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Home > Technical Resources > Drought Planning Toolbox > DroughtIndices  


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Primary and secondary drought indices
Index WebsiteFilterIndex DescriptionFilter
Standard Precipitation Index (SPI)Use SHIFT+ENTER to open the menu (new window).
The SPI was developed at the Colorado Climate Center as a tool for defining and monitoring drought and is a robust index for describing drought patterns. The SPI is based only on current and historical precipitation data for a particular location. The computed SPI values are proportional to precipitation deviation from the “average” (surplus or deficit).The SPI is computed for several time scales, ranging from one month to 24 months, to capture the various scales of both short-term and long-term drought.  The SPI can be used to identify the beginning and end for each drought, as well as an intensity level for each month in which the drought occurs.
Revised Surface Water Supply Index (SWSI)Use SHIFT+ENTER to open the menu (new window).
The SWSI index was developed by the Office of the State Engineer and the USDA Natural Resources Conservation Service (NRCS). SWSI is an indicator of mountain-based water supply conditions in the major river basins.  SWSI values in January through June is based on reservoir storage and forecasted flow. July through September is based on reservoir storage and actual (native) flow.  October through December is only based on reservoir storage. The resulting scores generally range from +4.16 (abundant water supplies) to -4.16 (extreme drought conditions).
Revised Surface Water Supply Index (SWSI)Use SHIFT+ENTER to open the menu (new window).
In the early 1990s the NRCS refined the SWSI calculation to improve upon the limitations of the existing SWSI procedures. This method substitutes streamflow forecasts for the weighting factor variables, which is an objective, statistical assessment of the data relating to snowmelt runoff.  Streamflow forecasts are optimized from the data for the hydrologic components and implicitly contain optimal weighting of the components. The revised technique provides a more stable month to month transition which eliminates some of the illogical shifts in index values, which the existing SWSI sometimes produces as the variables change throughout the year.
Palmer Drought Severity Index (PDSI)Use SHIFT+ENTER to open the menu (new window).
A complex soil moisture calculation that has been used by federal agricultural agencies to determine when to provide drought assistance. It requires weekly or monthly precipitation and temperature data as inputs. The Palmer Index uses a +4  to -4 scale where 0 represents normal and negative reflects drought.
Colorado Modified Palmer Drought Severity IndexUse SHIFT+ENTER to open the menu (new window).
In an effort to improve the utility of the Palmer index in Colorado, Doesken et al., 1983, created the Colorado Modified Palmer Drought Severity Index (CMPDSI). The CMPDSI creates 25 geographical subregions of the state that are more climatically similar than the original 5 regions calculated on the national scale. The procedure for calculating the CMPDSI is the same method as described by Palmer (1965), it is only the regions that are being calculated that were modified. Please refer to Palmer (1965) for a complete description of how the index is calculated.
U.S. Drought MonitorUse SHIFT+ENTER to open the menu (new window).
The Drought Monitor is a consensus of federal, state and academic expert opinions. A national drought summary is produced weekly using a drought severity classification system to illustrate drought conditions.
Colorado Monthly Precipitation and Percent of Normal MapsUse SHIFT+ENTER to open the menu (new window).
The Colorado Climate Center develops monthly precipitation maps in or to track Water Year (October 1- September 30) precipitation to aid in drought monitoring, runoff forecasting, agricultural planning, and emergency management.  The percentage of normal precipitation is calculated by dividing actual precipitation by normal precipitation—typically considered to be a 30-year mean. This can be calculated for a variety of time scales. Usually these time scales range from a single month to a group of months representing a particular season, to an annual or water year. Normal precipitation for a specific location is considered to be 100%.
Crop Moisture IndexUse SHIFT+ENTER to open the menu (new window).
This index was developed from the Palmer Index, and was designed to evaluate short-term moisture conditions across major crop producing regions. It uses the average temperature and total precipitation for each week and compares the calculated index with the previous week. This is a better index to measure rapidly changing conditions and for comparing different locations but the gross scale of the climate divisions (only 5 for Colorado) makes it a less useful index for Colorado.
Keetch-Byram Soil Moisture IndexUse SHIFT+ENTER to open the menu (new window).
A drought index specifically designed for fire potential assessment. This index is based on weather station latitude, mean annual precipitation, maximum dry bulb temperature, and the last 24 hours of rainfall. The resulting index relates to the flammability of organic material on the ground. Scores range from 0 to 800 where zero is the point of no moisture deficiency and 800 is the maximum drought.