How the MBDI Compares
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Multiple time scales
Drought occurs at a variety of time scales, from short-term periods that wilt plants to long-term periods that drain reservoirs. The complexity of drought creates the need for multiple ways to interpret its occurrence or future likelihood for management purposes.
The Moisture Balance Drought Index (MBDI) offers users an easy way to examine drought at a variety of scales, from one month to four years.
For example, a one-month comparison for June would rank the MBDI values for that month alone (as in the example under “Rank of drought severity”). Meanwhile, a three-month value for June would compare the April-June values for MBDI, creating rankings for based on every April-June in the record since 1895.
A time series plot of MBDI rankings will compare each subsequent month the same way. So the three month value for June (April-June) will be followed by the three month value for July (May-July). Similarly, the 12-month value for June (July-June) will be followed by the 12-month value for July (August-July).
Because longer term values will use many of the same months in the comparison, the graph will show less variability at the 12-month scale than at the 1-month scale, as the graphic below illustrates.
Different time scales yield different results, as described in the text.
Credit: Graphic design by Matthew Pace
These two examples, using the town of Payson for illustration, show a decade-long time series of MBDI at different scales. At the one-month time scale (top), the index captures monthly fluctuations, with peaks and dips showing brief excursions (relative to similar months since 1895).
The one-year scale shows the cumulative effect of drought on the landscape across the course of a year, relative to comparable 12-month periods in the record (bottom). Thus values for every September, for example, will show how the previous 12 months ranked on the MBDI scale relative to every previous October-September period in the record back to 1895.