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Water Budget Calculator

Calculate field water balance for irrigation planning. Free online calculator with instant results — no signup required.

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Inputs that matter
Rainfall, Irrigation Applied, Evapotranspiration, Runoff, and 1 more
Output to expect
Water Budget Calculator
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Water Balance Equation

The field water budget balances precipitation, irrigation, and capillary rise against evapotranspiration, runoff, and deep percolation over a time period.

ΔS = (P + I + CR) − (ET + R + DP)

Evapotranspiration (ET)

Crop water use depends on reference ET (ETo from weather data) multiplied by a crop coefficient (Kc) that varies by growth stage.

ETc = ETo × Kc

Irrigation Requirement

When soil moisture drops below the management allowable depletion (MAD), irrigation must replenish the root zone to field capacity.

Irrigation = ETc − P − ΔS_available, when ΔS < MAD threshold

Updated: July 2026

Example Scenarios

A 40-hectare corn field during tasseling with sandy loam soil in a week with 15 mm rainfall.

ETo: 6 mm/dayKc (mid-season): 1.15Precipitation: 15 mm/weekRoot Zone Depth: 0.8 m

→ ETc: ~48 mm/week; net irrigation need: ~33 mm (~330 m³/ha)

An almond orchard in hull split stage applying 80% ETc replacement.

ETo: 7.5 mm/dayKc: 0.95RDI Factor: 0.80Precipitation: 0 mm

→ Full ETc: 50 mm/week; applied irrigation: 40 mm/week

Tomatoes on drip with 95% application efficiency, high Kc during fruiting.

ETo: 5 mm/dayKc: 1.05Application Efficiency: 95%Precipitation: 2 mm

→ ETc: 5.25 mm; irrigation: 3.4 mm/day gross applied

Common Mistakes to Avoid

Using annual average ET instead of growth-stage Kc

Crop coefficient varies from 0.3 at emergence to 1.15 at mid-season. Using a single Kc value over- or under-estimates irrigation by 30–50%.

Ignoring irrigation system efficiency

Flood irrigation is 50–70% efficient; sprinklers 75–85%; drip 90–95%. Divide net water need by efficiency to get gross application volume.

Not accounting for soil water holding capacity

Sandy soils hold 50–80 mm/m; clay loams hold 150–200 mm/m. Root zone storage determines how often you must irrigate, not just how much.

FAQ

ETo (reference evapotranspiration) is water use by a standard grass surface from weather data. ETc (crop evapotranspiration) equals ETo multiplied by the crop coefficient (Kc), giving actual crop water demand.

Irrigate when available soil moisture drops below the management allowable depletion (MAD), typically 40–50% of available water in the root zone. Use soil moisture sensors or the water balance method.

FAO Penman-Monteith ETo is available from local weather stations, CIMIS (California), AgWeatherNet, or NASA POWER. Daily values are preferred over monthly averages for irrigation scheduling.

Organic mulch reduces soil evaporation by 30–50%, effectively lowering the Kc by 0.05–0.15. Plastic mulch nearly eliminates soil evaporation and is common in vegetable production.

Yes. Tracking cumulative ET against precipitation and irrigation prevents applying water beyond field capacity, which causes runoff, nutrient leaching, and root zone anaerobic conditions.

About Water Budget Calculator

A field water budget tracks all water inputs and outputs to determine whether a crop receives adequate moisture or needs supplemental irrigation. Hydrologists and agronomists use water balance calculations to optimize irrigation scheduling and prevent both drought stress and waterlogging.