Sunday, March 3, 2013

Gap filling techniques to use for OzFlux eddy covariance data by Natalia Restrepo-Coupe, Alfredo Huete and Kevin Davis

Our goal is to use the OzFlux network micrometeorological flux measurements of carbon dioxide exchange between different Australian ecosystems (biosphere) and the atmosphere, to better understand the physical meaning of satellite data generally associated to photosynthetic capacity. The Eddy Covariance Method (EC) used at all OzFlux sites, measures Net Ecosystem Exchange (NEE) on an hourly basis. NEE can be separated on its 2 main components, Ecosystem Respiration (Re) and assimilation (photosynthesis), the latter assumed to be equivalent to Gross Ecosystem Productivity (GEP). Therefore, GEP = NEE + Re. For detailed information about the EC method see Goulden et al. (1996).

Figure 1. Rectangular hyperbola fitted to 8-day worth of Gross Ecosystem Productivity (GEP) and Short Wave Incoming Radiation (SWin) data measured at Calperum-Chowilla flux tower data (OZflux). Photosynthetic Capacity (Pc), Light Use Efficiency (LUE) and GEP at saturation (GEPsat) are calculated, as shown. Special thanks to Prof D. Chittleborough, Prof W. Meyer, Dr. G. Whiteman and T. Luckbe

In order to harmonize and standardize the flux data we carry out the following process: (1) clean the data (e.g. remove data collected during rainfall) and remove existent outliers, (2) define a site specific turbulence threshold, as fluxes need to be corrected (removed) if measured during low turbulence periods, when the basic assumptions of the EC method are not satisfied, (3) NEE is partitioned in Re and GEP, were Re is derived from nighttime NEE (when there is no photosynthesis, GEP=0), and finally, (4) fit a rectangular hyperbola to the relationship between GEP and incoming solar radiation and using the fit to obtain different measures of ecosystem photosynthetic capacity (e.g. productivity at saturating light, GEP
sat) see Figure 1.

We compare different MODIS vegetation indices and reflectances to the flux derived data (Figure 2), to determine the product that best (similar seasonality and year-to-year variations) matches the insitu measures of ecosystem photosynthetic capacity, required by the proposed Australian terrestrial GPP near real-time monitoring system. For more information regarding MODIS vegetation indices, see Measuring Vegetation NDVI and EVI, by Weier and Herring (2000)

Figure 2.Annual cycle (16-day mean) for Calperum-Chowilla flux station (a) tower measured Gross Ecosystem Productivity, GEP (black line) and Ecosystem Light Use Efficiency, LUE (red line). Special thanks to Prof D. Chittleborough, Prof W. Meyer, Dr. G. Whiteman and T. Luckbe. (b) MODIS Enhanced Vegetation Index, EVI (black line) and Normalized Vegetation Index, NDVI (red line) by Weier and Herring (2000)