Plan irrigation with precise daily evapotranspiration numbers. Compare conditions, coefficients, and periods fast. Make smarter water decisions for every project site.
Enter site weather data and crop coefficient. Provide solar radiation or sunshine hours. Values are calculated daily, then scaled by your period.
Use this sample to sanity-check your inputs. Numbers are illustrative only.
| Latitude (deg) | Altitude (m) | Day | Tmax (degC) | Tmin (degC) | RHmax (%) | RHmin (%) | u2 (m/s) | Rs (MJ/m^2/day) | Kc | Period (days) |
|---|---|---|---|---|---|---|---|---|---|---|
| 31.5 | 200 | 150 | 33 | 21 | 85 | 40 | 2.2 | 22.0 | 1.05 | 7 |
| 24.0 | 50 | 250 | 36 | 26 | 80 | 55 | 1.6 | 19.5 | 0.90 | 10 |
Crop evapotranspiration: ETc = Kc * ETo
Reference evapotranspiration (daily, FAO-56 Penman-Monteith):
ETo = [0.408*Delta*(Rn - G) + Gamma*(900/(T+273))*u2*(es - ea)] / [Delta + Gamma*(1 + 0.34*u2)]
Where Rn is net radiation, G is soil heat flux (about 0 daily), Delta is slope of saturation vapour pressure curve, Gamma is psychrometric constant, and (es - ea) is vapour pressure deficit.
Solar radiation input: If Rs is not provided, Rs is estimated from sunshine hours:
Rs = (as + bs * n/N) * Ra, with as=0.25 and bs=0.50
Crop evapotranspiration links agronomy to design decisions on canals, pipelines, and storage. ETc converts climate demand into daily water depth, so engineers can estimate required supply and pumping hours. Underestimating ETc risks yield loss and complaints, while overestimating inflates energy and infrastructure costs. This calculator standardizes calculations and produces repeatable documentation for project files and reporting. It supports transparent reviews and approvals.
Reliable results depend on accurate weather inputs. Use station data measured near the site, ideally at two meter height for wind speed. Keep temperature and humidity aligned to the same day. Enter solar radiation when available, because it reduces uncertainty. If only sunshine hours are known, the radiation estimate is acceptable for planning, but field verification is recommended before final sizing. Log instrument heights and maintenance.
The calculator first computes reference evapotranspiration, ETo, using net radiation, vapor pressure deficit, temperature, and wind effects. ETo represents a well watered reference surface, not your crop. Multiply by the crop coefficient Kc to obtain ETc. Kc changes by growth stage, canopy cover, and management. Use local agronomic guides or calibrated values for best performance. Document chosen Kc sources in notes.
Daily ETc is useful for operations, but design often needs totals. Select a period that matches your scheduling window, such as seven or ten days. Period ETc gives expected depth to replace with irrigation or effective rainfall. If you enter irrigation efficiency, the tool converts net demand into gross depth, helping size valves, emitters, and delivery rates with realistic losses. Use consistent units across teams.
Treat results as a baseline and document your assumptions. Adjust for microclimate, salinity management, mulching, or partial ground cover by refining Kc. After rain, reduce applied water, but remember infiltration limits and distribution uniformity. In early design, use conservative Kc and efficiency to avoid undersupply. During commissioning, compare calculated ETc with field observations and refine inputs. Schedule audits at peak demand.
ETc is the crop water use rate, expressed as millimeters of water per day. It is calculated as ETc = Kc * ETo, where Kc reflects crop stage and canopy conditions.
Kc converts reference demand into crop-specific demand. Using the wrong Kc can bias water estimates even when weather data are correct. Update Kc as the crop develops and as management changes.
Yes. If Rs is unavailable, sunshine hours estimate Rs using an empirical relationship with extraterrestrial radiation. This is suitable for planning, but measured Rs is preferred for final design and audits.
Pick a period that matches scheduling and storage decisions, such as 7 to 10 days. Longer periods smooth daily variation but can hide short peaks that influence pumping and conveyance sizing.
Gross depth equals net ETc over the period divided by irrigation efficiency. Efficiency accounts for conveyance and application losses. If you leave efficiency blank, the calculator reports net demand only.
Keep inputs in the units shown on the form. Temperatures are in degrees Celsius, wind speed is meters per second at 2 meters, and radiation is megajoules per square meter per day. Outputs are millimeters per day and per period.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.