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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)Crop water use depends on reference ET (ETo from weather data) multiplied by a crop coefficient (Kc) that varies by growth stage.
ETc = ETo × KcWhen 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 thresholdUpdated: July 2026
A 40-hectare corn field during tasseling with sandy loam soil in a week with 15 mm rainfall.
→ ETc: ~48 mm/week; net irrigation need: ~33 mm (~330 m³/ha)
An almond orchard in hull split stage applying 80% ETc replacement.
→ Full ETc: 50 mm/week; applied irrigation: 40 mm/week
Tomatoes on drip with 95% application efficiency, high Kc during fruiting.
→ ETc: 5.25 mm; irrigation: 3.4 mm/day gross applied
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%.
Flood irrigation is 50–70% efficient; sprinklers 75–85%; drip 90–95%. Divide net water need by efficiency to get gross application volume.
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.
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.