NOAA and international partners assess indices and indicators of drought used across the United States, Canada, and Mexico
Numerous indicators and indices are used to characterize and assess drought. Their effectiveness and accuracy can vary significantly, however, based on different geographical areas and climates, availability of data, and many other distinctions. That’s why a new study was initiated to assess the effectiveness of drought indicators and indices across North America.
NOAA scientists, in coordination with the North America Commission for Environmental Cooperation (CEC), collaborated with domestic and international partners to engage users across the continent in an assessment of indices and indicators that are used for drought monitoring in the United States, Canada, and Mexico.
The study, lasting two years (2019–20), evaluated drought indices and indicators compiled by the World Meteorological Organization (WMO) in its Handbook of Drought Indicators and Indices, as well as indices and indicators not included in the WMO Handbook.
New Guidance on Drought Indicators and Indices
The WMO Handbook only lists the drought indices and indicators; it does not provide any guidance on their use. This study supplements the WMO Handbook by providing information on the utility and effectiveness of the drought indices and indicators in North America.
The project engaged drought users and stakeholders across North America through a series of surveys and virtual meetings to assess how drought indices and indicators were used in the diverse climates of the continent.
In the study, each drought index or drought indicator was rated by the respondents as to its effectiveness in their climate zone(s). The ratings ranged from less effective (1) to very effective (5). The indices and indicators in the WMO Handbook were rated for short-term drought and long-term drought.
Diverse Climates of the North American Continent
The respondents characterized the climates of their regions using the Köppen Climate Classification System. Across North America, they identified more than 30 different climate zones representing their areas of responsibility on the continent, including zones ranging from Tropical Rainforest (Af) to Ice Cap Climate (EF), among many others.
This high level of diversity just in climate zones is an example of why numerous indices are used to assess drought.
Utility of the Indices and Indicators
The two-year study assessed the utility of the drought indices and indicators by climate zone, location (geography), and time of year (season).
For most climate zones, respondents indicated that indicators do not perform equally well across their respective geographical areas or across different seasons. Soil Moisture was the only indicator considered to be very effective in every climate zone.
Six other indicators were found to be very effective in most, but not all climate zones:
- Percent of Normal Precipitation
- Standardized Precipitation Evapotranspiration Index
- Standardized Precipitation Index
- Crop Status
- Reservoir Storage
- U. S. Drought Monitor
Finally, a majority of study respondents indicated that reports of drought impacts were highly effective indicators for drought monitoring for their region/area. Impacts were categorized as economic, environmental, or societal. Examples of reported impacts that survey respondents found useful for drought monitoring included increased consumer, business, and food costs; declines in farming, timber, and recreational income; destruction of wildlife habitat and increased wildlife disease; increased wildfires; and human health problems/loss of life, among many others.
The full results of the study are summarized in a Guide to Drought Indices and Indicators Used in North America, published by the CEC, and the assessment’s summary statistics, available online from NCEI.