Drought
Environmental Indicators:
Societal Indicators:
Disclaimer: This tool is best viewed in Chrome, Firefox, Microsoft Edge, and Safari browsers. It does not work with Internet Explorer.
The initial map shows the number of drought events by climate division from 1900 to 2023.
Each of the above highlighted attributes can be changed in the grey sidebar to the left of the map. Click the Update Map button to refresh the map based on your selections.
Click a location on the map and scroll down to see how drought changes over time at that location. Click on the tabs to view trends and additional information. Click on the magnifying glass icon on the right side of the map to search for a location.
PHDI over time (Left Graph) displays the Palmer Hydrologic Drought Index (PHDI) over time at the selected location. Drought occurred when the PHDI was -2.0 or lower (dark red). Lighter shades of blue and red are considered "normal" (PHDI is between -2.0 and 2.0). Dark blue indicates wet conditions (PHDI ≥ 2.0)
When drought occurred, how long and how strong? (Right Graph) plots each drought event and its median strength (PHDI) over time. Drought events began with three consecutive months with a PHDI ≤ -2.0 and ended with a PHDI ≥ -2.0 for three consecutive months.
Drought Extent displays the median PHDI for all climate divisions during the time period of the selected drought event. Be patient, the map takes a few seconds to load.
Select a drought event from the list above. The map may take a few seconds to load.
When do droughts start or end (Top Left) shows the percent of drought events that started and/or ended by month at the selected location and across the nation.
Is the magnitude or duration of droughts changing (Top Right) shows trends in the duration (number of months) or magnitude (median PHDI) of drought events.
How much time is spent in drought? (Bottom Left) shows the percent of each year that overlapping political boundaries spend in drought.
How does the frequency, magnitude, and duration of events compare? (Bottom Right) shows the frequency (number of events), magnitude (strength of events), and duration (length of events). The solid line represents the selected location. The boxplot shows the 25th to 75th percentile of comparable political boundaries.
Over which time periods were there significant trends? displays the slope (longer duration and higher PHDI are positive values while shorter duration and lower PHDI are negative values) and p-values (significance) for different time periods with 30+ years of data and more than 5 drought events. This shows how consistent changes in the duration or magnitude of drought events have been over time. Highlighted cells are for the selected start year (row).
Slope:
| Duration (months/yr) | Slope ≥ 0.5 | -0.5 < Slope < 0.5 | Slope ≤ -0.5 |
| Magnitude (phdi/yr) | Slope ≥ 0.1 | -0.1 < Slope < 0.1 | Slope ≤ -0.1 |
P-value:
| Pval > 0.10 | 0.05 < Pval ≤ 0.10 | Pval ≤ 0.05 |
Drought occurs when there is an abnormal deficit in precipitation, soil moisture, or surface water resources. The Palmer Drought Hydrologic Indicator (PHDI) is a long-standing metric to understand drought that impacts surface water resources. The more traditional Palmer Drought Severity Index (PDSI) responds rapidly to changes in precipitation, while the PHDI measures hydrologic impacts and responds slower to changing conditions. The impact of drought on reservoirs that provide water supply to communities is of particular concern. A warming climate is predicted to increase the frequency of drought conditions. Understanding the frequency, duration, and extent of hydrologic drought (and how this may be changing) can be a helpful tool for water managers.