Eddy Covariance and Weather Data

Available Data
Wet Season 2005 Dry Season 2006

Methods

Evapotranspiration measurements from the Cerro Pelado area were made with an eddy covariance system mounted to a tower reaching above the canopy. The location of the tower is indicated by a "T" on the Gamboa Site Map and has the advantage of being located in a saddle location on the hill. The eddy covariance system is supplied by Campbell Scientific including a 3-D sonic anemometer, a Licor LI-7500 open path H20 / CO2 gas analyzer, and a CR5000 data logger capable of computing the required fluxes online. Further supplementing the eddy covariance components listed above is a fine wire thermocouple that when coupled with the sonic anemometer provide sensible heat estimates. Typically Campbell accomplishes this with a RH probe, however no RH can accurately measure humidity above 95% a common occurence in Panama during the wet season and likely the case every night. Additionally a Zipp & Konen CNR1 net radiometer capable of measuring incoming/outgoing shortwave and longwave radiation is found on the tower. CNR1 measurements allow additional quantities such as the albedo of the jungle canopy to be estimated. Combining latent and sensible heat fluxes with in/outoing radiative fluxes allows an evaluation of the closure obtained in the energy budget.

Thoughtful tower placement of the main components to be used in latent and sensible heat fluxes measurements must be made to ensure that the appropriate scale of measurement is made. Average vegetative height in the area surrounding the sensors must first be estimated to determine the sensor height and subsequent fetch. Immediately surrounding the tower the effective canopy height is approximately 25 m, therefore we decided to coincidently place the sonic, Licor, and fine-wire thermocouple at approximately 27 m.

Theory

Evapotranspiration (ET) is the process that dominates the vertical transport of water from ground to atmosphere when the ground surface is vegetated. There are several approaches available to estimate ET: the water balance method, turbulence methods, aerodynamic profile methods, energy balance methods, combination methods, and empirical methods with each method having its own advantages and disadvantages.  A more rigorous measurement system, but expensive and data intensive is known as the Eddy-Covariance system, a turbulence method. This system recognizes that the transport of heat, moisture, and momentum in the boundary layer is governed almost entirely by turbulence. In order to quantify turbulent parameters with any success instruments capable of sampling with a high frequency (10-20 Hz) must be employed. Development of the eddy covariance system as a measurement technique is described by Baldocchi [1988] and is in wide spread use globally by the FLUXNET [2004] global network. Following the development of Baldocchi, the eddy covariance method is derived from conservation of mass equations, where the mixing ratio is used since it is conservative quantity.  Considering the time rate of change of a given quantity at a fixed point in space can be given as:

where c is the mixing ratio of a given quantity (mass quantity per unit mass of dry air), u, v, and w are wind velocity components acting in the longitudinal (x), lateral (y), and vertical (z) directions respectively. D and S represent the molecular diffusion and source or sink terms respectively. Overbars represent time-averaged quantities and primes denote fluctuations from the mean. Qualitatively the first three terms on the right hand side of the equation represent the mean horizontal and vertical advection of c. The next three terms are the convergence/divergence of the turbulent flux of c.

Application of Eq. 1 is ideally supposed to be applied to horizontal and level land surfaces, free of sources or sinks of c, and the concentration should not vary significantly with time. Application of these assumptions along with two more steps from Baldocchi allows us to arrive at the mean vertical turbulent flux of c:

 
where r_a is the dry air density. According to the convention used in the derivation a value of F>0 indicates an upward flux.