Testing the applicability of neural networks as a gap-filling method using CH4 flux data from high latitude wetlands
- 1University of Helsinki, Department of Physics, P.O. Box 48, 00014 University of Helsinki, Finland
- 2University of Sheffield, Department of Animal and Plant Sciences, Western Bank, Sheffield S10 2TN, UK
- 3San Diego State University, San Diego, CA. 92182, USA
- 4Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, 14473 Potsdam, Germany
- 5Finnish Meteorological Institute, P.O. Box 503, 00101, Helsinki, Finland
- 6Center for Permafrost, Department of Geosciences and Natural Resources Management, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark
- 7Department of Physical Geography and Ecosystem Science, Lund University, Sölvegatan 12, 223 62 Lund, Sweden
Abstract. Since the advancement in CH4 gas analyser technology and its applicability to eddy covariance flux measurements, monitoring of CH4 emissions is becoming more widespread. In order to accurately determine the greenhouse gas balance, high quality gap-free data is required. Currently there is still no consensus on CH4 gap-filling methods, and methods applied are still study-dependent and often carried out on low resolution, daily data.
In the current study, we applied artificial neural networks to six distinctively different CH4 time series from high latitudes, explain the method and test its functionality. We discuss the applicability of neural networks in CH4 flux studies, the advantages and disadvantages of this method, and what information we were able to extract from such models.
Three different approaches were tested by including drivers such as air and soil temperature, barometric air pressure, solar radiation, wind direction (indicator of source location) and in addition the lagged effect of water table depth and precipitation. In keeping with the principle of parsimony, we included up to five of these variables traditionally measured at CH4 flux measurement sites. Fuzzy sets were included representing the seasonal change and time of day. High Pearson correlation coefficients (r) of up to 0.97 achieved in the final analysis are indicative for the high performance of neural networks and their applicability as a gap-filling method for CH4 flux data time series. This novel approach which we show to be appropriate for CH4 fluxes is a step towards standardising CH4 gap-filling protocols.