Temporal Interpolation of the Standardized Precipitation Index between Accumulation Intervals and Ending Dates for Computational Efficiency in the Dynamic Drought Index Tool

The feasibility of interpolating gamma distribution parameters between different precipitation accumulation intervals (durations) is statistically evaluated. Similarly the interpolation of these parameters for a specific accumulation interval, but ending on different dates is also assessed. Such interpolation facilitates the computation of Standardized Precipitation Index (SPI) in drought monitoring tools that require the index be computed for any user specified accumulation period on any given day. For instance, through interpolation SPI can be computed for a 135-day precipitation accumulation ending on July 15 based on stored gamma distribution parameters for 90- and 180-day precipitation accumulations ending on June 30 and July 31. This increases the computational efficiency of such tools. Spatial interpolation of the distribution parameters is also assessed.