NASA’s Observatorium: Hydrologic CycleThe movement of water between the land surface, oceans and atmosphere is called the hydrologic cycle. Water in the atmosphere is transported to the land surface and oceans as precipitation (rain, snow or sleet). Upon reaching the land surface, water may immediately become streamflow, or it may infiltrate into the soil where it may later be taken up by plants or it can percolate to the groundwater. Surface streamflow and groundwater flow move water from the land surface to lakes and the ocean. Water re-enters the atmosphere as vapor either via evaporation from surface waters (ocean, lakes, etc) or transpiration from plants. This cyclical movement of water is driven by solar energy. An increase in net solar radiation or temperature will effectively speed up the processes within this cycle (evaporation, condensation, precipitation, etc).
Figure 1: The Water CycleClick on Thumbnail for full size image Due to complex interactions of changes in the hydrologic cycle with global circulation patterns and local weather patterns, an increase in energy in the hydrologic cycle does not necessarily translate into an increase in precipitation in all geographic regions. It is difficult to predict future changes in regional precipitation patterns. Predicting regional changes in streamflow and groundwater recharge due to climate change also remains challenging, particularly because of the uncertainty in regional projections of how precipitation may change (IPCC, 2007).
Changes in temperature, precipitation patterns and snowmelt can have impacts on water availability. Temperature is predicted to rise in most areas, but is generally expected to increase more in inland areas and at higher latitudes. Higher temperatures will increase loss of water through evaporation. The net impact on water supplies will depend on changes in precipitation (including changes in the total amount, form, and seasonal timing of precipitation). Generally speaking, in areas where precipitation increases sufficiently, net water supplies may not be affected or they may even increase. In other areas where precipitation remains the same or decreases, net water supplies would decrease. Where water supplies decrease, there is also likely to be an increase in demand, which could be particularly significant for agriculture (the largest consumer of water) and also for municipal, industrial and other uses.
Increases in temperature can affect the amount and duration of snow cover which, in turn, can affect timing of streamflow. Glaciers are expected to continue retreating, and many small glaciers may disappear entirely. Peak streamflow may move from late spring to early spring/late winter in those areas where snowpack is important in determining water availability. Changes in streamflow have important implications for water and flood management, irrigation, and planning. If supplies are reduced, off-stream users of water such as irrigated agriculture and in-stream users such as hydropower, fisheries, recreation and navigation, could be most directly affected (IPCC, 2007).
ReferencesIPCC, 2007: Climate Change 2007: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Parry, Martin L., Canziani, Osvaldo F., Palutikof, Jean P., van der Linden, Paul J., and Hanson, Clair E. (eds.)]. Cambridge University Press, Cambridge, United Kingdom, 1000 pp.
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