Crop Water Requirement (ET) Calculator
ANA›Life Services Authority›National Calculator Authority›Crop Water Requirement (ET) Calculator
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Crop Water Requirement (ET) Calculator
Calculate crop evapotranspiration (ETc) using the FAO-56 Penman-Monteith reference evapotranspiration (ETo) and crop coefficient (Kc) method.
Location & Climate Inputs
Maximum Temperature (°C)
Minimum Temperature (°C)
Mean Relative Humidity (%)
Wind Speed at 2 m height (m/s)
Solar Radiation (MJ/m²/day)
Elevation above Sea Level (m)
Latitude (decimal degrees, negative = South)
Day of Year (1–365)
Crop & Field Inputs
Crop Coefficient Kc
Field Area (hectares)
Irrigation Period (days)
Irrigation Efficiency (%)
Calculate
function croCalc() { const err = (msg) => { document.getElementById("cro-result").innerHTML = '⚠ ' + msg + ''; };
// --- Read inputs --- const Tmax = parseFloat(document.getElementById("cro-tmax").value); const Tmin = parseFloat(document.getElementById("cro-tmin").value); const RH = parseFloat(document.getElementById("cro-rh").value); const u2 = parseFloat(document.getElementById("cro-ws").value); const Rs = parseFloat(document.getElementById("cro-rs").value); const elev = parseFloat(document.getElementById("cro-elev").value); const lat = parseFloat(document.getElementById("cro-lat").value); const doy = parseInt(document.getElementById("cro-doy").value); const Kc = parseFloat(document.getElementById("cro-kc").value); const area = parseFloat(document.getElementById("cro-area").value); const days = parseInt(document.getElementById("cro-days").value); const eff = parseFloat(document.getElementById("cro-eff").value);
// --- Validation --- if (isNaN(Tmax) || isNaN(Tmin)) return err("Enter valid max and min temperatures."); if (Tmax 100) return err("Relative humidity must be between 1 and 100%."); if (isNaN(u2) || u2 90) return err("Latitude must be between -90 and 90."); if (isNaN(doy) || doy 365) return err("Day of year must be between 1 and 365."); if (isNaN(Kc) || Kc 2.0) return err("Crop coefficient Kc must be between 0.1 and 2.0."); if (isNaN(area) || area 0 hectares."); if (isNaN(days) || days 100) return err("Irrigation efficiency must be between 1 and 100%.");
// ============================================================ // FAO-56 Penman-Monteith ET₀ Calculation // ============================================================
const Tmean = (Tmax + Tmin) / 2;
// 1. Atmospheric pressure (kPa) — FAO-56 Eq. 7 const P = 101.3 * Math.pow((293 - 0.0065 * elev) / 293, 5.26);
// 2. Psychrometric constant γ (kPa/°C) — FAO-56 Eq. 8 const gamma = 0.000665 * P;
// 3. Saturation vapour pressure (kPa) — FAO-56 Eq. 11 const es_Tmax = 0.6108 * Math.exp((17.27 * Tmax) / (Tmax + 237.3)); const es_Tmin = 0.6108 * Math.exp((17.27 * Tmin) / (Tmin + 237.3)); const es = (es_Tmax + es_Tmin) / 2;
// 4. Actual vapour pressure ea (kPa) — FAO-56 Eq. 17 const ea = (RH / 100) * es;
// 5. Vapour pressure deficit (kPa) const vpd = es - ea;
// 6. Slope of saturation vapour pressure curve Δ (kPa/°C) — FAO-56 Eq. 13 const Delta = (4098 * 0.6108 * Math.exp((17.27 * Tmean) / (Tmean + 237.3))) / Math.pow(Tmean + 237.3, 2);
