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Water scarcity is intensified by drought, a phenomenon that impacts many sectors of society and affects virtually all climate zones. The Palmer drought indices are widely used by scientists and policy makers to understand drought and model its components. Despite the spatial heterogeneity and variability in variables required by the Palmer model, regional index values are most commonly used for real-time drought assessment. Local stakeholders charged with developing flexible and tailored water management policies have articulated the need for drought indices calculated at finer spatial resolutions than a regional scale. We use the Pacific Northwest United States (U.S.) as a study area to demonstrate the differences between drought indices calculated for U.S. climate divisions with those calculated at a 0.5° by 0.5° latitude/longitude resolution. Our results indicate that regional values of the two cumulative Palmer drought indices do not represent finer-resolution values well. For half of the study area, the pictures of drought (as determined by regional and finer-resolution values) are drastically different more than 30% of the time. Thus, quite often water managers do not have a clear understanding of the relative severity of drought in their area, which can have serious implications for drought mitigation and adaptation.

This paper presents a tool for calculating the Palmer Drought Severity Index (PDSI) and associated drought indices. The PDSI is a widely used drought index, yet the complexity and lack of transparency associated with the calculation of the PDSI makes it difficult for a researcher to independently calculate the index. Researchers are often forced to use PDSI values supplied by the National Climatic Data Center (NCDC) of the National Oceanic and Atmospheric Administration (NOAA) or some other third party. The MATLAB tool presented here is easy to use, thoroughly documented, and transparent. The tool was developed by checking an independently developed code against NCDC's FORTRAN code. Discrepancies between the MATLAB tool and the NCDC code are documented. Researchers using the tool will be able to easily calculate the Palmer drought indices for data inputs of any length and at any spatial scale. [ABSTRACT FROM AUTHOR]