Evapotranspiration (ET) & Watering Schedule Calculator

ANA›Life Services Authority›National Calculator Authority›Evapotranspiration (ET) & Watering Schedule Calculator

.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }

Evapotranspiration (ET) & Watering Schedule Calculator

Estimate daily reference evapotranspiration (ET₀) using the Hargreaves-Samani equation and generate a weekly watering schedule based on your crop, soil type, and local climate conditions.

### 📍 Location & Climate

Latitude (°N/S)

Month

January February March April May June July August September October November December

Mean Daily Max Temperature (°C)

Mean Daily Min Temperature (°C)

Mean Relative Humidity (%)

Mean Wind Speed (m/s)

### 🌱 Crop & Soil

Crop Type

Bare Soil / Fallow (Kc=0.30) Lawn / Turf Grass (Kc=0.50) Small Vegetables (Kc=0.70) Tomatoes / Peppers (Kc=0.85) Corn / Maize (Kc=1.00) Wheat / Grain (Kc=1.05) Alfalfa / Clover (Kc=1.15) Fruit Trees (Kc=0.65) Cotton (Kc=0.90) Rice (Kc=1.20)

Soil Type

Sandy (FC=10%, low retention) Sandy Loam (FC=18%) Loam (FC=26%) Clay Loam (FC=32%) Clay (FC=38%, high retention)

Root Zone Depth (cm)

Allowable Soil Depletion (%)

Irrigation Efficiency (%)

Flood / Furrow (60%) Sprinkler (75%) Drip / Micro (90%) Hand Watering (100%)

Irrigated Area (m²)

Calculate Watering Schedule

function evaCalc() { // --- Read inputs --- const lat = parseFloat(document.getElementById('eva-latitude').value); const month = parseInt(document.getElementById('eva-month').value); const tmax = parseFloat(document.getElementById('eva-tmax').value); const tmin = parseFloat(document.getElementById('eva-tmin').value); const rh = parseFloat(document.getElementById('eva-humidity').value); const wind = parseFloat(document.getElementById('eva-wind').value); const kc = parseFloat(document.getElementById('eva-crop').value); const fc = parseFloat(document.getElementById('eva-soil').value); const rootDepth = parseFloat(document.getElementById('eva-rootdepth').value); const depletion = parseFloat(document.getElementById('eva-depletion').value) / 100; const eff = parseFloat(document.getElementById('eva-efficiency').value); const area = parseFloat(document.getElementById('eva-area').value);

// --- Validation --- const errors = []; if (isNaN(lat) || lat 90) errors.push("Latitude must be between -90 and 90."); if (isNaN(tmax) || tmax 60) errors.push("Max temperature must be between -50 and 60°C."); if (isNaN(tmin) || tmin 60) errors.push("Min temperature must be between -50 and 60°C."); if (tmax 100) errors.push("Relative humidity must be between 1 and 100%."); if (isNaN(wind) || wind 50) errors.push("Wind speed must be between 0 and 50 m/s."); if (isNaN(rootDepth) || rootDepth 0) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Please fix the following:' + errors.map(e => '').join('') + ''; return; }

// ============================================================ // STEP 1: Extraterrestrial Radiation (Ra) — FAO-56 method // ============================================================ // Day of year (mid-month approximation) const midDay = [15,46,75,105,135,162,198,228,258,288,318,344]; const J = midDay[month - 1];

// Solar declination (radians) const delta = 0.409 * Math.sin(2 * Math.PI * J / 365 - 1.39);

// Latitude in radians const phi = lat * Math.PI / 180;

// Sunset hour angle (radians) const ws = Math.acos(-Math.tan(phi) * Math.tan(delta));

// Inverse relative distance Earth-Sun const dr = 1 + 0.033 * Math.cos(2 * Math.PI * J / 365);

// Extraterrestrial radiation (MJ/m²/day) // Ra = (2460/π) * Gsc * dr * [wssin(φ)sin(δ) + cos(φ)cos(δ)*sin(ws)] const Gsc = 0.0820; // Solar constant MJ/m²/min const Ra = (24 * 60 / Math.PI) * Gsc * dr * (ws * Math.sin(phi) * Math.sin(delta) + Math.cos(phi) * Math.cos(delta) * Math.sin(ws));

// ============================================================ // STEP 2: Reference ET₀ — Hargreaves-Samani (1985) // ============================================================ // ET₀ = 0.0023 * Ra * (Tmean + 17.8) * TD^0.5 // where TD = Tmax - Tmin (temperature range) const Tmean = (tmax + tmin) / 2; const TD = tmax - tmin; const ET0_hs = 0.0023 * Ra * (Tmean + 17.8) * Math.pow(TD, 0.5);

