We measured the radiation and turbulent flux densities during the fieldtrip – Rn is net radiation, G is ground heat flux, H is sensible heat flux and LE is latent heat flux.The graph shows data for our sunniest day, 8th September 2010.The open symbols are the measurements, and the lines are predictions using the Penman-Monteith equation to estimate evaporation. The estimate of LE (which is the evaporated moisture flux multiplied by the latent heat of vaporisation) looks very good compared to the measurements, whilst H appears to be overestimated. It was a bit of a surprise that the agreement for LE was that good, as the PM method makes a lot of assumptions!!
The Penman-Monteith equation uses standard screen meteorological measurements (i.e. temperature, humidity, windspeed) to estimate potential evaporation. Making direct measurements of evaporation, or the moisture flux transported upward by turbulence, is a tricky thing. You need an instrument which is accurate, and has a fast response – making an observation 10 to 20 times a second. We used a Licor 7500 infra red gas analyser to measure LE, alongside a sonic anemometer. This combination is used all over the world to measure not only moisture flux, but also carbon dioxide flux in the effort to quantify our emissions into the atmosphere.
Click here for more info on Fluxnet, the global network of carbon dioxide flux measurements.