Evap Pans - the good old way

Old ways sometimes die hard.

The old way of measuring evaporation was to manually fill a shallow 1.2 m diameter pan with water to a depth of 200 mm, then measure the ‘top-up’ requirements each day at 9 am as a record of the number of millimetres of daily evaporation.

Years ago MEA automated this whole process, using pumps and valves and water reservoirs, plus an ultrasonic depth sensor to measure sub-millimetre changes in water depth throughout the day.

MEA automated evap pans compute daily evaporation and rainfall (the latter by measuring the depth of water ‘gained’ by the pan). After daily totals are calculated, the system refills the pan precisely, or empties it precisely if rainfall has exceeded evaporation.

Is this clever or what?

The result is clever, but undertaking the design project might not have been; the software required was complex and all-encompassing. It took some years to get right and to test thoroughly.

Is the MEA automated evap pan the best way to measure evapotranspiration?

 Unless you are a stickler for a traceable daily evaporation figure, there is considerably less maintenance and cost in using an MEA weather station to calculate – not measure – evapotranspiration using internationally-recognized mathematical models built into MEA’s operating software.

MEA Automated Evaporation Pan
An MEA Automated Evaporation Pan situated in a quarry.


01 May 2015
Although the Class A pan's are used globally and automatic control and measuring of the water level is positive contrubution the scientific results are at least moderate accurate. Due to self heating the measurements overestimate the real open water evaporation. Alternative measurements as i.e. Eddy Correlation are complex and expensive. The large amount scientific model calculations are also restricted in accuracy due to local variations in field surface. Only a Lysimeter will deliver ultimate accuracy for measuring evapotranspiration.
- Hans van Rheenen

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