Synoptic climatology of the early 21st century drought in the Colorado River Basin and relationships to reservoir water levels

Abstract

ABSTRACTSince the year 2000, streamflows throughout the Colorado River Basin (CRB) in the southwestern United States have been greatly impacted by a prolonged drought. As a primarily mountain snowmelt‐sourced basin, the climatic drivers of annual runoff in the CRB bear significant influence on the hydrologic status of its network of reservoirs. To analyse reservoir response to the drought, this study takes a holistic approach of relating the frequency of mid‐tropospheric circulation patterns (CPs) to water levels at Lake Powell and Lake Mead, the primary reservoirs in the CRB. A synoptic climatology of the region was constructed by classifying daily mean 500‐hPa geopotential height values from 1982 to 2012, a span that includes the early 21st century drought and the relatively wetter years preceding it, to determine how the drought manifests itself in the trends of CPs over time. The CPs were then statistically compared to several measures of reservoir water level, including yearly inflow and net changes in storage year‐to‐year, to examine potential relationships. The results of the classification reveal that the CRB is most notably characterized by dominant summertime ridge patterns, which have become slightly more common during the drought. Cool season troughs were generally linked with the presence of more water in the reservoirs, but ridges have stronger relationships to reservoir variables, including linkages to years with lower reservoir inflow, greater yearly losses, and net declines in storage year‐to‐year. Further, dry transitional patterns, CPs which represent zonal flow yielding little precipitation, are increasingly common in the spring, coming at the expense of wetter CPs. These results support previous research regarding the observed hastening of snowmelt, contributing to the early 21st century drought in the CRB as well as the overall trend in declining mountain snowpack across western North America.