How the MBDI Compares
Funder & Affiliates
About the MBDI
In much of the Colorado River Basin, high evaporation rates combine with relatively low precipitation rates to limit plant growth and river flow – with some years more extreme than others. Evaporation rates often run several times higher than precipitation rates. Precipitation alone, then, cannot fully illustrate the climatic conditions related to drought.
The MBDI considers both precipitation and potential evaporation rates relative to historic conditions for a location. The input data for calculating MBDI is the PRISM data set, which incorporates factors such as elevation, slope, and aspect to estimate precipitation and temperature in mountainous regions.
The Moisture Balance Drought Index takes into account the following factors when evaluating the severity of drought conditions:
- Both supply and demand – i.e., precipitation and evaporation –
influence drought impacts.
- Local conditions are ranked of drought severity based on climatic
conditions for the same locale and time frame.
- Drought operates at multiple time scales, with different impacts
relating to different time scales.
Potential Evapotranspiration (PE) is subtracted from Precipitation (P) to yield the values used to calculate MBDI.
Credit: Graphic design by Jorge Arteaga
The appropriate time scale for use of the MBDI depends on what is being considered. For instance, water resources, especially groundwater and reservoir storage, tend to reflect long-term climatic conditions (e.g., the cumulative impact over 24 or 36 months). On the other hand, vegetation greenness tends to reflect shorter-term conditions of less than a year. Wildfire hazard can reflect conditions both short or long time frames, depending on fuel build-up, a complexity that makes interpretation challenging. Researchers continue to work to identify which time scales best reflect different drought impacts.
The MBDI uses an approach similar to the Standardized Precipitation Index, but brings evaporation into the mix. It is more transparent and flexible compared to some other drought indices, such as the Palmer Drought Severity Index.
The MBDI was developed in a partnership between Arizona State University and the University of Arizona, with funding from the National Oceanic and Atmospheric Administration and the U.S. Bureau of Reclamation.