or ET. The application also calculates daily soil water balances for each field. Reports include indexes to help farmers determine when to irrigate.
University of Missouri Extension automatically downloads data each day from a network of agricultural weather stations across Missouri. ET is calculated from temperature, humidity, wind and solar radiation. This application uses the standardized short crop Penman-Monteith Evapotranspiration equation, or ETo, which was developed by a committee of the American Society of Civil Engineers. ET is the amount of combined water lost from a reference crop, such as grass, and soil evaporation.
ETo is multiplied by a crop coefficient, which is specific for the crop and growth stage. The crop coefficients, with the exception of rice, are derived from United Nations Food and Agriculture Organization publication FAO-56. Beginning at planting, growth stages are predicted from heat units for corn, rice and cotton. Calendar days are used for soybean. This information is used to estimate daily crop water use, or ETc.
Irrigation is a management tool to protect against yield loss from crop water stress in years with low rainfall. In that regard, irrigation can be viewed as an insurance policy. The cost of installing irrigation depends on the water source and whether applications are made by flood, furrow or sprinkler. To be profitable, the value of higher crop yields from watering, averaged over several years, should exceed the cost of the irrigation. Irrigation applications when crops do not need them are a waste of water, energy and money. In a drought though, they can save a farm from financial ruin.
Most of the cost of irrigation comes from the equipment, which is a multiple-year investment. Wells, pumps, pipes, valves and sprinklers are fixed costs even in years when they are not used. Variable costs for irrigation include fuel costs, repairs and changes depending on how many inches of water are applied in a given growing season.
Before deciding whether 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