ter 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 up with irrigation over the past month. If a field's soil water balance is consistently below the trigger line, you might need to irrigate more frequently.
Soil water is near field capacity.
Field is drying, but irrigation might not be needed.
Check soil and crops to verify irrigation is needed.
This is the second monitoring option in the Crop Water Use application that can help a farmer make plans in their fields for the next week, assuming no rainfall occurs. It can help you plan ahead for labor needs and help anticipate future irrigation needs that might interfere with planned activities.
The Soil Dryness Index in Farm Summary is derived from the soil water balance and the irrigation trigger and can simplify irrigation management to save farmers time. For growers with 100 or more fields, keeping up with each field's irrigation trigger value can be difficult. For example, two fields of a crop planted on the same day can differ even when soil water balances are the same because the soils are not the same. A field with silt loam soil might last one or two days longer without water stress than a field with sandy loam soil, which might need to be irrigated sooner.
To make the process easier to interpret, a dryness index with four levels based on a critical soil water balance for each field is calculated from rooting depth, available water holding capacity and allowable depletion. Table 1 is an example from research fields irrigated at University of Missouri. Field 4 Pivot E and Lee Field 12-1a needed irrigation that day. The others could wait until later in the week. A warning is displayed on the Farm Summary screen based on the soil water balance trigger for each field. If a field shows an OK dryness index, that means the balance is greater than 70 percent of the critical level. One, two or three asterisks were shown when the balance exceeds 70, 80 or 90 percent of the critical level, respectively. The farm summary displays XXX when the balance exceeds the critical level and irrigation is needed.
The dryness inde