The Weather Project · ETo & water planning
Evapotranspiration forecasts for water planning
Track daily reference evaporative demand across your region. Use the ETo outlook with rainfall, soil moisture and crop information to plan the next water-management checks.
Understand the product
What does reference ETo measure?
Reference evapotranspiration is an estimate of atmospheric water demand expressed in millimetres per day. These maps use the temperature-based Hargreaves-Samani method. They are model guidance, not measured evaporation, and are not the FAO Penman-Monteith method.
Read one local day at a time
Each daily map covers the local calendar day printed on the image. Check that the rainfall forecast you compare covers a compatible period. Compare the same region, date and model run before drawing conclusions about changing water demand.
Turn reference demand into a crop water budget
Reference ETo is a starting point. Crop coefficients and growth stage, effective rainfall, soil moisture, rooting depth and irrigation efficiency are needed to estimate water requirements. For example, 5 mm/day of ETo does not automatically mean applying 5 mm of irrigation.
Use trends and check local performance
A run of higher ETo days can flag increased pressure on stored soil water, while lower-demand days may offer more flexibility. Temperature-based estimates can differ from local reference measurements. Compare with reliable local data and calibrate your planning approach where needed.
See what the maps look like
Examples illustrate the format and legend. They are not a live forecast briefing; use the forecast viewer for the latest available run and valid time. Select an image to open it at full size.
From the map to your site
Open the forecast viewer, choose your region and product, then read the model run and local valid date. Compare related maps over matching periods and check local observations before making a time-sensitive decision. Longer-range forecasts are best used for planning and reassessed as the date approaches.
Common questions
Is ETo my crop’s exact irrigation requirement?
No. Convert reference demand using crop and soil information, account for effective rain and existing water storage, and allow for system efficiency.
Why might the first or last day be missing?
Daily products require sufficient model samples to represent a local day. A partial day should not be presented as a complete daily estimate.
Independent model guidance from The Weather Project. Check the map’s method, coverage and valid period. Regional guidance complements local measurements and crop-specific advice.