// ============================================================ // STEP 3: Humidity & Wind Correction (Samani 2000 adjustment) // Correction factor for arid/humid conditions // CF = 1 + 0.00185(50 - RH) + 0.0041(wind - 2) // ============================================================ const CF = 1 + 0.00185 * (50 - rh) + 0.0041 * (wind - 2.0); const ET0 = Math.max(0.1, ET0_hs * CF);

// ============================================================ // STEP 4: Crop ET (ETc) // ETc = Kc × ET₀ // ============================================================ const ETc = kc * ET0;

// ============================================================ // STEP 5: Soil Water Balance & Irrigation Interval // Total Available Water (TAW) = FC × root depth (mm) // Readily Available Water (RAW) = depletion × TAW // Net irrigation depth = RAW (mm) // Gross irrigation = Net / efficiency // Interval (days) = RAW / ETc // ============================================================ const TAW_mm = fc * rootDepth * 10; // fc fraction × depth(cm) × 10 = mm const RAW_mm = depletion * TAW_mm; // mm const netIrr = RAW_mm; // mm per irrigation event const grossIrr = netIrr / eff; // mm per event (accounting for losses) const interval = Math.max(1, RAW_mm / ETc); // days between irrigations

// Volume per event const volumePerEvent_L = grossIrr * area; // mm × m² = liters

// Weekly water need const weeklyET_mm = ETc * 7; const weeklyGross_mm = weeklyET_mm / eff; const weeklyVol_L = weeklyGross_mm * area;

// Events per week const eventsPerWeek = 7 / interval;

// ============================================================ // STEP 6: Build watering schedule (7-day) // ============================================================ const days = ['Mon','Tue','Wed','Thu','Fri','Sat','Sun']; let schedule = []; let daysSinceLastWater = 0; for (let i = 0; i = interval) { schedule.push({ day: days[i], water: true, amount: grossIrr.toFixed(1) }); daysSinceLastWater = 0; } else { schedule.push({ day: days[i], water: false, amount: 0 }); } daysSinceLastWater++; }

// ============================================================ // STEP 7: Render results // ============================================================ const scheduleRows = schedule.map(d => '' + '' + d.day + '' + '' + (d.water ? '💧 Water' : '—') + '' + '' + (d.water ? d.amount + ' mm' : '—') + '' + '' + (d.water ? (parseFloat(d.amount) * area).toFixed(0) + ' L' : '—') + '' + '' ).join('');

resultDiv.style.display = 'block'; resultDiv.innerHTML = ` ### 📊 ET & Irrigation Results

Extraterrestrial Radiation (Ra) ${Ra.toFixed(2)} MJ/m²/day

Reference ET₀ (Hargreaves) ${ET0_hs.toFixed(2)} mm/day

Corrected ET₀ (humidity + wind) ${ET0.toFixed(2)} mm/day

Crop ET (ETc = Kc × ET₀) ${ETc.toFixed(2)} mm/day

Total Available Water (TAW) ${TAW_mm.toFixed(1)} mm

Readily Available Water (RAW) ${RAW_mm.toFixed(1)} mm

Gross Irrigation per Event ${grossIrr.toFixed(1)} mm

Irrigation Interval ${interval.toFixed(1)} days

Volume per Event (${area} m²) ${volumePerEvent_L.toFixed(0)} liters

Weekly Water Need (gross) ${weeklyVol_L.toFixed(0)} liters/week

### 📅 Sample Weekly Watering Schedule

DayActionDepthVolume (${area} m²) ${scheduleRows}

Note: Schedule starts watering on Monday. Adjust start day to match your local conditions. Humidity correction factor applied: ${CF.toFixed(3)}. ${interval ⚠️ ETc exceeds RAW daily — consider increasing root depth, adjusting depletion, or irrigating every day.' : ''}

`; }

#### Formulas Used

1. Extraterrestrial Radiation (Ra) — FAO-56:

Ra = (24·60/π) · Gsc · dr · [ωs·sin(φ)·sin(δ) + cos(φ)·cos(δ)·sin(ωs)]

2. Reference ET₀ — Hargreaves-Samani (1985):

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

3. Humidity & Wind Correction (Samani 2000):

CF = 1 + 0.00185·(50 − RH) + 0.0041·(u − 2)  →  ET₀corrected = ET₀ · CF

4. Crop Evapotranspiration:

ETc = Kc · ET₀

5. Soil Water Balance:

#### Assumptions & References

More Calculators

Read Next

Study Time Planner Authority Network America › Life Services Authority › National Calculator Authority .calc-container { max-width: 640px;...

References

Related Authorities

Life Services Authority › Professional Services Authority › Trade Services Authority › United States Authority ›