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GCOS Essential Climate Variables (ECV) Introduction: Surface water storage occurs in lakes, reservoirs (and also wetland areas) for water in its liquid phase. The volume of water in a surface storage unit at any one time is an integrator variable, reflecting both atmospheric (precipitation, evaporation-energy) and hydrologic (surface water recharge, discharge and ground water tables) conditions. Depending on the storage capacity of a reservoir, it may primarily reflect human control. However, if lakes and wetland areas are not being affected by excessive withdrawal, they are strongly driven by extant climate conditions and are important for assessing net climate effects over time. If climate change is leading to a hotter and drier mode, then lakes and wetlands should reflect this promptly. (from the FAO/GTOS web site) Many modelling activities in Earth and environmental sciences, telecommunications and civil engineering increasingly require accurate, high resolution and comprehensive topographical databases with, indication of changes over time, where relevant. The information is also used by, amongst others, land use planners for civil planning and development, and by hydrologists to predict the drainage of water and likelihood of floods, especially in coastal areas. In its Fourth Assessment Report in 2007, the IPCC predicts that (by conservative estimation techniques) global mean sea level may rise as much as 28–43 cm by the end of the 21st century. Potentially, sea level rise will cause severe flooding, with disastrous impacts on large, densely populated, low-lying coastal cities and deltaic areas, such as Bangladesh. (from the CEOS web site) Satellite Observations: Satellite techniques offer a unique, cost-effective and comprehensive source of landscape topography data. At present, most information is obtained primarily from multi-band optical imagers and synthetic aperture radar (SAR) instruments with stereo image capabilities. The pointing capability of some optical instruments allows the production of stereo images from data gathered on a single orbit (e.g. by ASTER) or multiple orbits (e.g. by SPOT series).These are then used to create digital elevation maps, which give a more accurate depiction of terrain. Since SARs can also be used in interferometric mode to detect very small changes in topography, they have important applications in monitoring of volcanoes, landslides, earthquake displacements and urban subsidence. Current missions include Envisat, RADARSAT-2, TerraSAR-X and ALOS (which carries both high precision optical and SAR topographic mapping instruments). In future, ESA’s Sentinel-1 mission will also contribute to such information. Radar altimeters can also provide coarse topographic mapping over land. They have been supplemented by a new generation of laser altimeters, such as GLAS (on ICESat) which can provide landscape topography products with height accuracies of order 50–100 cm, depending on slope. The role of these satellites and their importance in mitigating geo-hazards, such as earthquakes, landslides, and volcanic eruptions, is the focus of the IGOS Geo-hazards Theme. The Geo-hazards Theme report is available from www.igospartners.org. GCOS notes that measurements of lake area and lake level give an indication of the volume of the lake, an integrator variable that reflects both atmospheric (precipitation, evaporation-energy) and hydrological (surface water recharge, discharge and ground water tables) conditions. GCOS threshold requirements for these variables are currently met by existing missions. (Satellite Missions) (from the CEOS web site) Data, Product, Metadata and Information Access
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