5
mm/day
3
mm/day
4
mm/day
300
m³/day
400
m³/day
12,000
m³
4.63
L/s
5
mm/day
3
mm/day
4
mm/day
300
m³/day
400
m³/day
12,000
m³
4.63
L/s
The Irrigation Water Requirement Calculator determines how much supplemental water a crop needs beyond what rainfall provides. It uses the FAO Penman-Monteith approach where crop evapotranspiration (ETc) is calculated from reference evapotranspiration (ETo) and a crop coefficient (Kc). The net irrigation requirement is the difference between crop water demand and effective rainfall.
This tool helps farmers, irrigation engineers, and water resource managers plan water allocations, size irrigation systems, and schedule water deliveries. Proper irrigation scheduling prevents both water waste from over-irrigation and yield loss from under-irrigation.
The calculator follows the FAO-56 methodology:
Where ETo is reference evapotranspiration (from weather data), Kc is the crop coefficient that varies by crop and growth stage, and effective rainfall is the portion of rainfall actually available to the crop (typically 70-80% of gross rainfall). The factor of 10 converts mm-ha to m³.
Inputs
Results
Mid-season maize (Kc=1.15) with 5 mm/day ETo and 2 mm/day rain needs 3.75 mm/day irrigation, or 375 m³/day for 10 ha.
Inputs
Results
A citrus orchard (Kc=0.65) with no rain needs its full ETc of 2.6 mm/day irrigated, or 130 m³/day for 5 ha.
Kc represents the ratio of crop evapotranspiration to reference evapotranspiration. It varies by crop type and growth stage: initial stage (Kc 0.3-0.5), crop development (increasing), mid-season (peak Kc 0.9-1.3), and late season (declining Kc 0.3-0.9). FAO Irrigation and Drainage Paper No. 56 provides comprehensive Kc tables for hundreds of crops.
ETo is calculated from weather data (temperature, humidity, wind speed, solar radiation) using the FAO Penman-Monteith equation. Many weather stations and agricultural services provide daily ETo values. You can also estimate ETo from temperature alone using the Hargreaves equation. Typical ETo ranges from 1-3 mm/day in cool humid climates to 6-10 mm/day in hot arid regions.
Not all rainfall is available to crops. Some is lost to runoff, deep percolation, and interception. Effective rainfall is the portion that actually infiltrates the root zone and is available for crop use. A common rule of thumb is that effective rainfall equals 70-80% of gross rainfall, but this varies with soil type, slope, rainfall intensity, and crop rooting depth. The USDA SCS method provides more precise estimates.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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