Multivariate Analysis of Drought Conditions in the United States: 1895-2016

Drought events are important to understand, primarily because of their potential to reduce agricultural production, result in economic losses, and contribute to environmental degradation. In addition, drought events can create public health consequences, including adverse effects on air quality, decreased drinking water quantity and quality, compromised food availability and nutrition, and increased illness and disease. Over the past few decades, there have been efforts to quantify drought through simple indices that take into account both meteorological and hydrologic phenomena. The Palmer Drought Severity Index (PDSI; Palmer 1965) was the first index of its kind and included information such as prior precipitation, moisture supply, runoff, and evaporation demand at the surface level. While widely accepted, PDSI limitations include spatial comparability, strong influence of the calibration period, and fixed temporal scale. The Standard Precipitation Index (SPI; McKee et al. 1993, McKee et al. 1995) was developed to address these concerns, and can be calculated at different time scales to monitor useable water resources to assess drought. More recently, the Standard Precipitation Evapotranspiration Index (SPEI; Vincente-Serrano et al. 2010) was developed as an extension to SPI, adding both precipitation and temperature data, and an extensive climatic water balance by incorporating the differences in potential evapotranspiration over multiple periods of time.

NOAA’s National Centers for Environmental Information (NCEI) has developed gridded values of PDSI, SPI and SPEI from 1895 to 2016. While all three indices provide independent, robust analyses of drought, these data are at different spatial resolutions that require manipulation and pre-processing before they are accessible for studies to determine the effects of drought on health. To support efforts by the Centers for Disease Control and Prevention’s Health Studies Branch to assess the effects of drought on health outcomes, North Carolina Institute for Climate Studies is working on a project to assess existing drought data and increase the usefulness and accessibility of drought data by public health professionals. The first step of this project is to develop a consistent approach to evaluate monthly drought data at the county level for the continental United States. Next, statistical techniques will be applied to analyze onset, severity, and end of drought events, using all three indices, for each county in the continental United States. Percentiles will be calculated to note where drought is more common, and trends will be analyzed to assess how drought conditions are changing in the United States over time. By identifying trends in drought and providing drought data in a homogenous temporal and spatial format consistent with health data, public health professionals can better assess relationships between exposures to drought and potential health conditions, particularly at the county level. Better understanding of linkages allows health departments to recognize potential impacts of drought in their communities and to identify opportunities to develop intervention strategies to improve health and save lives.