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GCOS Terrestrial ECV T3 Introduction: An appreciable amount – nearly 30% – of the world’s total freshwater resources (i.e., including snow/ice) is estimated to be stored as groundwater. Today, groundwater it is the source of about one third of global water withdrawals. Groundwater is by far the largest available reservoir of liquid freshwater (approx. 10.5 million km3). Estimates of the number of people who depend on groundwater supplies for drinking range from 1.5 to 3 billion. Global groundwater abstraction grew ten-fold in the last 50 years, concentrated in agriculture (approx. 90%), in particular in Asia. Groundwater use, in relative terms, has increased in the recent decades as compared to surface water use. On the one hand, groundwater use is technically more complicated and more expensive than surface water use but, on the other hand, it is more reliable and safer. Groundwater storage, recharge, and discharge are important aspects of climate change impacts and adaptation assessments. Over the past several years, important progress has been made, facilitated through the International Groundwater Resources Assessment Centre (IGRAC), in global-scale groundwater monitoring with in situ well observations as a foundation, and more is expected over the next decade through the establishment of a Global Groundwater Monitoring System (GGMS). In particular, the feasibility of satellite observation of groundwater storage variations using the Gravity Recovery and Climate Experiment (GRACE) mission has been demonstrated. The representation of groundwater storage in land surface models has advanced significantly. In the longer term, full implementation of the Global Terrestrial Network – Groundwater (GTN-GW) should be accomplished, including contributions from satellites (e.g., gravity missions like GRACE).(Source: WMO/IOC Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523) Satellite Observations: Gravity field measurements from space provide the most promising advances for improved measurement of the ‘geoid’ and its time variations. The geoid is the surface of equal gravitational potential at mean sea level, and reflects the irregularities in the Earth’s gravity field at the planet’s surface caused by the inhomogeneous mass and density distribution in the interior. Such measurements are vital for quantitative determination – in combination with satellite altimetry – of ocean currents, improved global height references, estimates of the thickness of the polar ice sheets and its variations, and estimates of the mass/volume redistribution of fresh water in order to better understand the hydrological cycle. Gravity field measurement packages on satellites often utilise combinations of different instrument types in order to derive the necessary information: single or multiple accelerometers; precise satellite orbit determination systems; and satellite to satellite tracking systems. DLR’s CHAMP gravity package (since 2000) and the NASA/DLR twin satellite GRACE mission (since 2002) have been providing new information that has resulted in new and unique models of the Earth’s gravity field and its variability over time, and determination of the geoid to centimetre accuracy at length scales of several hundred kilometres. GRACE has demonstrated that satellites can detect groundwater variations by measuring subtle temporal variations in gravity. From 2009, this data is supplemented by ESA’s GOCE satellite, which is designed to make significant advances in our understanding of ocean circulation and the crucial role which it plays in regulating the climate, as well as sea level rise and processes occurring in the Earth’s interior. GOCE data will also find a broad range of applications in the field of geodesy and surveying. A number of Earth missions, including Australia’s Fedsat, launched in 2002, have carried sensors to study the electromagnetic environment of spacecraft. Satellite-borne magnetometers provide information on the strength and direction of the Earth’s internal and external magnetic field and its time variations. Such instruments are on board the Ørsted satellite, which is Denmark’s first satellite dedicated to the magnetic field, launched in 1999. The CHAMP mission also provides these measurements, which are of value in a range of applications, including navigation systems, resource exploration drilling, spacecraft attitude control systems and assessments of the impact of ‘space weather’. Further missions are under way or planned for more in-depth, dedicated studies of magnetic field. They include DEMETER (launched June 2004), which is investigating links between earthquakes and magnetic field variations, and Swarm (from 2010), which aims to provide the best ever survey of the geomagnetic field and its temporal evolution, providing new insights by improving our knowledge of the Earth’s interior and climate. (Satellite Missions) (Source: CEOS EO Handbook - Earth Observations Plans by Measurement) References:
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