There are apparent trends in streamflow volume—both increases and decreases—in many regions. These trends cannot all be definitively attributed to changes in regional temperature or precipitation. However, widespread accelerated glacier retreat and shifts in streamflow timing in many areas from spring to winter are more likely to be associated with climate change.

• The effect of climate change on streamflow and groundwater recharge varies regionally and between scenarios, largely following projected changes in precipitation. In some parts of the world, the direction of change is consistent between scenarios, although the magnitude is not. In other parts of the world, the direction of change is uncertain.
• Peak streamflow is likely to move from spring to winter in many areas where snowfall currently is an important component of the water balance.
• Glacier retreat is likely to continue, and many small glaciers may disappear.
• Water quality is likely generally to be degraded by higher water temperature, but this may be offset regionally by increased flows. Lower flows will enhance degradation of water quality.
• Flood magnitude and frequency are likely to increase in most regions, and low flows are likely to decrease in many regions.
• Demand for water generally is increasing as a result of population growth and economic development, but it is falling in some countries. Climate change is unlikely to have a large effect on municipal and industrial demands but may substantially affect irrigation withdrawals.
• The impact of climate change on water resources depends not only on changes in the volume, timing, and quality of streamflow and recharge but also on system characteristics, changing pressures on the system, how the management of the system evolves, and what adaptations to climate change are implemented. Nonclimatic changes may have a greater impact on water resources than climate change.
• Unmanaged systems are likely to be most vulnerable to climate change.
• Climate change challenges existing water resources management practices by adding additional uncertainty. Integrated water resources management will enhance the potential for adaptation to change.
• Adaptive capacity (specifically, the ability to implement integrated water resources management), however, is distributed very unevenly across the world.

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