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Perhaps the most comprehensive set of studies conducted to date was published by Lettenmaier et al. (1999). This study looked at potential climate change impacts on six dimensions of water resource (power generation, municipal water supply, recreation, flood defense, navigation, and environmental flow regulation) in six major U.S. basins, using realistic models of the water system operation and scenarios for possible future nonclimatic changes in demands and objectives (not all the six dimensions were assessed in each basin). Table 4-9 summarizes the results, showing changes in a range of indicators (which varied between basins) by 2050 under three climate change scenarios and a varying number of operational and demand scenarios. The results clearly show considerable variability between scenarios and catchments; they also show that climate change might produce water resources benefits. The results also indicate that, in most sectors and basins, the impacts of different demand and operational assumptions by 2050 are greater than, or of similar magnitude to, the potential impacts of climate change.

Table 4-9: Impact of climate change scenarios by 2050 on various water resource indices in six U.S. basins (Lettenmaier et al., 1999).a
	Savannah	Apalachicola- Chattanooga- Flint (ACF)	Missouri	Columbia	Tacoma	Boston Water Supply
Energy production
– GFDL – Hadley Centre – MPI – Demand/operational	+26% +3% -5% -3%	+5% -10% -1% -5 to +3%	-8% -13% -33% not calculated	-5% -4% -12% -10 to -15%
Municipal and industrial supply
– GFDL – Hadley Centre – MPI – Demand/Operational			-9% -5% -15% not calculated	-5% -1% -14% 0	0 0 0 -15%	0 0 -5% -3 to -40%
Flood risk
– GFDL – Hadley Centre – MPI	+70% +50% +16%	+32% -3% -10%	+4% +10% +12%		+40% +40% +35%
Navigation
– GFDL – Hadley Centre – MPI – Demand/operational		+3% -2% -1% -2 to -17%	-8% -10% -10% not calculated	-2% -5% +5% -4%
Instream flow requirements
– GFDL – Hadley Centre – MPI – Demand/operational	0 -1% -3% -1 to -3%			- 4% -5% -10% 0 to +12%	-1% -6% -8% -8 to -9%
Recreation
– GFDL – Hadley Centre – MPI – Demand/operational	+7% +5% +3% +18%	+22% -5% +4% -25 to +28%	0 0 0 not calculated	-10% -8% -12% -9 to +3%
a See original paper for detailed summary of scenarios used.

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