Soil Water Content & Irrigation Depth Calculator

Walks from a soil sample's mass water content, to volumetric water content, to an equivalent depth of water in the soil, to how much more water (if any) is needed to reach a target — the same chain of conversions behind every "how much should I irrigate" question.

Step 1 — Mass Water Content

θ_m = M_w / M_s

Enter mass water content directly instead

Wet soil mass

Dry soil mass (M_s)

Mass water content (θ_m)

—

g water / g dry soil

Step 2 — Volumetric Water Content

θ_v = θ_m × ρ_b / ρ_w

Enter bulk density directly instead

Dry soil mass (M_s)

Bulk sample volume (V_b)

Bulk density (ρ_b)

g/cm³

—

cm³ water / cm³ soil (θ_v)

Step 3 — Equivalent Depth of Water

d = θ_v × L

Soil layer depth (L)

—

Current water depth in this layer

Step 4 — Irrigation Depth Needed

Depth to add = (θ_{v, target} − θ_{v, current}) × L

Target volumetric water content

—

About this calculator

Mass water content (θ_m) is the easiest to measure directly (weigh, dry, reweigh) but isn't directly useful for irrigation decisions. Converting it to volumetric water content (θ_v) using bulk density, then to an equivalent depth of water over a soil layer, makes it directly comparable to an irrigation or rainfall depth — which is what Step 4 uses to figure out how much more water (if any) is needed to reach a target water content.

Textbook Example 2.2: An 80 cm³ soil sample weighs 120 g wet and 100 g dry. θ_m = 0.20, θ_v = 0.25, current depth in the top 12 in (1 ft) = 3.0 in. Raising θ_v to 0.30 requires 0.6 in more water. Click "Load textbook Example 2.2" above to see it worked through.