Eddy Covariance Method for Evapotranspiration Measurements

Abstract:

Water vapor is one of the most important but often overlooked greenhouse gas in the atmosphere. Meanwhile, in today's uneven distribution of water resources, the evapotranspiration is going to better assist with water management. Eddy covariance methodology has been proven to be an effective way of measuring evapotranspiration. HealthyPhoton's HT-1800 Open-oath Laser Hygrometer is designed for the eddy covariance evapotranspiration measurement.

Why is Evapotranspiration important?

    Studies have shown that the specific heat of water and its huge content on the earth's surface has a great impact on climate change through transpiration and precipitation. The greenhouse effect is about 60%-70% of the overall greenhouse effections. The water cycle is the largest material flow in the biosphere (Chahine, 1992). Evaporation causes water to be transferred from soil and water bodies, and the plants lose water through stomata. These two processes are the greatest movement of water transferring to the atmosphere, collectively called Evapotranspiration.

    Globally, about 65% of land surface precipitation returns to the atmosphere through evapotranspiration (Trenberth, 2007). More understanding of evapotranspiration is important for water management, endangered species conservation, and the analysis of the causes of droughts, floods, wildfires, and other natural disasters. Besides, evapotranspiration consumes about 50% of the solar radiation absorbed by the Earth's surface (Trenberth, 2009). This affects climate and hydrology on a local, regional and global scale. For these reasons, it is important to measure evapotranspiration accurately. 

Eddy Covariance Techniques

    Evapotranspiration is difficult to measure because it is affected by a variety of environmental factors, including solar radiation, temperature, win speed, and humidity (Drexler, 2004). The Eddy covariance technique has been popular for micrometeorology observation in recent years. It calculates the covariance of the vertical wind speed and the fluctuation of the physical quantity to be measured to obtain the energy and matter, flux caused by turbulence. This method can directly measure the exchanges of carbon, water and heat between the atmosphere, soil and plat. With current instrumental technology, it is possible to measure small fluctuations, on different time scales (hours, days, seasons, and years) and a spatial scale of 100-2000m. The eddy covariance technique has been proven to be the most effective method for measuring the interaction between the terrestrial biosphere and the atmosphere on an ecological scale (Friend, 2006; Baldocchi, 2008). 

The advantages of the eddy covariance method:

1. Measuring the evapotranspiration any time at any place.

2. Minimal interference to the target measurement area.

3. Providing average evapotranspiration measurement at the mesoscale (100- 1000m).

4. Automatic and continuous long-term measurement.

Eddy Covariance Method for Evapotranspiration Measurements.

    Eddy covariance method is highly approved by micro meteorologists as the standard method for determining energy and matter because it has few theoretical assumptions. It also is the most direct method for measuring evapotranspiration (Jung, 2010). The basic instruments required for eddy covariance measurements include a water vapor analyzer and a three-dimensional ultrasonic anemometer. The two instruments must be capable of making high-frequency measurements (usually at 10 Hz or faster to capture fast-moving turbulence).

HT-1800 Open-path Laser-Based Hygrometer

Figure 1: HT-1800 open-path hygrometer and the control box

Figure 2: HT-1800 and Gill® Windsmaster ultrasonic anemometer installation under typical conditions

    HT-1800 is an excellent choice for measuring evapotranspiration since its open-path design can avoid the absorption effect of water vapor. It also avoids the power consumption of the gas sampling pump and only requires solar energy for operation. The HT-1800 is designed specifically for eddy covariance. This instrument fully meets the requirements of eddy covariance measurement in terms of high sensitivity and high speed (signal output of 20Hz frequency). It is worth mentioning that the measurement of water vapor is also fundamental in flux analysis and flux correction for other gases. 

HT-1800's product feature:

- Light weight, low cost, easy installation and maintenance

- Ideal for eddy covariance and evapotranspiration measurements

- Open path configuration, with up to 20Hz measurement frequency

- Ultra-sensitive laser spectroscopy brings very low drift

- No cross-interference from CO₂ and other gas molecules

- No moving parts with strong vibration resistance

- Robust design for versatile field deployment

- Low power consumption (10 W) that can be supplied by a solar panel

Related Resources

    Compatible ultrasonic anemometers are available from the following manufacturers, including Gill®, Campbell Scientific®, Metek, RM Young Company, and Kaijo-Denki.
    Many software programs process eddy covariance data and provide the testing results such as latent heat, sensible heat flux, momentum, and gas flux. Here is a list of some resources:

Commercial software: EddyPro®
Ope-source programs: ECO2S and ECPACK;
Open-source packages: Eddyuh, EDIRE, TK3, Alteddy, and Eddy soft, etc

References
Chahine, M.T. 1992. The hydrological cycle and its influence on climate. Nature, 359:373-380
Drexler J.Z., R.L. Snyder, D. Spano, U.K.T. Paw. 2004. A review of models and meteorological methods used to estimate wetland evapotranspiration. Hydrological Processes, 18; 2071–2101.
Jung M., M. Reichstein, P. Ciais, etal. 2010. A recent decline in the global land evapotranspiration trend due to limited moisture supply. Nature, 467:951-954.
Trenberth K.E., L.Smith ,T. Qian, A.Dai, J.Fasullo. 2007. Estimates of the global water
budget and its annual cycle using observational and model data. Journal of Hydrometeorology, 8:758-769.
Trenberth K.E., J.T .Fasullo, J.Kiehl. 2009. Earth's global energy budget. Bulletin of the American Meteorological Society, 90;311-324.