Drought has many definitions. The Glossary of Meteorology defines drought as “A period of abnormally dry weather sufficiently prolonged for the lack of water to cause serious hydrologic imbalance.” In lay terms, it’s generally defined as a period when water is insufficient for existing demands. In Kansas, drought declarations, which trigger specific actions, are issued by the Governor’s office.
Palmer Drought Severity Index
One of the most common tools to measure drought is the Palmer Drought Severity Index (PDSI). The PDSI was originally developed by Palmer [1965] with the intent to measure the cumulative departure in surface water balance. It incorporates antecedent (past) and current moisture supply (precipitation, P) and demand (potential evapotranspiration, PE) into a hydrological accounting system, which includes a 2-layer bucket-type model for soil moisture calculations. The PDSI is a standardized measure, ranging from about -10 (dry) to +10 (wet) with values below -3 representing severe to extreme drought (Dai, 2017). The Kansas Climate Office has animated maps of the PDSI from the 1900s through 2011. A snapshot of PDSI values for Kansas from August 1936 is shown in Figure 1.
Standardized Precipitation Index
Another drought tracking tool is the Standardized Precipitation Index (SPI). Developed at Colorado State University, the SPI is a tool which allows an analyst to determine the rarity of a drought at a given time-scale (temporal resolution) of interest for any rainfall station with historic data. The SPI allows more temporal flexibility than the PDSI. The latest SPI map shows how the northwest corner of Kansas has a positive SPI for the last 120 days ending on April 11, 2018 (Figure 2).
U.S. Drought Monitor
In 1999, the U.S. Drought Monitor was developed. This resource is produced jointly by the National Oceanic and Atmospheric Administration (NOAA), the U.S. Department of Agriculture (USDA), and the National Drought Mitigation Center at the University of Nebraska-Lincoln. The U.S. Drought Monitor website is hosted and maintained by the National Drought Mitigation Center. The weekly monitor integrates the PDSI and the SPI, as well as hundreds of other measures, including stream flow, evaporative demand, soil moisture and vegetative health. Inputs are cutoff on Tuesday and the new map is issued on Thursday morning.
The U.S. Drought Monitor is used by policymakers and media in discussions of drought and in allocations of drought relief. As part of its response to the drought of 2012, the U.S. Department of Agriculture streamlined the process for secretarial disaster declarations, making declarations nearly automatic for a county shown in severe drought on the U.S. Drought Monitor for eight consecutive weeks.
References:
Dai, Aiguo & National Center for Atmospheric Research Staff (Eds). Last modified 12 Jul 2017. "The Climate Data Guide: Palmer Drought Severity Index (PDSI)."
http://drought.unl.edu/monitoringtools/climatedivisionspi.aspx
Mary Knapp, Weather Data Library
mknapp@ksu.edu
Chip Redmond, Kansas Mesonet Manager
christopherredmond@ksu.edu
Xiaomao Lin, State Climatologist
xlin@ksu.edu