Submitted to: ASA-CSSA-SSSA Annual Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: August 9, 2010
Publication Date: November 4, 2010
Citation: Meek, D.W. 2010. Are Eddy Covariance series stationary [CD-ROM]? In: ASA-CSSA-SSSA Annual Meeting Abstracts, Oct. 31 - Nov. 4, 2010, Long Beach, CA. Technical Abstract: Spectral analysis via a discrete Fourier transform is used often to examine eddy covariance series for cycles (eddies) of interest. Generally the analysis is performed on hourly or half-hourly data sets collected at 10 or 20 Hz. Each original series is often assumed to be stationary. Also automated filtering in frequency space is usually then utilized. Through the examination of multiple data sets, this study examines the 'stationary' assumption. The x vector wind speed (horizontal) and CO2 flux series for multiple hourly and half-hourly 20 Hz records from the 2008 Bushland Evapotranspiration and Remote Sensing Experiment (BEAREX) are tested for trend via two nonparametric procedures - Kendall's tau and Spearman's rank correlation coefficient. The analyses reveal that strong trend is almost always present in both variables for both time intervals. Night time wind speeds tend to have higher absolute values for both tau and rank correlation coefficient compared to their corresponding day time values. The coefficients for the CO2 flux series generally exhibit a diurnal pattern with negative values during the day time hours and positive otherwise. Eddy covariance series for horizontal wind speed and CO2 flux from hourly and half-hourly periods are, thus, not generally stationary. Some examples of removing the trend or dealing with the trend will be presented.