Potential effects of climate change on the Columbia River Basin: Hydrology and water resources Dennis P. Lettenmaier Department of Civil and Environmental Engineering and Climate Impacts Group University of Washington UW School of Communications/Knight Foundation Seminar for Journalists June 28 2002 JISAO Climate Impacts Group Joint Institute for the Study of Atmosphere and Oceans
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Potential effects of climate change on the Columbia River Basin: Hydrology and water resources Dennis P. Lettenmaier Department of Civil and Environmental.
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Potential effects of climate change on the Columbia River Basin: Hydrology and
water resources
Dennis P. LettenmaierDepartment of Civil and Environmental Engineering
and Climate Impacts Group
University of Washington
UW School of Communications/Knight Foundation
Seminar for JournalistsJune 28 2002
JISAO Climate Impacts GroupJoint Institute for the Study of Atmosphere and Oceans
A regional perspective on a global problem
What is known? On a global basis, temperature has been increasing,
appears to be well out of the range of natural variability
Picture is less clear for hydrology Modeling is the basis for future projections
Models essentially all agree on direction of temperature change and regional coherence, less so on precipitation
Some consistency in direction of model predictions for precipitation change over land at intermediate to high latitudes
Source: IPCC 2001
Source: IPCC 2001
What is a climate model?
• Abstraction of the true climate (“average weather”) system, based on the laws of physics
• Typically a set of equations, solved numerically, over a (global) grid, for “climate time scales” (decades to centuries)
• In practice, so-called General Circulation Models (GCMs) are close cousins of numerical weather prediction models
Source: NRC 1975
Source: IPCC 2001
Source: IPCC 2001
Source: IPCC 2001
Interpreting the hydrologic implications
• Scale mismatch – the critical roadblock• Interpreting hydrologic consequences
requires a river basin perspective, GCM scale of degrees lat-long doesn’t resolve any but the very largest rivers
• Topographic effects on precipitation and temperature at poorly resolved by global GCMs
Topography as a constraint (or why the situation is not hopeless)• In the PNW (and most of the West) streamflow originates
predominantly as winter snowfall (> 70% westwide)• Topography exerts a strong control on partitioning of
precipitation into rain vs snow (~ 6oC/1000 m mean lapse rate), and amount of precipitation (orographic enhancement)
• Hence hydrologic implications of warming can be extracted from large scale information about warming, as can some information about spatial distribution of precipitation (changes will be most important over mountainous source areas, especially in interior of west where lowlands tend to be arid or semi-arid)
Climate Scenarios
Transient GCM Simulations for Increasing CO2 and Aerosols