We all know that in order to properly irrigate a piece of land, we must have some specific information, such as how much water is in the soil to begin with and how much can be depleted. To show you how to get this information, the following example takes you through each step of determining a soil profile's total available water capacity, depletable water, and soil water reserves.

First, determine the soil profile depth. This will be the same as the crop's root depth, since any water past the root depth is not available to the plants. For this example, we will assume the crop is alfalfa, so the root zone and profile depth is 4'. Now, determine the soil texture and amount of water that specific texture will hold. Our hypothetical soil will be a silt loam, which holds 2" of water per foot of profile. Water holding capacity is available for your specific soil in your county's soil survey. The following table gives this information for the general soil textures.

Soil Texture	Inches of Water/Foot of Moist Soil
Sands, Fine sands	.75
Very fine sands, Loamy sand	1.0
Sandy loam	1.5
Loam	1.9
Silt loam, Silt	2.2
Silty clay loam	1.9-2.2
Clay loam, Sandy clay loam	1.7-2.2

Calculate the profile's total available water capacity by multiplying the profile depth by the amount of water held per foot of profile. In our example, each foot of the profile holds 2", and the profile is 4' deep, so we have 4' x (2"/foot) = 8" available water.

Determine management allowed depletion. This percentage describes how much water can be removed from the soil before the plant becomes stressed to the point where yield is effected. For alfalfa, 50% of the available water can be removed, making the management allowed depletion 50%.

Multiply the total available water by the management allowed depletion. This gives the depletable water, or the amount that can be removed before the plant becomes stressed. In our example, the amount of depletable water is 8"x0.5 = 4".

The soil water reserve threshold is simply the point at which all the depletable water is gone. It is determined by subtracting the depletable water from the total available water. For our alfalfa example, the soil water reserve threshold occurs at 8"- 4"=4". When the soil water reaches this threshold at the point where you begin irrigating the field, you should be irrigating.

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