Evapotranspiration (Bowen Ratio) - Psychrometer

Parameter to be measured:

The Bowen ratio is the parameter to be determined. It is the ratio between sensible heat flux (H) and latent heat flux (LE) on a reference atmosphere layer. In our experimental field, we were interested in near ground (below the canopy) evapotranspiration and therefore our reference layer is 1.5 m above ground.

Method:

Neither sensible nor latent heat flux are easily measurable. The bowen ratio allows us to replace these two parameters from the energy balance equation (see e.g. Dingman's Physical Hydrology). For that matter, the bowen ratio H/LE can be computed as psy*dT/de, where psy is the psychrometric constant, "dT" is the profile air temperature difference and "de" the profile water vapour pressure difference.

A temperature profile can be easily established with a dry bulb thermometer at two levels. Combining the dry bulb with a wet bulb thermometer it is possible to determine the water vapour pressure as well.

Equipment:

2 Psychrometers (two sets of dry and wet bulb ventilated thermometers) 2 Anemometers Soil Heat Flux plate 4-component Radiometer Logger

All equipment was provided by the company Theodor Friedrichs

Advantages:

This is a direct measurement of evapotranspiration. It is not invasive. Measures ET from a relatively limited area (fetch to height ratio can be as low as 20:1)

Disadvantages:

Relatively complicated set up (although all the sensors are standard meteorological sensors). Relatively high energy consumption (especially the ventilators) that can reach 0.2 A at 13 V when logging.

What to watch out for:

Water containers are small. During a comparably mild summer in southern europe a weekly visit and refill was necessary. If the bottle is empty for a long period of time, the wick that carries moisture from the bottle to the thermometer will dry out and be ruined permanently. Watch out for the way you handle the Wick!

Price:

Approximate price including battery and solar panel: 15000 EUR

Links

Projects that used the above equipment:

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