How the MBDI Compares
Funder & Affiliates
Comparison to other indices
The drought index historically used by climatologists – the Palmer Drought Severity Index (PDSI) – can be challenging to use because it’s unclear what time frame it represents. Based upon a soil moisture model, it requires knowledge of local soil characteristics.
The PDSI’s interpretation varies by region. Unlike other indices, the PDSI’s scale is not grounded in a statistical distribution that lends itself to a straightforward interpretation. The PDSI was developed in the Midwest, and thus it is calibrated for climate conditions quite different than those of the mountainous West.
In contrast, the MBDI was developed based on conditions in the Colorado River Basin. At the same time, the approach used by the MBDI – to normalize results based on local conditions – makes it applicable across the nation and beyond. Another advantage is MBDI values are available at a smaller scale than the climate-division-sized PDSI.
The Standardized Precipitation Index (SPI) grew in popularity as climatogists grew frustrated with the challenges of using the Palmer index. Like the MBDI, the SPI uses a rating system that ranks individual years or months in comparison to local norms for that same time frame. This makes it straightforward to interpret and compare among regions.
The SPI, however, does not consider evaporation. It focuses only on the supply side of the equation – precipitation – without accounting for the demand from evaporation. The MBDI, in contrast, includes an estimate for evaporation.
Arizona Drought Monitor
The Arizona Drought Monitor considers the role of evaporation, in effect, by including input on streamflow levels in addition to precipitation when considering drought impacts on the landscape.
However, the Arizona Drought Monitor limits consideration of drought to two categories, “short-term” and “long-term.” The MBDI offers more options for considering drought, with seven options ranging from one month to four years.
The MBDI also allows for consideration of finer spatial scales. While the Drought Monitor reports values at the watershed level, the MBDI can report them down to the resolution of the input data – roughly 4,000 acres – in addition to the watershed level. The MBDI does this by using a sophisticated dataset known as PRISM to interpolate climatic conditions between weather stations. This can be important in topographically varied regions such as the West.