Periodic Variability in Baseflow in Headwater Streams of the Upper Colorado River: Implications for Runoff Efficiency

ABSTRACT The majority of the discharge in the Colorado River originates as snowmelt in headwater catchments. Flow from these streams exhibits significant year‐to‐year variability (Coefficient of Variation: 0.2–1.18), challenging the management of a critical water and energy resource for millions. Understanding the factors driving this variability will become even more important in a rapidly changing climate. To address this need, we examine regional patterns in winter baseflow, a metric for catchment groundwater storage, in 52 USGS‐gauged watersheds in the Upper Colorado River Basin. Coherent 12‐ to 14‐year patterns emerge in baseflow in 5 headwater regions, alongside an apparent 8‐year periodicity in 2 regions, suggesting groundwater recharge is responding to a regional climate forcing. Subsequent analyses identified a statistically significant, positive relationship between antecedent winter baseflow and runoff efficiency (RE) in 22 of 24 headwaters with concurrent precipitation and streamflow data available since 1980. These relationships indicate that winter baseflow can be used to reduce uncertainty in RE and streamflow predictions months before snowmelt begins.