Drought Indices 101

About Drought

Drought is simply a shortage in water supply relative to demand; however, due to the wide range of types of water “demands”, quantifying drought is much more complex. More than 100 drought indices exist, with purposes that include characterizing drought for a particular sector or audience, providing early warning of drought, and improving understanding of how drought severity relates to drought impacts (Zargar et al. 2011). In our work, we selected indices that correspond to different parts of the hydrological cycle to gain a better understanding of what drives drought-related dynamics in this ecosystem (Figure 1).

Figure 1. The hydrological cycle, highlighting where in the cycle a particular drought index is focused. Source for background graphic of the hydrologic cycle is NOAA-National Weather Service, https://www.weather.gov/jetstream/hydro.

Drought Indices

EDDI - Evaporative Demand Drought Index

EDDI is a drought index based on the “thirst” of the atmosphere, or more technically, the anomaly (deviation from “normal”) in evaporative demand. It is calculated from temperature, humidity, windspeed, and solar radiation. EDDI calculations can indicate drought conditions earlier than other indices, and has been used to characterize sudden onset (“flash”) drought, and to inform seasonal fire potential.

LERI - Landscape Evaporative Response Index

LERI is a drought-monitoring and early warning guidance tool that measures the anomaly in the actual evapotranspiration (ETa) from the land surface. It uses remotely-sensed data to measure how much water is transpiring from vegetation and land surfaces, and can be considered a proxy for soil moisture.

SPI - Standardized Precipitation Index

SPI is a commonly-reported drought index that is based solely on precipitation data. It uses a comparison of precipitation during a certain time period to the multi-year average to calculate drought levels. SPI can be depicted for various time scales, and typically it is applied for the 3, 6, 12, 24, and 48 month time periods.

FDSI - Forest Drought-Stress Index

FDSI is a relatively new drought index that is based on empirical growth-climate relationships derived from tree rings widths and observed precipitation and vapor pressure deficit during 1896-2007. The FDSI corresponds to measures of forest productivity, mortality, bark-beetle outbreak and wildfire. Because vapor-pressure deficit is strongly driven by warming temperatures, the mean FDSI by the 2050s will exceed that of the most severe droughts in the past 1,000 years (Williams et al. 2013).

SWE - Snow Water Equivalent

Snow water equivalent, or SWE, is the amount of water contained within the snowpack on April 1. SWE hasn’t traditionally been considered a drought index, but as we move into the 21st century, we are experiencing more “snow drought” due to either lower precipitation during winter, or warmer temperatures during winter that prevent precipitation from falling as snow.