2.5: Available Water and the Soil Water Reservoir

Irrigation managers can view the soil as a reservoir for holding water. Figure 2.8 illustrates the analogy between a reservoir and a soil. Soil without any water would be like an empty reservoir (Figure 2.8a). Pores in the soil, measured as porosity, provide space for the storage of water. When saturated, the entire void space (reservoir) is filled with water as in Figure 2.8b. After 1 to 3 days of drainage, the water content reaches field capacity (Figure 2.8c). Water leaves through the drain tube on the side by gravity in the analogy. A plant can easily extract water between field capacity and a specific water content represented by the bottom of the large tube extending into the reservoir (Figure 2.8d). This specific water content is referred to as minimum balance and the difference between field capacity and minimum balance is allowable depletion (AD). As the water content decreases below the minimum balance (Figure 2.8e), the plant must work harder to extract the water it requires. The stress a plant experiences below the minimum balance causes a reduction in yield potential. If the reservoir is not replenished, the water content will continue to decrease and eventually reach permanent wilting, which is represented by the bottom of the small tube in Figure 2.8f. Once this point is reached, a plant can no longer recover even if water is added.

Figure 2.8 Reservoir analogy of soil water.

A soil volume and a corresponding water reservoir container are shown for each of six soil water statuses. The first is a dry soil and empty reservoir, the second is a saturated soil and corresponding filled reservoir, third is a soil at field capacity and corresponding partially filled reservoir (showing approximately 65% fill level), fourth showing a stress-free plant and the soil and reservoir with the allowable depletion volume of water in the reservoir, which is the fraction of water between field capacity and the maximum allowable depletion (water content at which plant stress begins), the fifth shows a plant experiencing water stress and the fraction of water stored in the soil and reservoir between the maximum allowable depletion and the permanent wilting point, while the last figure shows the soil water storage below the permanent wilting point.

The water held between field capacity and the permanent wilting point is called the available water capacity (AWC), i.e., available for plant use. The AWC of the soil is expressed in units of depth of available water per unit depth of soil, for example in/in or cm/cm. The AWC is calculated by:

\(AWC=θ_{fc}-θ_{wp}\) (2.8)

For the fine sandy loam soil shown in Figure 2.6, the volumetric water content at field capacity (θ_fc) is 0.23 and the volumetric water content at WP (θ_wp) is 0.10. Thus, the available water capacity for that soil is 0.13 in/in or cm/cm (0.23 – 0.10). You should read this as 0.13 inches of water per inch of soil depth. Field soils are generally at a water content between the FC and the WP. Commonly used terminology in irrigation management is soil water depletion (SWD) or soil water deficit (SWD). SWD refers to how much of the available water has been removed, i.e., the difference between θ_fc and θ_v, the actual soil water content. The difference between θ_v and θ_wp is the amount of available water remaining. Often the depleted and remaining water values are expressed as a fraction or percentage. The equations for determining the fraction of available water depleted and the fraction of available water remaining are as follows:

fraction of available water depleted = \(f_d=\left(\dfrac{θ_{fc}-θ_v}{θ_{fc}-θ_{wp}}\right)\) (2.9)

and

fraction of available water remaining = \(f_r=\left(\dfrac{θ_v-θ_{wp}}{θ_{fc}-θ_{wp}}\right)\) (2.10)

Also,

\(f_r=1–f_d\) (2.11a)

or

\(f_d=1-f_r\) (2.11b)

It is very useful in irrigation management to know the depth of water required to fill a layer of soil to field capacity. This depth is equal to the SWD. Do you see why? SWD can be calculated by:

\(SWD=f_d(AWC)L\) (2.12)

By substituting Equation 2.9 into Equation 2.12 you will find that this is equivalent to:

\(SWD=(θ_{fc}–θ_v)L\) (2.13)

The capacity of the available soil water reservoir, total available water (TAW), depends on both the AWC and the depth that the plant roots have penetrated. The relationship is:

\(TAW= AWC(R_d)\) (2.14)

where: TAW = total available water capacity within the plant root zone (cm), and R_d = depth of the plant root zone (cm).

Plant root zone depths will be discussed further in Chapter 6. Equation 2.14 is applicable to soils that have the same soil texture throughout the root zone. In the field, soil textures change with soil depth. Thus, TAW is calculated by determining SWD for each soil layer throughout the root zone and adding them together.

Example \(\PageIndex{1}\)

A sample of the silt loam soil characterized in Figure 2.6 has a volumetric water content of 0.26. Calculated f_d, f_r, AWC, and
SWD. Assume the soil is 36 inches deep.
Given: θ_fc = 0.34 (from Figure 2.6)
θ_wp = 0.16 (from Figure 2.6)
Find: f_d and f_r
Available water capacity (AWC)
Depth of soil water depletion (SWD)

Solution

\(f_d=\dfrac{θ_{fc}-θ_v}{θ_{fc}-θ_{wp}}=\dfrac{0.34-0.26}{0.34-0.16}=0.44\)

\(f_r = 1 – f_d = 1 – 0.44 = 0.56 \)
\(AWC= θ_{fc}-θ_{wp}= 0.34 - 0.16 = 0.18\text{in/in}\)
\(SWD=f_d(AWC)L= 0.44 (0.18 \text{in/in})36 \text{in} = 2.85\text{in}\)