Depletion Threshold Help

SoyWater Users: Below are some helpful hints regarding setting a soil water depletion threshold. Click HERE at any time you want to leave this HELP page and return to the SoyWater Chart Page.

You can choose the soil water depletion percentage that you want SoyWater to use as YOUR "irrigation trigger", BUT first please read the below bullets of information.
The inches of soil water depletion (SWD) below 100% field capacity (FC) is shown each day in the last column of the SoyWater Table. This depletion arises from the crop's daily water use.
The difference between the amount of water held in a soil at 100% FC and the amount of water held in that soil at the permanent wilting point (PWP) which is 0% FC, is known as plant-available water (PAW).
In irrigated soybean, it is best avoid a soil water depletion of much more than 50% of PAW unless (1) the crop is near R7 maturity, or (2) you have training in the art of deferred or deficit irrigation.
Once your "irrigation trigger" percentage depletion value is chosen, SoyWater will yellow-highlight any cell in the last column that exceeds your chosen value, thus denoting calendar dates that SoyWater is saying to you: Pull the trigger! (i.e., IRRIGATE!).
Note that if takes 4 days to move the pivot arm around the field circle, then start the pivot at least 2 days before the yellow-highlighted day.
The capacity of your system in terms of delivering X amount of irrigation within the Y days of a complete circle of your pivot will have to be considered in selecting depletion percentage value.
The menu default value of 35% is a conservative irrigation trigger choice, but will result in more frequent irrigation. A 50% choice results in less frequent irrigation, but is more risky (what if your center pivot breaks down and takes several days to fix?). Your choice is at your own risk!
You can change your soil water depletion threshold at any time. Just press the apply button each time, you change it.

Soil Water Categories — Figure 1. This graphic displays the key soil water terms and concepts relative to any given “layer” of soil irrespective of whether that that layer has a thickness of one inch, one foot, or two or more feet of soil. A "layer" of soil can contain water that falls into basically three categories of (a) temporary gravitational water that will drains downward from this soil layer into (the next) non-saturated soil layer, (b) plant available water (PAW) held by the soil against gravity, up its maximum water-holding capacity (WHC) when it is stated to be at a 100% field capacity (FC), and down to its minimum plant available water amount known as the permanent wilting point (PWP), which is inferentially 0% FC, and c) all remaining water below the PWP that is held so tightly by the soil that plant root hairs cannot extract it and removal of that remaining water requires oven-drying the soil. This general graphic applies to every soil textures, though the amounts of soil water in each of the three categories will vary amongst the 12 soil textures classes - see the next figure.

Soil Water Categories by Soil Texture Type — Figure 2. The above soil water categorization concept is used in this stacked bar chart, where each of the 12 bars on the horizontal axis is a specific soil texture type, and the left axis has units of inches per foot of a soil layer. This graphic was created from data provided in Table 2 and Figure 4 of the Extension Publication authored by Irmak and Djaman. 2015. EC2009. Note the substantive differences in the water-holding capacity shown in each soil texture bar (i.e., total plant available water in the both 100 to 50% FC green color zone and the 50% to 0% FC (PWP) olive green color zone). See the next figure relative to the clay, sand, & silt components of each of the 12 soil texture classes.

Figure 4. The below graphic is a modified version of the foregoing soil texture triangle that depicts the rate of water infiltration relative to the percentages of clay, sand, and silt in a given soil. The infiltration rate is an important consideration when considering the irrigation application rate intensity to avoid run-off or too much standing water on the soil surface when irrigating.

Soil Texture Triangle - Water Infiltration Rates