Hydrologic Conditions Description


The Standardized Precipitation Index (SPI) of McKee et al. (1993) is a statistical rainfall assessment, useful for summarizing how wet or dry a region is compared to its historical average. Unusually dry regions receive negative index values, and unusually wet regions receive positive index values. SPI indices between -1.0 and -2.0 represent moderate drought, and indices <-2.0 represent extreme drought.

The Western US Drought Monitor calculates SPI on ten accumulation periods, calculated on the 1/16 degree VIC model.


The Standardized Runoff Index (SRI) of Shukla and Wood (2008) is a statistical runoff assessment that summarizes the amount of runoff in a region compared to its historical average. Regions with unusually low runoff are given negative values, while regions with unusually high runoff are given positive values. This index is calculated using the same approach as the SPI, but with runoff instead of precipitation as input.


Soil Moisture (SM) is an index for the water content within a unit of soil. Low soil moisture represents dry soils, and high soil moisture represents wet soils. The Western US Drought Monitor displays soil moisture as a percentile compared to the historical soil moisture values from 1920-2010. A cumulative distribution function (CDF) is generated from the historical soil moisture data. Then the percentile of a region's current soil moisture is the corresponding percentile value in this CDF.


Snow-Water Equivalent (SWE) represents the amount of potential liquid water that is currently stored as snow. Snowmelt can transform SWE into liquid water (typically as runoff). For many regions around the world, snowmelt provides a significant percentage of annual water and it can be vital to quantify.

SWE is plotted here using the same methodology as SM. If at a grid cell, the historical and current SWE values are both less than 10mm, then the percentile value will not be plotted for this grid (displayed as grey on the graph), which means typically there is almost no snow stored at this grid cell.


Total Moisture (STOT) is a moisture index calculated by adding SM and SWE. STOT represents the total water content of a region. STOT is plotted on the PNW Drought Monitor using the same methodology as SM and SWE.