As cold regions warm, boreal forests become more vulnerable to drought and wildfire. The widely used Palmer Drought Severity Index (PDSI) is poorly suited to assess drought in cold regions because it utilizes a primitive water balance model that does not take snow, glaciers, or frozen ground into account. This study extends the applicability of the Palmer method to cold regions by replacing the primitive water balance with one that represents snow, sublimation, glacier runoff, and the variable permafrost active layer. In addition, the new water balance model can be calibrated to external observations of snow cover, streamflow, glacier mass balance, evapotranspiration, and total water storage. By linking a calibrated cold-region water balance model to the PDSI, characterization of current and future drought in Alaska can be greatly improved. The greatest benefit derives from representing the storage of winter precipitation as snow and the replenishment of soil moisture by snow melt in the spring. Simulation of the seasonal active layer further refines drought estimates in permafrost regions. The cold-region Palmer drought methodology is demonstrated here in the forested valley of the upper Copper River watershed.
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