What is the Gross Irrigation Requirement (GIR)?

GIR is the total water depth (mm/day) that must be applied at the field inlet to meet the crop's net water need after accounting for system losses. The formula is GIR = (ETc - Re) / Ea, where ETc is crop evapotranspiration, Re is effective rainfall, and Ea is application efficiency. Because no system delivers water perfectly, GIR is always greater than the net requirement.

What is ET0 and how is it estimated?

ET0 (reference evapotranspiration) is the water use rate of a hypothetical well-watered reference grass surface under local climate conditions, expressed in mm/day. The most accurate method is the FAO Penman-Monteith equation, which needs net radiation, wind speed, humidity, and temperature. When only temperature data are available, this calculator uses the Hargreaves-Samani (1985) formula (ET0 = 0.0023 x Ra x (Tmean + 17.8) x (Tmax - Tmin) raised to the power 0.5), which requires only daily maximum and minimum temperature plus extraterrestrial radiation estimated from latitude and day of year.

What is the Kc (crop coefficient)?

Kc is a dimensionless factor that converts ET0 to actual crop water use (ETc = ET0 x Kc). It varies by crop species and growth stage. This calculator uses mid-season Kc values from FAO Irrigation and Drainage Paper 56 (Allen et al., 1998). Mid-season represents peak demand; early-season and late-season Kc values are lower. If you have measured local Kc data, use the Custom option.

Why does the volume use a factor of 10?

One millimetre of water depth applied over one hectare equals exactly 10 cubic metres: 1 mm = 0.001 m, and 1 ha = 10,000 m2, so 0.001 m x 10,000 m2 = 10 m3. Multiplying GIR (mm/day) by area (ha) by 10 converts the irrigation depth into a volumetric flow the pump or canal must supply.

What irrigation efficiency values should I use?

Typical application efficiencies are 85-95% for well-maintained drip and micro-irrigation, 70-80% for sprinklers, 55-65% for properly managed furrow, and 40-50% for flood or basin irrigation. Higher efficiency means less water is lost to runoff and deep percolation. If you have water-balance measurements for your specific field, use the Override option to enter a more precise value.

How do I convert mm/day GIR to a pump flow rate?

Volume (m3/day) = GIR x area x 10. Divide by irrigation hours per day to get m3/h, then multiply by 1000 for L/h. For example, GIR = 8 mm/day on a 2 ha field with 8 h/day irrigation gives 8 x 2 x 10 = 160 m3/day, or 20 m3/h (20,000 L/h), which is about 333 L/min. Size your main line, pump curve, and pressure rating for this flow.

Irrigation Water Calculator

Planning how much water to apply — and when — is one of the most consequential decisions in crop production. Too little causes yield stress; too much wastes a scarce resource, leaches nutrients past the root zone, and raises pumping costs. The Irrigation Water Calculator uses the internationally accepted FAO-56 framework to compute your Gross Irrigation Requirement (GIR) in mm/day, convert it to a daily water volume in cubic metres and litres, and size the flow rate your pump or canal must deliver.

How it works

The calculation follows three steps.

Step 1 — Crop water demand (ETc). Reference evapotranspiration (ET0) describes how fast a well-watered grass reference crop loses water under local climate conditions. You can enter a known ET0 value from a weather station or the FAO CLIMWAT database, or switch to Calculate mode and let the tool estimate it using the Hargreaves-Samani (1985) formula:

ET0 = 0.0023 × Ra × (Tmean + 17.8) × (Tmax − Tmin)^0.5

where Ra is extraterrestrial radiation (MJ/m²/day) derived from your latitude and the day of year. Multiplying ET0 by the crop coefficient Kc gives actual crop evapotranspiration: ETc = ET0 × Kc. Kc values are taken from FAO Paper 56 and represent mid-season peak demand.

Step 2 — Net irrigation need. Effective rainfall (the portion of precipitation retained in the root zone) partly satisfies crop demand. Net irrigation requirement = ETc − Re.

Step 3 — Gross Irrigation Requirement. No irrigation system applies water perfectly. Application efficiency Ea (fraction) accounts for losses to runoff, drift, and deep percolation:

GIR (mm/day) = (ETc − Re) / Ea

Volume and flow rate follow from a simple unit conversion: one millimetre of water over one hectare equals exactly 10 m³. Volume (m³/day) = GIR × area (ha) × 10. Dividing by daily operating hours gives the required pump or canal flow rate in m³/h and L/h.

Worked example

A 3 ha maize field in a semi-arid region. Weather data give ET0 = 7 mm/day. Maize mid-season Kc = 1.20, so ETc = 7 × 1.20 = 8.4 mm/day. Effective rainfall = 1.5 mm/day. Net irrigation need = 8.4 − 1.5 = 6.9 mm/day. The farm uses a sprinkler system at 75% efficiency:

GIR = 6.9 / 0.75 = 9.2 mm/day

Daily volume = 9.2 × 3 × 10 = 276 m³/day (276,000 litres). Running the system 10 hours per day requires a flow rate of 27.6 m³/h (7.7 L/s). Over a 90-day grain-fill season that totals roughly 24,840 m³ — enough to fill about ten Olympic-sized swimming pools.

Switching to drip irrigation (90% efficiency) cuts GIR to 7.7 mm/day, reducing the seasonal total to around 20,800 m³ — a saving of more than 4,000 m³ per season on this field alone.

Formula note

All formulae are from or consistent with FAO Irrigation and Drainage Paper 56 (Allen, Pereira, Raes and Smith, 1998). The Hargreaves-Samani ET0 estimate is known to over-predict in humid conditions and slightly under-predict in windy arid zones; when reliable psychrometric and radiation data are available, use the full Penman-Monteith equation instead. Kc values used here are mid-season; consult FAO-56 Table 12 for initial and end-season values to build a full seasonal irrigation schedule.