Inland waters are sensitive indicators of global change because their internal physical or biogeochemical processes can amplify the signals of changes in energy, water, and chemical fluxes. Because inland waters also function as hot spots of biogeochemical transformations influencing the carbon and nitrogen cycle, this responsiveness to global change can function as an important signal or feedback to resource managers.
While the accumulation of greenhouse gases in the atmosphere is expected with some certainty to lead to 15 °C increases in temperature worldwide in the next
50 years, one of the greatest uncertainties in predicting climate change is how the water cycle will respond in terms of changes in both mean conditions and extreme events. Rates and patterns of precipitation and runoff are gradually changing with general circulation models predicting that these changes will continue as global CO2 levels rise. Further, simulations in global circulation models show that the direction of change in the water cycle will likely vary regionally, with different regions becoming either dryer or wetter and experiencing concomitant increases in drought or flood events. Other climate change uncertainties are variations in the frequency of disturbances. Because of the importance of hydrodynamics in inland waters, all of these changes can be expected to cause cascades of internal changes in river ecosystems.
Climate change will alter the seasonal pattern of runoff coming into streams and river networks. The seasonal pattern of high and low flow in river and streams has been modified to a large extent, however, by flow regulation in many regions of the world. Therefore, the changes in stream and river systems will depend upon how water resource management practices change.
There are two contrasting developments in terms of the impact of flow regulation on stream and river systems. In recognition that flow regulation has sustained ecological and biogeochemical effects, many smaller older dams are being decommissioned rather than being upgraded. On the other hand, the willingness to allocate water for in-stream flow may change under less predictable hydro-logic regimes. Specifically, the physical infrastructure for flow regulation and the system of water allocation were designed to operate within a range of expected hydrologic variation that will likely be exceeded under a changing climate. The maintenance of in-stream flow during certain seasons may depend upon significant changes in water use within a region. Understanding the response of streams and rivers to these changes in flow regime will require enhanced capabilities to measure and model the hydrodynamics of these systems. Biogeochemical processes in streams and rivers will also be affected by changes in temperature and changes in riparian vegetation.
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