Testing the applicability of neural networks as a gap-filling method using CH4 flux data from high latitude wetlands.

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. [ABSTRACT FROM AUTHOR]