Frequency and intensity of extreme drought in the Delaware Basin, 1600–2002

Abstract

The frequency and severity of drought in the Delaware Basin between 1600 and 2002 are examined using the Palmer Drought Severity Index (PDSI) estimated from tree ring data and correlated with reconstructed annual low flows. In the Delaware Basin, the most severe drought in nearly a century occurred during 1995–2002 as the Brandywine River, Delaware's largest surface water supply, ran dry at its mouth and declined to the lowest flow on record since 1912. To evaluate the long‐term context of the 1995–2002 droughts given a variable hydroclimate, tree ring and PDSI data were correlated to reconstruct flows along the river to 1600, the beginning of European exploration to the Delaware Bay. Reconstructed PDSI and low flows were fit using general extreme value (GEV) distributions to estimate drought frequency. Some variability is present as reconstructed low flows tend to overestimate recorded streamflow in severe dry years, a finding reported by others. Some uncertainty appears in the correlations as the coefficient of multiple determination (CRSQ) between recorded and estimated PDSI from tree ring data is 0.50–0.54, a level of variance considered to be “quite good,” and the coefficient of determination (r2) between PDSI and low flow is 0.52. Given the uncertainty, PDSI and reconstructed low flow data both agree that the most extreme drought in 400 years occurred during 1635, and the drought of 1995–2000 was historically extreme with differences only in the degree of severity. On the basis of PDSI, the 2002, 1999, and 1995 droughts were the sixth, twelfth, and seventeenth most severe in 400 years with frequencies of once every 50, 33, and 16 years, respectively. Based on low flow, the 2002, 1999, and 1995 droughts were the second, fourth, and ninth most severe since 1600 with frequencies of once every 200, 100, and 50 years, respectively. The record drought of 2002 has a low probability of reoccurring in any given year (2.0% by PDSI and 0.5% by low flow), but droughts nearly as severe have occurred during the 1630s, 1680s, 1820s, 1840s, 1860s, 1930s, 1940s, and 1960s. Increased intensities of drought low flows in Delaware during the late twentieth century through 2002 were coincident with population growth, watershed urbanization, and atmospheric warming although these associations were not correlated and further study is needed. Over 400 years of tree ring, PDSI, and reconstructed streamflow data indicate that the Delaware Basin record drought of 1995–2002 was a historically severe event with important implications for water supply and drought management. Droughts more severe than the record 2002 event have occurred in the past, and droughts may become even more intense should watershed urbanization and atmospheric warming continue in the future.

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