Long-term trends in the atmospheric water vapor content estimated from ground-based GPS data

We have used 10 years of ground-based data from the Global Positioning System (GPS) to estimate time series of the excess propagation path due to the gases in the neutral atmosphere. We first derive the excess path caused by water vapor which in turn is used to infer the water vapor content above each one of 33 GPS receiver sites in Finland and Sweden. Although a 10 year period is much too short to search for climate change we use the data set to assess the stability and consistency of the linear trend of the water vapor content that can be estimated from the data. The linear trends in the integrated water vapor content range from -0.2 to +1.0 kg m(-2) decade(-1). As one may expect we find different systematic patterns for summer and winter data. The formal uncertainty of these trends, taking the temporal correlation of the variability about the estimated model into account, are of the order of 0.4 kg m(-2) decade(-1). Mostly, this uncertainty is due to the natural short-term variability in the water vapor content, while the formal uncertainties in the GPS measurements have only a small impact on the trend errors.