About Evapotranspiration

Introduction

Evapotranspiration is not the same as evaporation. Evapotranspiration is the term used to describe the part of the water cycle which removes liquid water from an area with vegetation and into the atmosphere by the processes of both transpiration and evaporation.

Evaporation occurs when liquid water is converted to water vapour and hence removed from a surface, such as a lake, soil or wet vegetation, into the air. Transpiration occurs when water in plant tissues is lost to the atmosphere, predominantly through the small opening in the leaves of plants and grasses called stomata.

By knowing the rate of water loss from a region farmers will be better placed to efficiently manage the available water for their crop.

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Calculation

The Bureau's evapotranspiration values have been calculated using the Penman-Monteith equation, as recommended by the United Nations Food and Agriculture Organisation in their Irrigation and Drainage paper 56 (FAO56). The FAO56 method is also the approved standard for the U.N. World Meteorological Organization, of which the Bureau is a member agency.

The calculation of evapotranspiration using this method has been made relative to a standard "reference" crop, and is often referred to as "reference crop evapotranspiration" or "reference evapotranspiration" denoted as ETo. This crop approximates a wide area of green grass about 12cm in height, well watered and actively growing which is completely covering the ground.

For crops of a different type, evapotranspiration will vary according to their height, colour, leaf area, age and condition. Estimation of evapotranspiration for different crop types can be made by multiplying the ETo by a crop factor. Tables of crop factors can be found in the FAO56 document.

ETo values have been calculated at Bureau of Meteorology automatic weather stations (AWS) using standard wind speed measurements 10 metres above ground level, air temperature, humidity and daily solar radiation values from satellite data. The wind speed is then adjusted down to the 2 metre height as needed by the FAO56 method, using the logarithmic wind law assuming neutral stability conditions in the atmosphere. Neutral stability conditions generally occur during the daytime when the atmosphere is mixed thoroughly by heating and by wind turbulence. A local comparison of the Bureau ETo values against measurement by other weather stations using direct wind measurements at 2 metres above ground level indicated that the Bureau values can be up to 20% higher. This may be due to a number of factors and further research is likely to help understand these differences.

Please note that local environmental factors such as hills or nearby water bodies can also impact ETo values, so ETo at locations away from Bureau of Meteorology automatic weather stations may vary considerably, and some local knowledge and calibration between the station and the location of interest may be required to help with decision making at the property level. In some cases where alternative weather station information is available on-farm then some local calibration of the ETo values provided by the Bureau should be possible.

Satellite measurements cover the period from midnight to midnight, which differs from the standard meteorological day, which extends for the 24 hours from 9am. The ETo values presented in the maps and tables are for the period midnight to midnight and may differ from those that have been calculated using the standard meteorological day.

The satellite model used to derive the solar exposure is applicable only over land-filled grid cells. Offshore cells, inland water bodies and some coastal cells are assigned values from the nearest land-filled cell as a convenience for users. However, users should recognise that in these cases the values are derived for a location 5 to 70 km away.

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Daily Evapotranspiration Maps and Tables

On the main page a map of Australia is presented with a series of tabs at the top for your state or territory. To access the daily data click the map of your state or territory and move the mouse over individual points on the map to see the daily values.

To access the table of all the locations within your state or territory click on the tab above the map. These tables list all the values used to derive ETo and, in some places, the pan evaporation. To see the details for any location for the month to date click on the location name and a table of values for that location will be presented.

Underneath the current month table there is a monthly archive going back to January 2009.

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Data Updates

ETo data is updated 3 times a day at around 3pm, 5pm and 9pm EST (0500, 0700 and 1100 UTC).

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References

Allen, R.G., Pereira, L.S., Raes, D., and M. Smith Crop evapotranspiration - Guidelines for computing crop water requirements - FAO Irrigation and drainage paper 56, 1998.

Webb, C.P. Bureau of Meteorology Reference Evapotranspiration Calculations , pp20, 2010.

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