Crop Water Requirement Calculator
Crop Selection
Leave blank to use the FAO-56 default for selected crop and stage. Enter a value to override.
Reference ET (ETo)
Enter ETo from a local weather station or FAO climate table. Typical values: 2–4 (temperate), 5–7 (semi-arid), 7–10 (hot arid).
Field & Rainfall
FAO-56 typical season: 150 days for selected crop.
Irrigation System
Enter Crop and Climate Data
Select a crop type, growth stage, and reference ETo to calculate daily ETc, irrigation requirements, seasonal water volumes, and scheduling intervals.
How to Use This Calculator
Select Crop and Growth Stage
Choose your crop from the dropdown (35+ FAO-56 crops organized by category). Then select the current growth stage: Initial (seedling/bare soil), Development (canopy expanding), Mid-season (full canopy, peak water use), or Late-season (maturing/senescent). The Kc value is automatically filled from FAO-56 Table 12 data — you can override it if you have a locally calibrated value.
Enter ETo and Rainfall
In Quick mode, enter the Reference ETo (mm/day) directly from a weather station, regional atlas, or FAO climate table. In ETo Calculator mode, enter daily max/min temperatures and your latitude to compute ETo via the Hargreaves-Samani equation — useful when only temperature data is available. Enter monthly rainfall (mm) to credit effective precipitation toward crop needs using the FAO USDA method.
Configure Field and Irrigation System
Enter your field area (hectares or acres) and growing season length in days. Select your irrigation method — the tool automatically applies the correct efficiency factor (drip 90%, center pivot 80%, sprinkler 70%, furrow 60%) to compute the gross irrigation requirement that accounts for system losses. Larger efficiency values mean less total water withdrawn from the source.
Review Results and Schedule
Read ETc, net and gross irrigation needs, and daily/seasonal water volumes from the results. Switch to Irrigation Schedule mode and add soil type, root zone depth, and MAD percentage to compute the irrigation interval (days between events) and application depth per event. Export the complete results to CSV for use in farm management records, water budgets, or permit applications.
Frequently Asked Questions
What is the difference between ETo and ETc?
ETo is the Reference Evapotranspiration — the water demand of a hypothetical, well-watered grass reference surface under given weather conditions. It reflects climate only, not the specific crop. ETc (Crop Evapotranspiration) is the actual water need of your specific crop, calculated as ETc = ETo × Kc. The crop coefficient Kc adjusts for the crop's canopy size, stomatal behavior, leaf area, and growth stage. A newly planted seedling with sparse canopy has a Kc of 0.3–0.5, while a crop at peak mid-season growth can reach Kc = 1.0–1.25. ETo can be obtained from a local weather station, a regional atlas, or calculated from temperature data using the Hargreaves method.
How accurate is the Hargreaves ETo method?
The Hargreaves-Samani equation estimates ETo from daily maximum and minimum temperature and extraterrestrial radiation (computed from latitude and date). It is significantly less accurate than the FAO-56 Penman-Monteith equation, which uses temperature, humidity, wind speed, and solar radiation. Hargreaves typically over-estimates ETo by 10–20% in humid climates with low wind and can under-estimate in hot, windy, low-humidity conditions. However, it is far better than no calculation at all when full weather data is unavailable. For critical applications (pump sizing, water rights), supplement with local weather station data or the FAO Penman-Monteith equation using measured climate variables.
What is effective rainfall and why is it less than total rainfall?
Effective rainfall (Pe) is the portion of total precipitation that actually infiltrates the soil and is available to crop roots. It is always less than total rainfall because some water runs off the surface before infiltrating, some evaporates from wet leaves or soil before it can be absorbed, and very intense rain may exceed soil infiltration capacity and run off. The FAO USDA formula used here estimates Pe as 80% × P − 25 mm for months with over 75 mm of rain, and 60% × P − 10 mm for drier months (Pe ≥ 0). This approximation works well for planning purposes but may need adjustment for very sandy soils (high runoff) or on sloped fields.
Why does rice (paddy) have a higher irrigation requirement?
Paddy rice irrigation needs are substantially higher than upland crops because they include not only ETc but also: (1) soil saturation before transplanting — typically 200 mm applied once at the start of the season; (2) continuous deep percolation losses through the soil profile — typically 4–8 mm/day even in compacted paddy soils; and (3) maintaining a standing water layer on the field surface — typically 100 mm initially. The total gross water requirement for paddy rice in tropical Asia commonly ranges from 800–2000 mm/season, compared to 400–800 mm for upland crops. This is why paddy rice is one of the most water-intensive staple crops and a major driver of irrigation water demand globally.
What is Management Allowable Depletion (MAD) and how does it affect irrigation timing?
MAD (Management Allowable Depletion) is the maximum fraction of Total Available Water (TAW) that should be depleted before irrigating. TAW is the total water held in the root zone between field capacity and the permanent wilting point. A MAD of 50% means you irrigate when 50% of TAW has been depleted — this is the recommended default for most crops. Lower MAD (30–40%) means more frequent but smaller irrigations, which keeps soil moisture more constant and suits high-value or shallow-rooted crops like lettuce or onions. Higher MAD (60–70%) means less frequent irrigation, acceptable for deep-rooted drought-tolerant crops like sorghum or alfalfa. Irrigation interval = dMAD / ETc, where dMAD = MAD × TAW.
How do I convert between mm/day and m³/hectare/day?
The conversion is straightforward: 1 mm/day applied over 1 hectare = 10 m³/day. This is because 1 hectare = 10,000 m² and 1 mm = 0.001 m, so 10,000 m² × 0.001 m = 10 m³. For other units: 1 mm/day per hectare = 10,000 liters/day; 1 mm/day per acre = 4,047 liters/day = 1,069 gallons/day; 1 inch/day per acre = 27,154 gallons/day = 102.8 m³/day. Seasonal totals simply multiply the daily rate by the number of growing days. These conversions are critical for matching your water source capacity (pump flow rate in m³/hour) to crop demand and for calculating water costs at utility rates per m³ or acre-foot.