1. Centre for Ecology & Hydrology; Lancaster Environment Centre; Library Avenue, Bailrigg Lancaster LA1 4AP UK
2. Shell International Exploration and Production; Shell Technology Center Houston; 3333 Highway 6 South Houston TX 77082-3101 USA
3. Institute of Biological and Environmental Sciences; University of Aberdeen; 23 St Machar Drive Aberdeen AB24 3UU UK
4. Department of Soil and Crop Sciences; Colorado State University; Fort Collins CO 80523 USA
5. Centre for Ecology & Hydrology; Maclean Building Wallingford OX10 8BB UK