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Jeffrey Cardille, Michael T. Coe, and Julie A. Vano

Abstract

Lakes are a major geologic feature in humid regions, and multiple lake hydrologic types exist with varying physical and chemical characteristics, connections among lakes, and relationships to the landscape. The authors developed a model of water fluxes through major components of groundwater-dominated lake catchments in a region containing thousands of lakes, the Northern Highland Lake District (NHLD) of northern Wisconsin and the Upper Peninsula of Michigan. The model was calibrated with data from widely differing lakes using the same set of simple equations to represent the hydrologic type, water residence time, and amount and timing of stream and groundwater flows of representative lakes in today's climate. The authors investigated the sensitivity of the water balance of a set of three connected representative lakes and their catchments to systematic increases and decreases in the precipitation regime, and contrasted results using lake-specific morphometry to those for a lake having size and shape parameters typical of the region. Results indicate that a common set of equations can successfully represent major water balance characteristics of the three basic lake hydrologic types (hydraulically mounded, groundwater flowthrough, and drainage) in the NHLD. Sensitivity of modeled lakes varied by lake type, with drainage lakes more strongly buffered against substantial hydrologic changes in extreme climate scenarios. Catchment-scale water budgets differed substantially among lakes of different types, yet can be understood along a continuum of relative catchment size. These results suggest that a simple model of lake and catchment water balance can be extended to entire lake districts, where the detailed morphometry of most lakes is not well known.

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Julie A. Vano, Tapash Das, and Dennis P. Lettenmaier

Abstract

The Colorado River is the primary water source for much of the rapidly growing southwestern United States. Recent studies have projected reductions in Colorado River flows from less than 10% to almost 50% by midcentury because of climate change—a range that has clouded potential management responses. These differences in projections are attributable to variations in climate model projections but also to differing land surface model (LSM) sensitivities. This second contribution to uncertainty—specifically, variations in LSM runoff change with respect to precipitation (elasticities) and temperature (sensitivities)—are evaluated here through comparisons of multidecadal simulations from five commonly used LSMs (Catchment, Community Land Model, Noah, Sacramento Soil Moisture Accounting model, and Variable Infiltration Capacity model) all applied over the Colorado River basin at ⅛° latitude by longitude spatial resolution. The annual elasticity of modeled runoff (fractional change in annual runoff divided by fractional change in annual precipitation) at Lees Ferry ranges from two to six for the different LSMs. Elasticities generally are higher in lower precipitation and/or runoff regimes; hence, the highest values are for models biased low in runoff production, and the range of elasticities is reduced to two to three when adjusted to current runoff climatology. Annual temperature sensitivities (percent change in annual runoff per degree change in annual temperature) range from declines of 2% to as much as 9% per degree Celsius increase at Lees Ferry. For some LSMs, small areas, primarily at midelevation, have increasing runoff with increasing temperature; however, on a spatial basis, most sensitivities are negative.

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Julie A. Vano, Kathleen Miller, Michael D. Dettinger, Rob Cifelli, David Curtis, Alexis Dufour, J. Rolf Olsen, and Anna M. Wilson
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Julie A. Vano, Bradley Udall, Daniel R. Cayan, Jonathan T. Overpeck, Levi D. Brekke, Tapash Das, Holly C. Hartmann, Hugo G. Hidalgo, Martin Hoerling, Gregory J. McCabe, Kiyomi Morino, Robert S. Webb, Kevin Werner, and Dennis P. Lettenmaier

The Colorado River is the primary water source for more than 30 million people in the United States and Mexico. Recent studies that project streamf low changes in the Colorado River all project annual declines, but the magnitude of the projected decreases range from less than 10% to 45% by the mid-twenty-first century. To understand these differences, we address the questions the management community has raised: Why is there such a wide range of projections of impacts of future climate change on Colorado River streamflow, and how should this uncertainty be interpreted? We identify four major sources of disparities among studies that arise from both methodological and model differences. In order of importance, these are differences in 1) the global climate models (GCMs) and emission scenarios used; 2) the ability of land surface and atmospheric models to simulate properly the high-elevation runoff source areas; 3) the sensitivities of land surface hydrology models to precipitation and temperature changes; and 4) the methods used to statistically downscale GCM scenarios. In accounting for these differences, there is substantial evidence across studies that future Colorado River streamflow will be reduced under the current trajectories of anthropogenic greenhouse gas emissions because of a combination of strong temperature-induced runoff curtailment and reduced annual precipitation. Reconstructions of preinstrumental streamflows provide additional insights; the greatest risk to Colorado River streamf lows is a multidecadal drought, like that observed in paleoreconstructions, exacerbated by a steady reduction in flows due to climate change. This could result in decades of sustained streamflows much lower than have been observed in the ~100 years of instrumental record.

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