D. Cripe J. Vogt M. Werner R. Heim M.J. Brewer J. Sheffield R. Westerhoff R. Stefanski M. Svoboda J. Verdin W. Pozzi 2011 [No abstract available] application/pdf 10.1109/GEOSS.2011.5948948 en Group on Earth Observations (GEO) global drought early warning information service text