“What is the current understanding of the changing streamflow in the Upper Colorado River Basin (UCRB) and the drivers of this change?” This question was the focus of a 2-day workshop hosted by the Physical Sciences Division of the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory, which brought together experts in global and regional climate modeling, hydrologic modeling and theory, and observational analysis. The timing of this workshop is opportune. Water year 2018 saw inflow into Lake Powell, a key indicator of the hydrology of the basin, ranking among the lowest in the historical record. This year comes on the heels of 18 years of cumulative low flows that have stressed an already overallocated system. Upcoming negotiations on water allocations during periods of shortage provide a further impetus. Therefore, it is critical to determine how much anthropogenic warming has already depleted flows, that is, the extent to which the recent low flows are “the new normal.”
The workshop began with a discussion of recent observational analyses that indicate a strong response of the basin to warming. This was followed by a more in depth discussion of the separate roles of temperature, precipitation, and other factors in driving streamflow trends based largely on modeling and theory. Although there was general agreement that the UCRB has already warmed by more than 1°C and that this warming goes hand in hand with reduced streamflow, the experts at the meeting collectively expressed uncertainty about the magnitude of this effect (Figure 1). Three broad areas were discussed that may account for differences among the approaches: model uncertainty, particularly in gridded meteorological data sets; sampling uncertainty due to having only one realization of the weather; and methodological uncertainty both within and between modeling approaches.
A major goal of the workshop was to motivate the research community to undertake new observational analyses and model experiments to better understand these differences with the hope of reducing the uncertainty. In the breakout sessions on the second day, attendees identified several potential research opportunities, including better characterization of uncertainty in the observational record and its effect on the attribution of historic changes in flow to temperature and precipitation trends; reconciling observational and model-based estimates of sensitivity; developing new metrics of model performance; conducting further inquiry into the roles of seasonality, timescales of variability, and spatial scale (e.g., horizontal resolution of models); and integrating the effects of nonmeteorological drivers of change into attribution of the causes of historical streamflow change, including dust on snow, tree mortality, and surface water–groundwater interaction.
A full report on the outcomes of the workshop is available from the conference website.
—Joseph J. Barsugli ([email protected]), Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado Boulder; also at Physical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration (NOAA), Boulder, Colo.; Martin P. Hoerling, Physical Sciences Division, Earth System Research Laboratory, NOAA, Boulder, Colo.; and Ben Livneh, CIRES, University of Colorado Boulder; also at Department of Civil, Environmental and Architectural Engineering, University of Colorado Boulder