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Understanding the global water cycle is a major research theme of the Hydrospheric and Biospheric Sciences Laboratory. The major components of the global water cycle include the evaporation from the land and ocean surfaces, precipitation onto the ocean and land surfaces, the net atmospheric transport of water from land areas to ocean, and the return flow of water from the land back into the ocean. The additional components of oceanic water transport are few, including the mixing of fresh water through the oceanic boundary layer, transport by ocean currents, and sea ice processes. On land the situation is considerably more complex, and includes the deposition of rain and snow on land; water flow in runoff; infiltration of water into the soil and groundwater; storage of water in soil, lakes and streams, and groundwater; polar and glacial ice; and use of water in vegetation and human activities.
The distribution of fresh water is highly uneven over the Earth, with strong latitudinal differences due to the atmospheric general circulation. There is also large variability due to landforms and the interaction of land with global weather systems. The annual global fresh water budget is a balance between evaporation, atmospheric transport, precipitation, runoff and storage. Although the available volume of fresh water on land is small, the short residence time of water in these fresh water reservoirs causes the flux of fresh water - through evaporation, atmospheric transport, precipitation and runoff - to be large. With a total atmospheric water store of ~13*1012m3, and an annual flux of ~460*1012m3/yr, the mean atmospheric residence time of water is ~10 days. River residence times are similar, biological are ~1 week; soil moisture is ~2 months; lakes and aquifers are highly variable, extending from weeks to years; and glaciers are generally decades to centuries. As the Earth’s climate varies through natural and man-induced causes, there is a potential for redistribution of water on Earth, and acceleration of the global hydrological cycle. The hypothesized speed-up of the global water cycle - thought to be associated with global warming - adds another element to be dealt with when considering the pressure placed upon water resources by the burgeoning human population, the variability of weather and climate, and concerns about anthropogenic impacts on global fresh water availability.
Knowledge of the key terms in the fresh water flux budget is inadequate and is a major topic of research within the Laboratory. Many components of the budget, e.g. precipitation, runoff, storage, are measured with variable accuracy across the globe, and we are just now improving the measurements of the major components of global fresh water storage in global ice and ground water. The easily accessible fresh water sources in rivers, lakes, snow and runoff are only adequately measured in the more affluent portions of the world. Present research activities in the Laboratory address new methods of measuring from space these and other components of the global fresh water cycle. Within the next decade, an experimental global water and energy cycle observation system combining operational environmental satellites, such as Cloudsat, CALIPSO, SMOS, Aquarius, GPM, and potential new exploratory missions - i.e. advanced remote sensing systems for solid precipitation, soil moisture, and ground water storage - may be feasible. These proposed new approaches are tantalizing, for knowledge of global fresh water availability under the effects of climate change is of increasing importance as the human population grows. Space measurements provide the only means of systematically observing the full Earth while maintaining the measurement accuracies needed to assess global variability.
A major Laboratory goal is to develop a systematic error budget for the complete global fresh water budget. Based on this, we will work with other agencies and organizations to develop the priorities for the future observational capabilities for a systematic global water cycle observational network of the future. This network will consist of observational technologies and assimilation of the measurements into forecast models. Based on present plans and the knowledge gained over the past years, we foresee that the capability to measure the full global fresh water cycle and its budget is tantalizingly close, and that, if designed as a complete observational-data analysis system, the full water cycle can be measured with known and planned technologies. The size and scope of this effort is comparable with current earth science space programs and therefore could be affordable as an international effort. Our ultimate water cycle research goal is to assist in quantifying this important potential.
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