Keeping tabs on drought

Continually tracking and evaluating conditions is critical, but complex, especially in a state as large and climatically diverse as Montana. Drought monitoring involves documenting drought data such as streamflow, snowpack, and precipitation. Drought assessment is the next step, requiring interpretation of science – plus local impact reports – to classify how bad the drought is. Drought monitoring and assessment blend science, local knowledge, and expert interpretation and it happens weekly, all year round, in Montana. 

In Montana, a group of five experts track and evaluate data-based drought metrics each week. They supplement their findings with local-level drought impact reports from Montanans – for example stories about grasshopper plagues or spring wheat failure. Then, the findings go out to a 50-member listserv for further review and feedback. After that, Montana sends a map delineating that week’s drought classes to the U.S. Drought Monitor, where it’s reviewed, checked for accuracy, and published. The U.S. Drought Monitor is the national standard for drought assessment, and it directly links to Montana’s drought stages as well as federal drought aid programs.  

Montana’s process has gained national attention for its collaborative nature and “convergence of evidence” approach. The convergence of evidence approach enables drought assessors to combine the many drivers, types, and impacts of drought into a single assessment. Although the approach considers both quantifiable and non-quantifiable variables, the foundation is data about current conditions and how they are changing over time.

Expand the tabs below to learn more about key drought metrics in Montana, and see the drought metrics in action on conditions tracking sites like the Upper Missouri River Basin Drought Indicators Dashboard and others available on our Resources page.


Precipitation percentiles describe the amount of precipitation received relative to what is expected over a period of interest. Percentiles are calculated against a historical record (reference period) to estimate expected precipitation amounts.
The Standardized Precipitation (SPI) was designed to standardize precipitation time series across a reference period to normalize precipitation anomalies in both time and space. SPI is different from precipitation percentiles as it explicitly models the probability of observing a specific amount of precipitation over a time scale of interest.
Precipitation and Reference Evapotranspiration - The Standardized Precipitation Evapotranspiration Index (SPEI) accounts for both precipitation (P) and reference evapotranspiration (ETr) to describe the wetness or dryness of a time period. During warmer times of the year, SPEI is advantageous to SPI since it accounts for atmospheric demands on moisture as well as precipitation inputs.
Reference Evapotranspiration - The Evaporative Demand Drought Index (EDDI) is similar to SPI and SPEI in its formulation; however, EDDI only accounts for reference evapotranspiration (ETr). EDDI describes anomalies in ETr over a timescale of interest with respect to a historical reference period.
Temperature percentiles describe the average daily maximum (or minimum) temperature experienced relative to what is expected over a timescale of interest. Percentiles are useful as they provide a historical context of any particular anomaly with respect to how it compares to observed variability.
Snow Water Equivalent (typically SNOTEL or SNODAS) - Snow water equivalent (SWE) is an indicator of liquid water stored as snow. SWE anomalies describe the observed SWE measured (or estimated) at a site relative to what is expected for a day or season of interest.
SNODAS and a digital elevation model – The standardized SWE index is an indicator of snow water equivalent (SWE) anomalies across Montana. It is helpful for understanding the spatial distribution of SWE and for identifying locations with lesser than average snowpack given the time of year. Hypsome-SWE estimates the cumulative SWE that occurs across elevation bands within Montana’s HUC 8 watersheds and calculates a percentage of normal. This analysis is useful for understanding high- vs. low-elevation snow water accumulation throughout the snow season.


Streamflow is an indicator of hydrological drought. Daily, weekly, and monthly streamflow percent of averages and percentiles are tracked as indicators of current hydrological conditions. Streamflow percentiles represent a ranking of current streamflow as compared with historical flow readings. Percent of average streamflow represents the current flow compared to the historical average for the day or period of interest.
Water Table Depth - Groundwater is an indicator of longer timescale hydrological drought. Groundwater table height percentiles represent water table deviation from normal and can be computed over various timescales. This metric is important to understand the availability of stored subsurface water but must be used in conjunction with storage characteristics of the aquifer.

Soil Moisture

Soil moisture percentiles or anomalies describe the amount of soil moisture in the soil reservoir relative to what is expected for period of interest.


The Normalized Difference Vegetation Index (NDVI) - The NDVI is a commonly used indicator of ecosystem photosynthesis and provides an indication of vegetation status. NDVI greenness anomalies are useful indicators of vegetation stress (or vigor) and can be calculated over various timescales, similar to other drought metrics.
The Vegetative Health Index (VHI) - The VHI is a proxy that characterizes vegetation health or a combined estimation of moisture and thermal conditions. Vegetation health is often used to estimate crop condition and anticipated yield. If the indices are below 40 (unfavorable), vegetation stress, losses of crop and pasture production might be expected; if the indices are above 60 (favorable conditions), plentiful production might be expected.