1. Environmental Science Department, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, LA1 4YQ, UK, Meteorological Service of Canada, Environment Canada, 4905 Dufferin Street, Downsview, Ontario M3H 5T4, Canada, Centre for Ecology and Hydrology, Natural Environment Research Council, Winfrith Technology Centre, Dorchester, Dorset DT2 8ZD, UK, and Rothamsted Experimental Station, Harpenden, Hertfordshire AL5 2JQ, UK