| Summary: | A global monthly data set of Palmer Drought Severity Index (PDSI) from 1870 to 2005 has been produced using historical observed surface air temperature and precipitation data for global land areas, except Antarctica and Greenland, on a 2.5 x 2.5 degree grid. Calibration (or reference) period is 1950-1979. The PDSI was created by Palmer (1965) with the intent to measure the cumulative departure (relative to local mean conditions) in atmospheric moisture supply and demand at the surface. It incorporates antecedent precipitation, moisture supply, and moisture demand into a hydrological accounting system. To calculate the global monthly PDSI, Climate Research Unit (CRU) surface air temperature data (Jones and Moberg, 2003 and updates) were regridded to a 2.5 x 2.5 degree grid. National Centers for Environmental Prediction (NCEP) precipitation data for 1948â“2003 from about 5,000 to 16,500 rain gauges during 1948â“97 and about 3,500 gauges thereafter (Chen et al., 2002) were gridded using the optimal interpolation scheme. For the pre-1948 period, precipitation data from Dai et al. (1997) were used. The monthly anomalies of Dai et al. (1997) were adjusted to have zero mean values for 1950â“79 and then added to the 1950â“79 mean of NCEP data to obtain the total precipitation used for the PDSI calculation. For field water-holding capacity (awc), a soil textureâ“based water-holding-capacity map from Webb et al. (1993) was used. The time series are thought to be reliable over most land areas. The relatively low resolution used, however, does not resolve small scale variations such as those over mountains. The global monthly PDSI data are provided by UCAR/NCAR as compressed binary, text, and netCDF files with an associated README file. Oak Ridge National Laboratory (ORNL) also provides the global monthly PDSI data set, but in GeoTiff format. ORNL converted the UCAR/NCAR data from netCDF format to GeoTIFF format. The processed GeoTIFF data were fed into ORNL DAAC Web Map Service v1.1.1 (WMS), Web Coverage Service v1.0.0 (WCS), and Spatial Data Access Tool (SDAT) to provide data visualization and distribution capabilities. Sources: Chen, M., P. Xie, J. E. Janowiak, and P. A. Arkin. 2002. Global land precipitation: A 50-yr monthly analysis based on gauge observations. J. Hydrometeor., 3, 249â“266. Dai, A., K. E. Trenberth, and T. Qian. 2004. A global data set of Palmer Drought Severity Index for 1870-2002: Relationship with soil moisture and effects of surface warming. J. Hydrometeorology, 5, 1117-1130. Dai, A., I. Y. Fung, and A. D. Del Genio. 1997. Surface observed global land precipitation variations during 1900â“88. J. Climate, 10, 2943â“2962. Jones, P. D., and A. Moberg. 2003. Hemispheric and large-scale surface air temperature variations: An extensive revision and an update to 2001. J. Climate, 16, 206â“223. Palmer, W. C. 1965. Meteorological drought. Research Paper 45, U.S. Dept. of Commerce, 58 pp. Webb, R. S., C. E. Rosenzweig, and E. R. Levine. 1993. Specifying land surface characteristics in general circulation models: Soil profile data set and derived water-holding capacities. Global Biogeochem. Cycles, 7, 97â“108.
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