Available soil-water storage in the root zone: A soil has field capacity = 25% and permanent wilting point = 15%. The specific dry unit weight is 1.5 (relative to water = 1.0). For a crop with an 80 cm root zone, what is the storage capacity (available water) expressed as an equivalent depth of water?

Introduction / Context:
In irrigation scheduling, the available water in the root zone determines when to irrigate. It depends on field capacity (FC), permanent wilting point (PWP), soil bulk density (dry unit weight), and the effective root depth of the crop.

Given Data / Assumptions:

• FC = 25% (by weight).
• PWP = 15% (by weight).
• Specific dry unit weight = 1.5 (i.e., 1.5 times water density).
• Root zone depth = 80 cm.

Concept / Approach:
Available water fraction by weight = (FC − PWP) = 10%. Convert to an equivalent depth using the ratio of soil dry unit weight to water density and the root depth. For weight-basis moisture content, an accepted conversion is:
Depth of available water = ((FC − PWP)/100) * (γ_d / γ_w) * Root depth.

Step-by-Step Solution:
Compute moisture difference: (25 − 15) = 10% = 0.10.Unit weight ratio: γ_d/γ_w = 1.5/1.0 = 1.5.Root depth = 80 cm.Depth = 0.10 * 1.5 * 80 = 12 cm.

Verification / Alternative check:
If FC and PWP were on a volume basis, the conversion would alter; here, the presence of specific dry unit weight indicates weight-basis data and justifies multiplying by γ_d/γ_w.

Why Other Options Are Wrong:
8 cm and 10 cm: Underestimate due to ignoring the 1.5 density factor or using a smaller effective depth.14 cm and 16 cm: Overestimates beyond the computed available water for the given data.

Common Pitfalls:

• Confusing weight-basis and volume-basis moisture contents.
• Forgetting to multiply by soil-to-water density ratio when using weight basis.

Final Answer:
12 cm