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Supporting Information
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for
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Seasonal asymmetry and the importance of warm
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season warming in driving streamflow changes in the
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western U.S.
Tapash Das1,4, David W. Pierce1, Daniel R. Cayan1,2, Julie A. Vano3, Dennis P.
Lettenmaier3
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Division of Climate, Atmospheric Sciences, and Physical Oceanography, Scripps
Institution of Oceanography, University of California San Diego, La Jolla, California,
USA.
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United States Geological Survey, La Jolla, California, U.S.A.
Department of Civil Engineering, University of Washington, Seattle,
Washington, U.S.A.
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Current address CH2MHILL, San Diego
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28 November 2011
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Version 3
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S1. River basins analyzed
The study area consists of the four regionally important river basins shown in Fig.
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S1. The Colorado River basin consists of the drainage area up to Colorado at Lees Ferry
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(Arizona). The Columbia River basin considered here consists of the drainage area up to
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Columbia at The Dalles (Oregon). The northern Sierra Nevada consists of the drainage
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areas upstream from gauges at the Sacramento River at Bend Bridge, the Feather River at
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Oroville, and the Yuba River at Smartville. The southern Sierra Nevada consists of the
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drainage areas from the four main tributaries of the San Joaquin river: the Stanislaus at
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New Melones Dam, the Tuolumne River at New Don Pedro, the Merced at Lake
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McClure and the San Joaquin at Millerton Lake. Geographical and climatic conditions of
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the basins are shown in Table S1.
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The Colorado River basin receives almost half of its precipitation in the warm
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season, the highest warm season precipitation fraction of each of the four basins, as
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shown in Fig. S2.
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The VIC simulated flow in the basins compares well with that from observations,
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reproducing the strong seasonality observed in each basin, shown in Fig. S3. Also, the
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VIC annual runoff efficiency, the ratio of aggregated annual runoff to aggregated annual
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precipitation in each basin is very close to that from observations (Fig S4). The Colorado
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River basin converts less than 20% of precipitation into runoff, compared about 40% in
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the Columbia basin and about50% in the Sierra Nevada basins. Thus, in the Colorado
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River basin about 80% of total precipitation is lost via evapotranspiration, compared to
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50% or more in the Columbia and Sierra Nevada, shown by the VIC evaporation
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efficiency in Fig. S4.
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S2. Meteorological forcing
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Daily gridded meteorological observations of precipitation, maximum daily
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temperature (Tmax), minimum temperature (Tmin), and wind speed at 1/8 degree latitude
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by longitude spatial resolution across the western U.S. were obtained from the Surface
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Water Modeling Group at the University of Washington
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[http://www.hydro.washington.edu; see also Hamlet and Lettenmaier, 2005] and cover
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the period 1915-2003. The data are based on the National Climatic Data Center’s co-
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operative network of weather observations stations, augmented by information from the
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higher quality Global Historical Climatology Network (GHCN) stations. The dataset also
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incorporates information from the monthly PRISM data fields [Daly et al., 1994] to
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adjust for elevation effects on precipitation and temperature. Such corrections are
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necessary because topography in the study region affects not only spatial patterns of
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precipitation (and temperature) but also basin-scale totals of precipitation.
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S3. Hydrological model configuration
The VIC model is a physically based, semi-distributed model that parameterizes
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the land surface heterogeneities (in topography, soils, and vegetation) that control the
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generation of runoff. It includes an energy balance snow accumulation and ablation
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model [Andreadis et al., 2009], which represents the interactions of snow and vegetation.
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VIC is configured with three soil layers and 5 elevation bands that are used to account for
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the effects of sub-gridcell topography. VIC was run with a daily time step in water-
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balance mode, meaning that for purposes of energy computations, the effective surface
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temperature is assumed to equal the surface air temperature.
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SI References
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Andreadis, K, Storck, P, and Lettenmaier D.P. (2009) Modeling snow accumulation and
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ablation processes in forested environments. Water Resourc. Res. 45, W05429,
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doi:10.1029/2008WR007042.
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Daly, C., Neilson, R.P., and Phillips D.L. (1994) A statistical-topographic model for
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mapping climatological precipitation over mountainous terrain. Journal of
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Applied Meteorology 33: 140-158.
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Hamlet, A. F., and Lettenmaier D. P. (2005) Production of temporally consistent gridded
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precipitation and temperature fields for the continental U.S., J. Hydrometeorol., 6,
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330–336, doi:10.1175/JHM420.1.
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SI Tables
Mean
elevation (m)
Drainage
area (km2)
Mean
Mean
annual T (C)
annual P (cm)
Colorado
2181
283,000
5.3
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Columbia
1398
615,000
4.1
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North Sierra
1362
45,800
8.1
90
South Sierra
1843
15,700
6.5
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Table S1. Geographical and climatic mean conditions of the basins studied.
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Fig. S1 The study area consists of four major river basins in the western U.S.: a)
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Upper Colorado River measured at Lees Ferry (blue); b) Columbia River measured at
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The Dalles (white); c) California's Northern Sierra Nevada (purple) measured as the sum
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of flows of the Sacramento River at Bend Bridge, the Feather River at Oroville, and the
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Yuba River at Smartville; d) California's Southern Sierra Nevada (red) measured as the
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sum of flows of the Stanislaus River at New Melones Dam, Merced River at Lake
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McClure, Tuolumne River at New Don Pedro, and San Joaquin River at Millerton Lake.
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Colors and contours show elevation (m).
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Fig.S2. Cold season (October through March) and warm season (April through
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September) precipitation as a percentage of total water year (October through September)
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precipitation, for our four river basins in the western U.S.
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Fig.S3. Red: VIC simulated monthly streamflows as driven by observed
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meteorology from Hamlet and Lettenmaier (2005). Black: Observed (naturalized)
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streamflows.
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Fig. S4. Runoff efficiency, R/P (defined as the ratio of area-averaged VIC
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simulated streamflows to observed precipitation) and evaporative efficiency, AET/P
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(defined as the ratio of VIC simulated actual evapotranspiration to observed
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precipitation) for our four river basins in the western U.S. Solid black color circles show
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the runoff efficiency computed from observed naturalized streamflows and precipitation.
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The values are for water year 1916-2002.
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