// 7. Extraterrestrial radiation Ra (MJ/m²/day) — FAO-56 Eq. 21 const phi = (Math.PI / 180) * lat; // lat in radians const dr = 1 + 0.033 * Math.cos((2 * Math.PI / 365) * doy); // inverse relative distance const delta = 0.409 * Math.sin((2 * Math.PI / 365) * doy - 1.39); // solar declination const ws = Math.acos(-Math.tan(phi) * Math.tan(delta)); // sunset hour angle const Ra = (24 * 60 / Math.PI) * 0.0820 * dr * (ws * Math.sin(phi) * Math.sin(delta) + Math.cos(phi) * Math.cos(delta) * Math.sin(ws));
// 8. Clear-sky solar radiation Rso (MJ/m²/day) — FAO-56 Eq. 37 const Rso = (0.75 + 2e-5 * elev) * Ra;
// 9. Net shortwave radiation Rns (MJ/m²/day) — FAO-56 Eq. 38 (albedo α = 0.23) const Rns = (1 - 0.23) * Rs;
// 10. Net longwave radiation Rnl (MJ/m²/day) — FAO-56 Eq. 39 const sigma = 4.903e-9; // Stefan-Boltzmann constant MJ/K⁴/m²/day const TmaxK = Tmax + 273.16; const TminK = Tmin + 273.16; const RsRso = Math.min(Rs / Rso, 1.0); const Rnl = sigma * ((Math.pow(TmaxK, 4) + Math.pow(TminK, 4)) / 2) * (0.34 - 0.14 * Math.sqrt(ea)) * (1.35 * RsRso - 0.35);
// 11. Net radiation Rn (MJ/m²/day) — FAO-56 Eq. 40 const Rn = Rns - Rnl;
// 12. Soil heat flux G (MJ/m²/day) — assumed 0 for daily calculation const G = 0;
// 13. FAO-56 Penman-Monteith ET₀ (mm/day) — FAO-56 Eq. 6 const ETo = (0.408 * Delta * (Rn - G) + gamma * (900 / (Tmean + 273)) * u2 * vpd) / (Delta + gamma * (1 + 0.34 * u2));
if (ETo
Parameter Value
Atmospheric Pressure (P) ${P.toFixed(2)} kPa Psychrometric Constant (γ) ${gamma.toFixed(4)} kPa/°C Saturation Vapour Pressure (es) ${es.toFixed(3)} kPa Actual Vapour Pressure (ea) ${ea.toFixed(3)} kPa Vapour Pressure Deficit (VPD) ${vpd.toFixed(3)} kPa Slope of Sat. VP Curve (Δ) ${Delta.toFixed(4)} kPa/°C Extraterrestrial Radiation (Ra) ${Ra.toFixed(2)} MJ/m²/day Net Radiation (Rn) ${Rn.toFixed(2)} MJ/m²/day
Reference ET (ETo)
${ETo.toFixed(2)} mm/day
Crop ET (ETc = Kc × ETo) ${ETc.toFixed(2)} mm/day Total ETc over ${days} days ${totalETc_mm.toFixed(1)} mm Net Water Requirement (${area} ha) ${totalETc_m3.toFixed(1)} m³
Gross Water Requirement (at ${eff}% efficiency) ${grossWater_m3.toFixed(1)} m³ (${(grossWater_L/1000).toFixed(1)} kL)
- Soil heat flux G assumed = 0 (standard for daily calculations).
`; }
#### Formulas Used
FAO-56 Penman-Monteith Reference Evapotranspiration (ETo):
ET₀ = [0.408·Δ·(Rn − G) + γ·(900/(T+273))·u₂·(es−ea)] / [Δ + γ·(1 + 0.34·u₂)]
- Δ = slope of saturation vapour pressure curve (kPa/°C)
- Rn = net radiation at crop surface (MJ/m²/day)
- G = soil heat flux (MJ/m²/day) — 0 for daily
- γ = psychrometric constant = 0.000665 × P
- T = mean daily air temperature (°C)
- u₂ = wind speed at 2 m height (m/s)
- es = saturation vapour pressure (kPa)
- ea = actual vapour pressure = (RH/100) × es
Crop Evapotranspiration: ETc = Kc × ETo
Gross Water Requirement: GWR = (ETc × days × area × 10) / (efficiency/100) [m³]
#### Assumptions & References
- Based on FAO Irrigation and Drainage Paper No. 56 (Allen et al., 1998).
- Soil heat flux G = 0 is standard for daily ET₀ calculations (FAO-56 §3.2).
- Wind speed input must be measured or corrected to 2 m height. Use u₂ = u_z × (4.87 / ln(67.8·z − 5.42)) to convert from height z.
- Typical Kc values (FAO-56 Table 12): Wheat mid-season 1.15, Maize mid-season 1.20, Cotton mid-season 1.20, Rice mid-season 1.20, Tomato mid-season 1.15, Potato mid-season 1.15.
- Extraterrestrial radiation Ra is computed from latitude and day of year (FAO-56 Eq. 21).
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