iculty 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 water available in soil is tightly bound to soil particles, so it's probably best to set the allowable depletion to trigger before the soil water "tank," to carry on the metaphor, goes past half empty.
The Crop Water Use application is designed to allow users to monitor the soil water balance by two methods. Each way has it pros and cons for making management decisions. Farmers who like to plan several days in advance usually like Farm Summary. If you prefer to just take one day at a time, you might prefer Field Status.
The Field Status button is on the main menu of the application. Farmers can scroll between fields each day to determine which needs to be irrigated. Fields are sorted driest to wettest based on their current estimated soil water balances. Use an index finger to swipe right to left across a smartphone screen to advance to the next field. To navigate to other fields on a computer screen, move the cursor close to center of the page. Then click and hold either button of the mouse while moving right to left to advance or left to right to go back.
Each field has two graphs in the field status display designed to resemble an electronic fuel gauge. The first shows the relative soil water balance to field capacity (wet) and permanent wilting point (dry) with the irrigation trigger in the center. It stops when the soil water balance declines past the trigger point in the center. Figures 1 through 3 offer examples of different soil water conditions. In the first and second figures, irrigation is not needed. In the third figure, a farmer should check the soil and crops in the field to determine whether irrigation is needed.
As a field gets drier, the graph will move toward the middle, which is the trigger point. The goal is to irrigate before a field reaches the trigger point in the center.
Also in Field Status, located below the bar graph, a line graph tracks how the soil water balance has done over the past four weeks relative to the irrigation trigger. This is a good way to see how well you have been keeping