A modified Standardized Precipitation Index for drought monitoring

The SPI Blend is a modified Standardized Precipitation Index (SPI) created for use in a newly developed, high-resolution drought monitoring tool, assessing drought using precipitation data on multiple time scales. Unlike the traditional SPI, the SPI Blend uses a linear weighting system that places a higher importance on recent precipitation within a time period. At each time scale, the MPE precipitation data are divided into several periods and the precipitation total from the most recent period is given the highest weight, with a linear decrease in the weights for succeeding periods. The high-resolution (4 km) precipitation data are obtained daily from the Advanced Hydrologic Prediction Service multi-sensor precipitation estimates (MPE) and are aggregated on several time scales. Cooperative Observer Program (COOP) daily station precipitation data provide the historical context for the MPE precipitation data. Pearson Type III distribution parameters were interpolated to the 4 km grid based on a regional frequency analysis of the COOP stations and L-moment ratios of the precipitation data. For a given time scale and grid point, the SPI Blend is a summation of the MPE weighted precipitation values placed on a historical cumulative distribution function derived from COOP precipitation data. The SPI Blend can be used as guidance for the United States Drought Monitor and provides an improvement over the traditional SPI because it places a greater importance on more recent precipitation.