Is Hot Drought a Risk in the <scp>US</scp> Mid‐Atlantic? A Potomac Basin Case Study

ABSTRACT Interannual variability of streamflow will increase under a future climate, but at the regional scale, there is uncertainty regarding changes in drought severity, and in particular, changes in extreme hydrological drought that could necessitate new water supply infrastructure. This is due to the wide range of regional projections for precipitation and the challenge of estimating statistics in a nonstationary climate. We assess changes in annual streamflow in the Potomac River Basin using a nonparametric approach based on a climate response function and the K‐nearest neighbor method, which is relied on to construct time series of sufficient length to compute extreme quantile values. Our results indicate that future Potomac River flows will be impacted by “hot drought”, that is, increasing drought severity caused by rising temperatures coupled with natural variability in precipitation. Average precipitation is projected to increase in the Potomac basin by 9%–12% in the period 2039–2069 and by 11%–16% by 2070–2099. Average streamflow increases more modestly, by 4%–7% in 2039–2069 and by 2 to 9% in 2070–2099, whereas annual flows in an extreme drought year decrease by 3 to 26% in 2039–2069 and by 2%–49% in 2070–2099, assuming a medium sensitivity of flow to temperature. Our approach can provide multi‐model consensus inputs for water supply planning models to support decision‐making regarding new infrastructure.