Groundwater Storage Variability in Headwater Catchments of the Upper Colorado River Basin: Improving Our Understanding of Why Runoff in Mountain Environments is Highly Variable

Snowmelt-dominated headwater streams in the upper Colorado River Basin(CRB) contribute the majority of the total flow in the Colorado River. Year-to-year variability in runoff efficiency can be greater than 50%, however the magnitude and direction of this variability is difficult to predict. Anomalously low or high flows compared to expected values based on amount of precipitation create challenges for millions of downstream water users, especially under the current drought conditions and rising temperatures. Although runoff efficiency generally is higher during wetter and colder years, precipitation and temperature together explain only 20% of interannual variability. Building on recent work in Northern Utah that demonstrates groundwater storage 1) exhibits coherent regional variability and 2) is a major control on runoff efficiency, we quantify spatial and temporal patterns in groundwater storage variability and evaluate the role of storage in controlling runoff efficiency in the headwaters of the Colorado River. Our work includes 65 guaged headwater catchments, with periods of recorded streamflow longer than 20 years in the upper CRB including: HUC: 1401 (Upper Colorado), HUC: 1402 (Upper Gunnison), HUC: 1404 (Upper Green), HUC: 1405(Upper Yampa) and HUC 1408 (San Juan). We use precipitation data from PRISM, aggregated using the GAUGES II database. We calculate temporal and spatial variability in groundwater storage using baseflow, calculated using the monthly mean lowest flow (and lowest variability) period from each catchment. Specifically, we ask: in Upper Colorado headwater catchments, is there periodicity in groundwater storage (inferred from baseflow), and does groundwater storage influence runoff efficiency? Initial results suggest that baseflow varies at a surprising 4 and 15-year periodicity (1958-2018), suggesting a multi-year carryover of groundwater storage in headwater catchments, which may be controlled by regional climate forcings. Further analysis of how periodicity in groundwater storage may amplify or reduce runoff efficiency could reduce uncertainty in runoff predictions. Additionally, connecting periodicity in baseflow to regional climate patterns could improve our understanding of upper Colorado River Basin response to global atmospheric climate patterns.