Soil Moisture Monitoring: What Does 65% Depletion Mean?

by Jim Bauder and Linzy Carlson

"Just what do you mean when you say that alfalfa doesn't need to be irrigated until 65% depletion of the soil moisture, yet beans and potatoes need to be irrigated at 40% depletion? I really can't visualize that or put a number on what that means in terms of when I need to irrigate. "

In simplest terms, the percent depletion is the percentage of the maximum amount of water the soil can hold that has already been used by the plant. The percent available water is the percentage of that same maximum amount that remains. So, "65% depletion" means that 65% of the soil's total water capacity has been used. In other words, 65% of the water that was present in the soil when it was saturated is now gone. At 65% depletion, there is 35% available water remaining. At 40% depletion, there is 60% available water remaining. These numbers are important because different plants start to suffer stress at different soil moisture levels. The alfalfa mentioned above can use 65% of the available water before stressing, while more sensitive crops, such as beans and potatoes, will stress at 40%.

Now, the question is how these depletion percentages can guide your irrigation scheduling. Before you begin, you need to know a couple things:

  1. The crop and stage of growth or date.
  2. The soil texture by depth. This is available from a county soil survey.

I. Find Available Water Capacity

The first step is to determine the available water capacity (AWC) of the soil profile. This is when you need to know the soil texture by depth as deep as the roots of the crop. The manageable rooting depth (the depth of soil you will be concerned with) is controlled either by the depth of soil or the crop rooting depth. Use the rooting depths from Table 1 if you have soils deeper than the crop roots. Otherwise, use the depth of the soil.

Table 2 shows the maximum amount of water each foot of the specific textures can hold. You already know how many feet of each soil type you have in your soil profile, so simply multiply the value for AWC per foot from the table by the depth of that type of soil. Do this for all the soil types and add up the total. This is the total available water capacity.

Table 1. Crop Rooting Depth and Maximum Allowable Depletion
Crop
Rooting Depth
Depletion Allowance
Alfalfa
4 feet
65%
Grass Meadow
3 feet
60%
Cereal Crops
2.5 feet
40-50%
Sugar Beets
2.5 feet
40%
Potatoes or Corn
2 feet
40%
Beans or Peas
1.5 feet
40%

Table 2. Soil Texture and Available Water Capacity in Inches/Foot II. Adjust for Rocks
Soil Texture
Feels Like . . .
AWC (inches/foot)
Coarse sand, sand
sand, grit
0.3-0.5
Fine sand, very fine sand
fine grit, sand
1.25
Loamy coarse sand, loamy sand
sandy, loamy
1.0
Loamy fine sand
smooth, fine grit
1.25
Coarse sandy soam, sandy loam
smooth with grit
1.25-1.5
Fine sandy soam
smooth, fine grit
1.5-2.0
Loam, silt loam
smooth
2.0
Silt, sandy clay loam
smooth, slippery
2.0-2.2
Clay loam, silty clay loam
sticky but smooth
2.2
Sandy clay, silty clay
smooth and sticky
2.0
Clay
sticky
2.0

Rocky soils cannot hold as much water as rock-free soils, so if you have lots of rocks in your soil, you must consider the effect this has on the amount of water available to the plants. First, estimate the percentage of rock in the soil. The rule of thumb is to assume that each percentage of rock reduces the amount of water the soil can hold by that percentage.

For example, a soil that is 20% rock holds 20% less water than a rock-free soil. In other words, it holds 80% of the water of a rock-free soil. Therefore, to get this hypothetical soil's true available water capacity, multiply the total available water capacity of a rock free soil by 0.8. (If the soil is 10% rock, multiply by 0.9. If it is 30% use a factor of 0.7, etc.)

II. Get a Number for Percent Depletion

You now have a number for available water adjusted for rocks. This is the total amount of water in the soil available to the crop. For each crop, there is an ‘accepted' depletion allowance. Those values are shown for the various crops in Table 1. Multiply this percentage by the available water adjusted for rocks. The result is the amount of water that the crop can remove before it needs to be irrigated.

III. Keep Track of How Much Water the Crop is Using

The AgriMet system gives a day-by-day account of how much water each crop has used throughout the growing season and projects crop water use for the next week. Look at the AgriMet data to see how much water the crop has used since the last irrigation. If it has used more than the allowable depletion amount, it's time to irrigate. If it hasn't, then project how far the water you have left will go, and plan to be irrigating close to the time when the depletable amount will be gone.

Example: The crop is alfalfa, and the soil survey says that the first foot of the profile is silty clay loam with 10% rock. The next foot and a half is fine sandy loam with 20% rock, and the next foot and a half is silty clay with 30% rock. For each soil type, multiply the depth by the number of inches of water held per foot of soil (Table 2) by the rock adjustment factor. This gives the total available water. Of that 6.3", only 65%, or 4.1", is actually available to the alfalfa crop. Therefore, you should be irrigating when the crop has used 4.1".

A look at the AgriMet plant water use data from the past couple weeks shows that the crop has used 3.1" since the last irrigation. This means that there is 1" of water still available to the crop before it begins to stress. Assuming the alfalfa uses .25" per day (a rough estimate), the crop will need water in about 4 days. After you irrigate, assume the soil profile is full and repeat the process of keeping track of how much water the crop is using, irrigating again when it has used 65% of the available water. It's as simple as that!
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