The Location of the Eddy Covariance System is Appropriate for Monitoring Shrub Flux using Footprint Analysis in Arid Areas

Eddy covariance (EC) measurements are applied widely to continuously monitor turbulent exchanges of mass and energy at the vegetation-atmosphere interface. However, the utilization and application of EC measurements can be problematic owing to difficulties/uncertainties in assessing/interpreting the associated measuring biases of EC data arising from the sensor’s views over heterogeneous natural vegetation and soils from different wind sectors.

Footprint models have been developed to determine the field of view (or source area) of EC measurements, thus, providing information on the spatial representativeness of the data. Determination of the footprint is necessary for interpretation of the results of EC measurements, especially when these are performed over a landscape of varying surface source strengths.

Arid areas are sensitive to global change. Tamarix ramosissima is a dominant native desert shrub in gravel desert, heavy textured desert soil and sandy desert. In the Fukang Station of Desert Ecology, EC is located in a region where T. ramosissima is the only dominant shrub species in the communities.

To highlight the importance of footprint analysis in identifying the area contributing to flux measurements, and to evaluate whether the location is appropriate for using EC to monitor the T. ramosissima shrub, ZHAO Jin et al. used EC to continuously monitor the turbulent exchanges of mass and energy in the Fukang station. Spatial distributions of footprint source areas were evaluated to reveal the relative flux contribution to the total flux over the entire observation period.

The results indicated that the applied footprint model (Kormann and Meixner model) provides accurate footprint estimates and the flux data can be used for the shrub flux estimations. The flux contribution from shrub land showed that the observed flux data were able to represent seasonal change in the flux of desert shrub ecosystems. The flux contribution rate of shrub was highest during May to July between 10:00 and 18:00. The location of the EC system in the Fukang station is appropriate for monitoring shrub flux.

Footprint analysis is necessary because it assesses the contribution of the target land-use type to the total flux for any user-defined period. The flux contribution rate was affected significantly by wind direction and the source region. The study was published in Environmental Earth Sciences in February 2014.