1. a Department of Environmental Science , University of Stirling , Stirling, FK 94LA, England, U.K.
2. b Westlakes Research Limited , The Princess Royal Building, Westlakes Science and Technology Park, Moor Row, Cumbria, CA 243LN, England, U.K.
3. c NERC Unit for Thematic Information Systems (NUTIS), University of Reading , Reading, RG 6 2AB, England, U.K.