Water Budget Calculator

Calculate seasonal irrigation water needs using Blaney-Criddle evapotranspiration, crop coefficients, and rainfall data.

Inputs

Mean temperature (°F)

Latitude (°N)

Crop coefficient (Kc)

Growing season (days)

Effective rainfall (in)

Irrigation efficiency (%)

Field area (acres)

Tab to move between fields

Results

Seasonal crop ET (in)

30.2

Gross irrigation need (in)

32.3

Reference ET (in/day)0.24

Crop ET (in/day)0.2

Net irrigation need (in)24.2

Total water (acre-ft)26.9

Total water (gallons)8,772,674

Monthly avg (acre-in)64.6

How to Use This Calculator

Enter mean daily temperature (°F) and site latitude for the Blaney-Criddle ETo estimate.
Set the crop coefficient (Kc) for your crop and enter the growing season length in days.
Enter effective seasonal rainfall and overall irrigation system efficiency (%).
Input total irrigated area in acres.
Review Gross irrigation need (in), Total water (acre-ft), and Monthly average to plan your water supply.

How the result changes with Mean temperature (°F)

Mean temperature (°F)	Seasonal crop ET (in)	Gross irrigation need (in)
41	16.5	14
63	25.4	25.8
89	35.9	39.8
111	44.8	51.7

What each input means

Mean temperature (°F): Average daily temperature during the growing season.
Latitude (°N): Site latitude in degrees north. Affects daylight hours.
Crop coefficient (Kc): Weighted average Kc for the crop over the season. Turf=0.8, corn=0.85, vegetables=0.7-1.0.
Growing season (days): Length of the irrigation season in days.
Effective rainfall (in): Total effective rainfall during the growing season (not all rain infiltrates).
Irrigation efficiency (%): Overall irrigation application efficiency. Drip=90%, sprinkler=75%, flood=60%.
Field area (acres): Total irrigated area.

What each result means

Reference ET (in/day): Blaney-Criddle reference evapotranspiration.
Crop ET (in/day): Adjusted crop ET = ETo × Kc.
Seasonal crop ET (in): Total crop water use over the growing season.
Net irrigation need (in): Seasonal ET minus effective rainfall.
Gross irrigation need (in): Water to apply accounting for system efficiency losses.
Total water (acre-ft): Total seasonal water volume in acre-feet.
Total water (gallons): Total seasonal water volume in gallons.
Monthly avg (acre-in): Average monthly water application in acre-inches.

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Mean temperature (°F) = 75, Latitude (°N) = 40, Crop coefficient (Kc) = 0.85, Growing season (days) = 150 = 7 input(s) provided
Calculate Seasonal crop ET
Seasonal crop ET = eTcInPerDay * growingSeasonDays
30.2 = 30.2
Calculate Gross irrigation need
Gross irrigation need = netIrrigationIn / appEfficiency
32.3 = 32.3
Calculate Reference ET
Reference ET = eToMmPerDay / 25.4
0.237 = 0.237
Calculate Crop ET
Crop ET = eToInPerDay * cropKc
0.202 = 0.202

Engine last updated July 26, 2026. Checked against 1 independently-derived test — how we verify calculators.

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