Potential Impacts of Multi-Decadal Persistence in Drought Planning

Recent examinations of long-term annual precipitation records and reconstructed streamflows from tree ring data in California suggest that the regional climate consistently drifts from periods of relative wet to periods of relative dry. This behavior is a recurring feature in California’s climate for, at least, the past 1000 years. The data suggest that changes in the multi-decadal average precipitation in the range of 25-50% are routine. Furthermore, the transitions between multi-decadal extreme wet and extreme dry regimes can occur quite quickly, over three to four decades in some cases.

Most of the directly observed precipitation records only cover the past 50-75 years or so. Since the last major hydro-climate minima in the 1930’s, average annual precipitation appears to have gradually increased by more than 25% during the period. The data suggest that by the early 2000’s, California had reached a hydro-climate maxima that appears to be in the top five such periods over the past millennium.

Despite being in a relatively wet era by historical standards, California has experienced four years of drought. The impacts of the current drought are exacerbated by higher temperatures and by population increases. Higher temperatures amplify evapotranspiration demand and irrigation needs.. California’s population was about 6 million during the last hydro-climate minima during the 1930’s. Today, approximately 38 million people along with vastly expanded agricultural and industrial activity place further stress on the state’s water management infrastructure.

The underlying upward bias in the observed precipitation record upon which current water management and drought planning rely on has masked some of the increased stress on California’s water infrastructure. This paper presents an assessment of current drought contingency planning for reservoir operation and provides a framework for improvement given our new understanding of the magnitude of historical hydro-climate variability.