The floating month drought index—a new drought monitoring tool

Drought indices evolved during the 20th century from simple precipitation-based indices to more complex water balance models which assessed the total moisture status. At the end of the century, the U.S. Drought Monitor (USDM) was developed to incorporate the various existing drought indicators, drought impacts information, and input from local field experts. A set of objective blends was created to integrate appropriately-scaled indices which assessed short-term and long-term moisture conditions. However, even with this comprehensive suite of drought monitoring tools, drought assessment still is sometimes difficult. The new Floating Month Drought Index (FMDI) initiates a drought based on unusually dry conditions (≤ 30th percentile) over 3 consecutive months. The dry spell ends when the total precipitation for the spell period no longer falls below the dry threshold, or the precipitation for the past 3 months was unusually wet (≥ 70th percentile). The FMDI computes the precipitation percentile for the current month and current N-month dry spell, the length and starting year/month of the current dry spell, and the Dx dry spell category based on USDM categories (and similar statistics for wet spells). The FMDI thus provides an objective decision-support tool for integrating the multiple time scales of drought.