Finding the Best Indices of Drought in the Northwest

By CIRCulator Editorial Staff

Reprinted from: The Climate CIRCulator

SOMETIMES A SIMPLE APPROACH works pretty well. That’s what CIRC PI John Abatzoglou at University of Idaho and his colleagues found when they held a scoring contest to gauge the streamflow estimating power of four drought indices in the Pacific Northwest — something that, of course, can be done at much more expense and time by running a sophisticated hydrologic model. The four indices they chose were: (1) standardized precipitation index; (2) standardized precipitation-evapotranspiration index; (3) Palmer Drought Severity Index; and (4) a water-balance runoff model.

The indices were tested on 21 unregulated Pacific Northwest river basins to determine which ones best represented year-to-year streamflow variability. The indices — all of which used weather data to produce a single number for each year — compared year-to-year variations in drought estimates with streamflow in basins ranging from warm to very cold. Scores were based on the percentage of year-to-year variability in October through September (“water year”) streamflow that could be explained by the contestant.

In most basins, precipitation-evapotranspiration and water-balance runoff were the best approaches, with the precipitation index coming in a distant third. The Palmer Drought Severity Index was nowhere near the frontrunners. Its poor showing, the researchers surmised, could be its failure to distinguish between rainfall and snowfall, and because it is designed for estimating soil moisture, not runoff. The water-balance index did best in mild coastal basins, where it often scored better than 90 percent. The toughest basin to model was Middle Fork Rock Creek in Montana, where the highest score (by the precipitation-evapotranspiration index) was 56 percent.

The Abatzoglou team concluded, among other things, that even these simple approaches depend on having enough measurements of hydrologic variables in the mountains. Operational forecasting of streamflow is much more sophisticated but also relies on mountain observations.

Abatzoglou, John T., Renaud Barbero, Jacob W. Wolf, and Zachary A. Holden (2014) Tracking Interannual Streamflow Variability with Drought Indices in the U.S. Pacific Northwest, J. Hydrometeor, 15, 1900–1912. Online Access

A version of this story originally appeared in the October 2014 issue of The Climate CIRCulator.