Palmer Drought Severity Index

Palmer Drought Severity Index
Name	Palmer Drought Severity Index
Developer	Milton B. Palmer
Introduced	1965
Discipline	Climatology

Palmer Drought Severity Index

The Palmer Drought Severity Index is a meteorological index created to quantify and monitor drought conditions across regions, widely used in United States climate assessments and agricultural planning. It links precipitation, temperature, and soil moisture to produce a standardized drought metric referenced in reports by agencies such as the National Oceanic and Atmospheric Administration and the United States Department of Agriculture. Developed during the mid-20th century, it remains influential in comparative studies involving indices like the Standardized Precipitation Index, the United Nations's drought monitoring efforts, and regional water-resource assessments in places such as California, Texas, and the Great Plains.

Overview

The index expresses drought severity through a numerical scale that classifies conditions from extreme wetness to extreme dryness, intended for use in climatology and operational monitoring by organizations including the National Weather Service, the United States Geological Survey, and state-level offices. It has been applied in analyses of historic events such as the Dust Bowl era and regional episodes affecting sectors represented by the American Farm Bureau Federation and the Food and Agriculture Organization. Users often compare it with measures developed by institutions like the World Meteorological Organization and research programs at universities such as Iowa State University and Texas A&M University.

Methodology

The algorithm combines empirical relationships among precipitation, temperature, and estimated soil moisture processes using parameters derived from long-term station records held by entities like the National Centers for Environmental Information and the Met Office. It computes a monthly moisture anomaly based on a two-tiered water-balance model incorporating potential evapotranspiration formulations influenced by procedures used in studies at Princeton University and Pennsylvania State University. Calibration relies on climatological averages similar to datasets maintained by the Climate Research Unit and observational networks such as the Global Historical Climatology Network. The resulting index values are scaled to represent degrees of dryness or wetness used in assessments by agencies like the Environmental Protection Agency and academic analyses published in journals associated with the American Meteorological Society.

Applications and Usage

Practitioners employ the index in drought monitoring by agencies including the National Drought Mitigation Center, the Federal Emergency Management Agency, and regional water authorities in states such as Colorado and Arizona. It has informed policy decisions related to agricultural assistance administered by the Farm Service Agency and water management plans overseen by the Bureau of Reclamation. Researchers use it in historical reconstructions alongside paleoclimate proxies examined by teams at institutions like Columbia University's Lamont-Doherty Earth Observatory and the University of Arizona. Comparative studies juxtapose it with indices developed by international bodies such as the European Centre for Medium-Range Weather Forecasts and used in multinational assessments coordinated by the Intergovernmental Panel on Climate Change.

Limitations and Criticisms

Critiques have highlighted sensitivity to the choice of climatological normals and limited responsiveness to short-term hydrologic changes noted in analyses from Stanford University and University of California, Berkeley. The index's reliance on temperature-based evapotranspiration estimates is contrasted with physically based approaches advocated by researchers at the Massachusetts Institute of Technology and the University of Washington. Operational users in regions like Sub-Saharan Africa and the Australian Outback have reported mismatches between index output and observed impacts, prompting evaluation by agencies such as the World Bank and NGOs including CARE International. Peer-reviewed critiques in journals affiliated with the Royal Society and the American Geophysical Union emphasize scale dependency and potential misclassification of drought onset and recovery compared with soil-moisture and streamflow indicators championed by the International Water Management Institute.

Variants and Related Indices

Extensions and alternative implementations include derivatives used in operational suites by the National Integrated Drought Information System and academic modifications developed at institutions like the University Corporation for Atmospheric Research and Iowa State University. Related indices frequently cited alongside it are the Crop Moisture Index, the Standardized Precipitation Index, the Reconnaissance Drought Index, and hydrologic measures used by the United Nations Educational, Scientific and Cultural Organization in transboundary water studies. Global drought monitoring programs from agencies such as the European Commission's Joint Research Centre and the World Meteorological Organization integrate multiple indices to address regional biases and sector-specific needs identified by groups including the International Monetary Fund and the Food and Agriculture Organization.

Historical Development

The index was formulated in the 1960s by a researcher associated with state and federal climatological services and published contemporaneously with other mid-century advances at laboratories and universities such as Cornell University, Michigan State University, and Iowa State University. Its adoption grew through incorporation into operational products by the National Weather Service and analysis frameworks used by the Soil Conservation Service and later the Natural Resources Conservation Service. Subsequent decades saw evaluations and reforms prompted by events like the 1988 United States drought and the 21st-century droughts in California, leading to methodological scrutiny in panels convened by organizations including the National Academy of Sciences and collaborative projects involving the National Science Foundation.

Category:Climatology