Water Budget Calculator

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

About this calculator

This calculator sizes seasonal irrigation water needs using the Blaney-Criddle method, formalized by the U.N. Food and Agriculture Organization in Irrigation and Drainage Paper No. 24, "Crop Water Requirements" — a decades-old but still widely used way to estimate reference evapotranspiration (ETo) when full weather-station data isn't available — it needs only mean temperature and latitude, unlike the more data-hungry Penman-Monteith approach (the method FAO's later Paper No. 56 recommends when fuller weather data is available). The p-factor in the formula represents the percentage of annual daytime hours falling in the period being estimated, which this calculator approximates as increasing with latitude to roughly capture longer summer daylight at higher latitudes; real Blaney-Criddle tables use month-by-month lookup values rather than this smooth approximation, so treat the ETo output as a reasonable planning estimate, not an agronomy-grade figure.

Reference ET is then scaled by a crop coefficient (Kc) — a multiplier reflecting how much water your specific crop actually uses relative to a reference grass surface — to get crop evapotranspiration (ETc), which is totaled across the growing season. Effective rainfall (only the portion of precipitation that actually infiltrates and is available to the crop, not total rainfall) is subtracted to find net irrigation requirement, and that's divided by application efficiency — how much of the water you apply the crop actually receives, with drip systems around 90% but flood irrigation as low as 60% — to get the gross amount you must actually deliver. The gross figure is finally converted to acre-feet and gallons using the standard 27,154-gallons-per-acre-inch conversion. The biggest lever for reducing total water pumped is usually application efficiency, not crop selection — switching from flood to drip can cut gross water need by nearly a third for the same crop.

Inputs

Mean temperature (°F)

°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

Figures current as of 1977. Source: Doorenbos, J. and Pruitt, W.O., "Crop Water Requirements," FAO Irrigation and Drainage Paper No. 24 (rev. 1977)

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)
38	15.3	12.4
56	22.6	22.1
113	45.6	52.8
120	48.4	56.6

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

Figures and sources

Modified Blaney-Criddle reference evapotranspiration method (1977) — Doorenbos, J. and Pruitt, W.O., "Crop Water Requirements," FAO Irrigation and Drainage Paper No. 24 (rev. 1977)

Engine last updated September 2, 2026. Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why does this calculator only need temperature and latitude, not full weather data?

It uses the Blaney-Criddle method, which estimates reference evapotranspiration as p x (0.46 x Tmean + 8.13), where Tmean is mean daily temperature in Celsius and p is a daylight-hours factor approximated here as increasing with latitude to reflect longer summer days farther from the equator. That's a deliberate tradeoff for sites without a weather station: the more data-hungry Penman-Monteith method would need wind speed, humidity, and solar radiation too, but Blaney-Criddle gets a usable planning estimate from just two easy-to-obtain inputs.

What's the difference between net and gross irrigation requirement in the results?

Net irrigation requirement is seasonal crop ET minus effective rainfall — the water the crop actually needs from irrigation after accounting for rain it already received. Gross irrigation requirement divides that net figure by your application efficiency, so a 75% efficient sprinkler system has to deliver net requirement / 0.75 to the field, since some of every gallon applied is lost to evaporation, wind drift, or runoff before it reaches the root zone.

Why does switching irrigation systems change gross water need so much for the same crop?

Gross irrigation need is net requirement divided directly by application efficiency, so efficiency sits in the denominator and has an outsized effect: at 60% efficiency (flood) you need net/0.6, while at 90% efficiency (drip) you only need net/0.9. For the same crop and net requirement, that's roughly a 33% reduction in gross water pumped just from the efficiency change, which is why the explainer flags irrigation system choice as the biggest lever, ahead of crop selection.

How does the calculator convert gross irrigation depth into gallons and acre-feet?

It multiplies gross irrigation depth in inches by field area in acres to get total acre-inches, then converts to gallons using the fixed factor of 27,154 gallons per acre-inch, and to acre-feet by dividing acre-inches by 12. All three figures describe the same total seasonal water volume, just in the units most useful for comparing against a well's pumping capacity (gallons), a reservoir allocation (acre-feet), or an on-field depth of applied water (inches).

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