A new tool for high-resolution drought assessment and monitoring

Although integrated drought assessment was first conducted on the climate division scale ten years ago via the U.S. Drought Monitor, various users soon began applying the Drought Monitor information as though it represented conditions on the county scale. The challenge in this instance was to develop a product that users were not explicitly demanding, but instead were already in a sense using.

High-resolution precipitation analyses were already available through the Advanced Hydrologic Prediction Service (AHPS), produced by National Weather Service (NWS) River Forecast Centers (RFCs) using a combination of gauge data, radar-estimated precipitation, and satellite-estimated precipitation. To convert this high-resolution precipitation information into high-resolution drought index (Standardized Precipitation Index, or SPI) values, we utilize historical observations from the Cooperative Observer (COOP) program and normal precipitation values from the PRISM dataset. We use an L-moments approach to determine the statistical parameters of the accumulated precipitation at COOP sites on a daily basis for a range of accumulation periods from 1 to 48 months. The L-moments are analyzed to the AHPS grid. SPI values are calculated on this grid using AHPS analyses and, optionally, aggregated to the county scale. The indices are validated against SPI values computed directly from COOP precipitation observations.

These high-resolution SPI values are subject to error where radar coverage is poor, such as in mountainous regions, regions of shallow precipitation, or regions with large gaps between adjacent radars. However, in flatter parts of the country with decent radar coverage, the products are reliable and have already been used as input to refine depictions of drought in the U.S. Drought Monitor in Texas, where the initial products were developed.