Measuring evaporation

In this video, University of Waikato Earth and Ocean Sciences master’s student Tehani Kuske explains how scientists use the eddy covariance instrument.

Precipitation and evaporation are two important processes in the water cycle. It is relatively easy to measure rainfall, but how can scientists measure water evaporating into the atmosphere? To do this, they use the eddy covariance instrument, which measures wind movement and water vapour in the air.

Scientists also measure the soil and air temperature, wind speed and incoming solar radiation.

Point of interest A wind eddy is a current of air, moving contrary to the direction of the main current, typically in a circular motion.

Learn more about eddy covariance systems in the article Measuring gases using eddy covariance.

Transcript

TEHANI KUSKE The water cycle consists of inputs and outputs. Usually the main input is rainfall, which we have heaps of measurements of – there is rain gauges everywhere, and we have a good measure of that. Evaporation, most of the time, we only use estimation methods, so equations to estimate these methods, and they are not always accurate, so to be able to get a measure of this evaporation using this eddy covariance instrument is a huge step forward in the science industry. The eddy covariance instrument measures evaporation using two instruments. One instrument measures the wind eddies – so the movement of the air in the 3-dimensional vertical axis and the horizontal axis – and then the other instrument measures the water vapour in the air being transported away from the surface. So that is what evaporation is – is the transport of water vapour away from the surface. So using those 2 measurements, we get a measure of what evaporation is leaving the Earth's surface. We also measure radiation, so the net radiation that is coming in from the sun and its different components. There is long wave radiation and short wave radiation. We also measure temperature and humidity, and we measure rainfall here, so that we can get an estimate of what is coming in compared to what we are losing, in terms of water vapour and rainfall. We are also measuring soil temperatures, and soil moisture contents, and we are measuring like wind speed and wind directions also.