Forecasts and Planning Tools

Climate Change Streamflow Scenario Tool

Methods

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Climate Change Scenarios Selection and Linkage to Hydrologic Models

The Climate Impacts Group developed the climate change streamflow scenarios tool using five different representations of future Pacific Northwest (PNW) climate. All five climate change scenarios for the PNW were simulated by four global coupled atmosphere-ocean climate models:

The fifth scenario is a composite climate change scenario that averages the results of the four individual model scenarios (see Figure 1). Climate change data are available here.

These models were selected primarily because of their higher spatial resolution for the atmospheric component and their use of relatively sophisticated land surface schemes. All simulations incorporated increased climate forcing by carbon dioxide ("IS92a" emissions scenario (IPCC 1996) or ~1% per year) and anthropogenic sulfate aerosols (IS92a).

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Figure 1 Decadal average changes in PNW temperature (top) and precipitation (bottom) projected by four global climate model simulations for the decades of the 2020s (left) and the 2040s (right). Temperature changes are shown as absolute increases relative to present climate; precipitation changes are shown as fractional changes. The red line indicates the monthly mean of the four GCM perturbations, that is, the composite climate change scenario.

To construct quantitative hydrologic scenarios, we use a simple downscaling technique frequently referred to as the "delta" method. Because it adjusts the observed (historical) climate record by the monthly shifts in mean climate projected for the future, this technique retains the fundamental temporal and spatial variability of observed regional climate. This modified climate record is then used as the input to a hydrologic model.

Climate Change Streamflow Scenarios

Monthly streamflows for the historic climate (1950-1989) and for the projected changes in monthly average temperature and precipitation (1950-1989 modified by the climate changes projected for the 2020s and 2040s) were simulated using the Variable Infiltration Capacity (VIC) hydrologic model implemented over the Columbia River basin and coastal areas in the PNW at 1/8 degree spatial resolution (see technical documentation).

Bias Correction

Hydrologic simulations invariably show some bias compared to observations. In planning studies, where model results of system performance derived from the historic record of streamflows are directly compared to results derived from hydrologic simulations, errors in the mean for particular months or significant errors in particular parts of the time series can be problematic.

In order to minimize these problems, we remove the bias from the simulated (“raw”) streamflow time series, using a probability-mapping technique (see technical documentation). This technique removes systematic bias and random simulation errors in the monthly time step hydrologic simulations by mapping between a simulated and observed probability distribution for each calendar month. The procedure involves careful treatment of values outside the range of historical simulations and insures that mass balance in the channel system (and across seasons) is preserved. The resulting climate change streamflow scenarios are numerically consistent with, and, for purposes of evaluating system performance, directly comparable to the observed streamflow time series.

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