Crop Water Requirement Calculator

Calculate net and gross crop water requirements (ETc) using FAO-56 stage-specific crop coefficients (Kc), reference ET0, and USDA effective rainfall credits.

Method reviewed by Plant Calculator Team·Updated August 2026

How this is worked out

ET<sub>c</sub> = ET<sub>0</sub> × K_c   ·   NIR = max(0, ET<sub>c</sub> - P_eff)   ·   GIR = NIR ÷ E_i   ·   Volume = GIR × Area × 27,154 gal/ac-in

Where ET<sub>0</sub> is reference evapotranspiration, K_c is growth-stage specific crop coefficient, P_eff is USDA effective rainfall, and E_i is irrigation application efficiency.

Assumptions

Crop coefficient (Kc) changes dynamically through initial vegetative, crop development, mid-season reproductive peak, and late season maturity.

USDA Soil Conservation Service effective rainfall model credits only precipitation that infiltrates into the root zone without causing runoff or deep percolation.

Gross Irrigation Requirement (GIR) accounts for system delivery efficiency (drip / micro-irrigation 90%, low-pressure center pivot 85%, solid-set rotary sprinklers 75%, fixed overhead sprayers 65%, surface furrow / flood 55%).

1 acre-inch of water equals precisely 27,154 US gallons (226,100 lbs of water).

How to calculate crop water requirements: FAO-56 Kc, ETc & rainfall credits

Accurate crop water budgeting prevents crop water stress during critical flowering and yield development phases while eliminating fertilizer nutrient leaching. The internationally recognized FAO-56 Dual Crop Coefficient method calculates seasonal water demand by combining atmospheric evaporative demand (ET₀) with physiological plant transpirational resistance (K_c).

The Four FAO-56 Crop Growth Stages

FAO-56 Crop Growth Stages and Evapotranspiration Characteristics
Growth Stage	Crop Development Activity	Typical Kc Range	Primary Water Driver
1. Initial (K_c,ini)	Germination, emergence, up to 10% canopy cover	0.30 – 0.50	Direct bare soil surface evaporation
2. Development (K_c,dev)	Rapid vegetative growth, 10% to 80% canopy cover	Linear rise (0.50 → 1.15)	Expanding root depth & leaf area
3. Mid-Season (K_c,mid)	Flowering, fruit set, yield formation	1.00 – 1.25 (Peak)	Maximum transpirational stomatal conductance
4. Late Season (K_c,end)	Maturation, leaf senescence, fruit ripening	0.60 – 0.85	Declining foliage & harvest drying

The Mathematical Equations

1. Crop Evapotranspiration: ET_c = ET₀ × K_c

2. Effective Rainfall (USDA Method): P_eff = MAX(0, 0.8 × P_tot - 0.45) (in inches)

3. Net Irrigation Requirement: NIR = MAX(0, ET_c - P_eff)

4. Gross Irrigation Requirement: GIR = NIR ÷ Irrigation Efficiency (E_a)

5. Volume in Acre-Inches: Acre-Inches = Acres × GIR (in)

6. Volume in Gallons: Volume = Acre-Inches × 27,154 gallons/acre-inch

Three Worked Crop Water Demand Examples

1. Commercial Tomato Field (10 Acres, Mid-Season Peak, Drip 90%)

July reference ET₀ = 7.0 inches. Tomato mid-season K_c = 1.15 → ET_c = 7.0 × 1.15 = 8.05 inches. Monthly rainfall P_tot = 1.0 inch → P_eff = (0.8 × 1.0) - 0.45 = 0.35 inches. Net demand NIR = 8.05 - 0.35 = 7.70 inches. Gross demand GIR = 7.70 ÷ 0.90 = 8.56 inches. Total Monthly Volume = 10 acres × 8.56 in × 27,154 = 2,324,382 Gallons (85.6 Acre-Inches).

2. Sweet Corn Plot (1 Acre, Center Pivot 85%, Moderate Rain)

Reference ET₀ = 6.0 in. Sweet corn peak K_c = 1.20 → ET_c = 7.20 in. Rainfall P_tot = 2.5 in → P_eff = (0.8 × 2.5) - 0.45 = 1.55 in. Net demand NIR = 7.20 - 1.55 = 5.65 in. Gross demand = 5.65 ÷ 0.85 = 6.65 inches. Total Volume = 180,574 Gallons (6.65 Acre-Inches).

3. Backyard Raised Bed Salad Greens (500 sq ft, Spring Initial Stage)

Reference ET₀ = 4.0 in. Lettuce initial K_c = 0.40 → ET_c = 1.60 in. Rainfall P_tot = 1.8 in → P_eff = 0.99 in. Net demand NIR = 1.60 - 0.99 = 0.61 in. Drip efficiency 90% → Gross depth = 0.61 ÷ 0.90 = 0.68 in. Volume = 500 sq ft × (0.68 ÷ 12) × 7.48 = 212 Gallons for the month.

Common Crop Water Budgeting Mistakes

Using a static mid-season Kc for the entire season: Applying peak water volume to young seedlings with K_c = 0.40 saturates the soil profile, suffocates roots, and leaches expensive nitrogen fertilizer beyond the root zone.
Crediting 100% of rain gauge precipitation: Light summer showers of 0.10 inches wet only surface dust and evaporate within two hours without replenishing root zone moisture. Always discount total rainfall to effective rainfall.
Confusing Net and Gross Water Requirements: Net water satisfies transpirational deficit; gross water accounts for irrigation distribution losses. Ignoring efficiency under-irrigates crops by 15% to 45%.

Determine local evapotranspiration and watering intervals with our Evapotranspiration Calculator, Irrigation Scheduling Calculator, and Watering Schedule Calculator.

Common questions

What is the Crop Coefficient (Kc)?

The Crop Coefficient (Kc) is an agronomic multiplier that relates actual crop water use (ETc) to a standard reference grass surface (ET0) via the equation ETc = ET0 × Kc. During early seedling stages, Kc is low (0.3 to 0.5); at peak flowering and fruit sizing, Kc peaks at 1.05 to 1.25; and at maturity/senescence, Kc drops to 0.60 to 0.80.

What is Effective Rainfall (P_eff)?

Effective rainfall is the fraction of total precipitation that actually infiltrates and is stored in the crop root zone without being lost to surface runoff or deep gravitational percolation below the roots. Under USDA-SCS methods, light rain showers under 0.20 inches often evaporate completely and do not contribute to effective crop water storage.

How much water is in one Acre-Inch?

One acre-inch is the volume of water required to cover one acre (43,560 sq ft) to a depth of one inch. It equals exactly 27,154 U.S. gallons (102.79 cubic meters or 102,790 liters).

Why does irrigation system efficiency impact gross water requirements?

Gross irrigation requirement (GIWR) divides the net crop deficit by the irrigation efficiency. An overhead sprinkler with 70% efficiency requires applying 1.43 inches of gross water to deliver 1.00 inch of net moisture to the root zone, whereas high-efficiency drip (90%) requires only 1.11 inches.

Crop Agronomy & Meteorological Parameters