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11 Water Resources and the Global Hydrologic Cycle OVERVIEW The global water cycle describes the circulation of water—a vital and dynamic substance—in its liquid, solid, and vapor phases as it moves through the atmosphere, the land, and the rivers, lakes, and oceans. Water affects everything—animal, vegetable, and mineral—on the surface of Earth and in the oceans. Life in its many forms exists because of water, and humans have flourished as a hydraulic civilization. Modern civilization depends on learning how to live within the constraints imposed by the availability of water—its excesses and its deficiencies (Figure 11.1). Water is the link among most dynamic processes on land. It controls the growth of plants through water availability related to soil moisture and through radiation reaching the land surface—controlled largely by clouds—that is available for photosynthesis. Evaporation and transpiration from plants act to transfer not only water vapor but also energy from the surface to the atmosphere, enabling a feedback that has important implications for precipitation over global land areas. The carbon, water, and energy cycles are strongly interdependent—latent heat flux is essentially proportional to evaporation, and photosynthesis is closely related to transpiration. Snow cover, glaciers, and sea ice affect climate through feedback between reflected solar energy and temperature. This feedback effect exists not only over the polar areas but also more seasonally or ephemerally over much of the Northern Hemisphere’s land area as well as high-elevation areas of the Southern Hemisphere. Glaciers and ice sheets store much of the freshwater on the planet, but changes in such storage occur on timescales of decades to centuries. The melting of ice sheets (mostly in Antarctica and Greenland) is a major contributor to sea-level rise, and mid- and low-latitude glaciers, although much smaller in comparison with polar ice storage, are important contributors to water supply in some parts of the globe. Those glaciers are almost all in retreat, and this will eventually lead to a loss of this source of usable water (see, e.g., Figure 11.2). On a global scale, there are important gaps in knowledge of where water is stored, where it is going, and how fast it is moving. Global measurements from space open a vision for the advancement of water science, or hydrology. This vision includes advances in understanding, data, and information that will
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FIGURE 11.1 Water in many parts of the United States, especially in the Southwest, is a critically scarce resource for most of the year. This view of the Snake Range was taken along the Great Basin National Park access road. Great Basin National Parkin eastern Nevada is known for its ecological diversity ranging from low, desert basin to high, alpine tundra, with many ecozones and habitats in between. SOURCE: Courtesy of the U.S. Geological Survey. improve the ability to manage water and to provide the water-related infrastructure that is needed to provide for human needs and to protect and enhance the natural environment and associated biological systems. The scientific challenge posed by the need to observe the global water cycle is to integrate in situ and space-borne observations to quantify the key water-cycle state variables and fluxes. The vision to address that challenge is a series of Earth observation missions that will measure the states, stocks, flows, and residence times of water on regional to global scales followed by a series of coordinated missions that will address the processes, on a global scale, that underlie variability and changes in water in all its three phases. The accompanying societal challenge is to foster the improved use of water data and information as a basis for enlightened management of water resources, to protect life and property from effects of extremes in the water cycle—especially droughts and floods. The recent western U.S. drought (see Box 11.1) has renewed a focus on more effective management of water resources in the perennially water-stressed West. More generally, a major change in thinking about water science that goes beyond its physics to include its
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FIGURE 11.2 Changes in the Qori Kalis Glacier, Quelccaya Ice Cap, Peru, from 1978 to 2000. SOURCE: Courtesy of L.Thompson, Byrd Polar Research Center.
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BOX 11.1 DROUGHT IN WESTERN NORTH AMERICA Drought is a nebulous concept for which there is no universal definition. All definitions—whether based on precipitation, soil moisture, or availability of water in rivers or reservoirs—are ultimately driven by conditions of abnormally low precipitation or high evaporative demand. Those conditions are particularly chronic in the western United States where water is scarce. The settlers of the 1800s found,for instance, that although land was in ample supply, the success of settlements depended heavily on ample rainfall. Post-Civil War settlers flourished during a period when precipitation generally was ample, but immense hardship followed in the generally dry decade of the 1880s. In modern history, the Dust Bowl years of the 1930s made an indelible impression on a generation of Americans. Although the 1930s drought was not restricted to the West (see Figure 11.1.1), its implications were most serious there (few of the major water systems now in place existed then). The drought of the 1950s was also widespread, but its effects were felt more in the Great Plains region than in the far West. The most recent western U.S. drought began in the late 1990s and persisted for at least 5 years over parts of the region. It has resulted in damages estimated at tens of billions of dollars. Reservoirs in the Colorado River system in particular have declined to near record low levels (see Figure 11.1.2). An important property of droughts in arid and semiarid regions is that small decreases in precipitation can produce large decreases in runoff. Figure 11.1.3 shows stream flow in the Rio Conchos River of northern Mexico, a major tributary of the Rio Grande. During the 1990s, precipitation fell short of its long-term average by only about 10 percent. Runoff, however, fell by about 50 percent. In contrast, in humid basins, a 10 percent dropoff in precipitation would produce only about the same decrease in runoff, which helps to explain why the severity and duration of droughts tend to be greater in the western than in the eastern United States. FIGURE 11.1.1 Drought extent in August 1934. Soil-moisture percentiles expressed relative to 1960–2003 climatology. SOURCE: See www.hydro.washington.edu/forecast/monitor.shtml. Courtesy of Land Surface Hydrology Research Group, University of Washington.
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FIGURE 11.1.2 Replicate photographs of Lake Powell at the confluence with the Dirty Devil River (entering from left). (Top) June 29, 2002. (Bottom) December 23, 2003. SOURCE: Photos by John C.Dohrenwend, Southwest Satellite Imaging, Moab, Utah.
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Industrialized societies have generally become less susceptible to drought because of their ability to provide buffers to water supply in the form of either reservoir storage or groundwater. Short, 1- or 2-year droughts in the Colorado River basin are barely noticed, for instance, because total reservoir storage exceeds four times the mean annual flow. But the explosion of population in the “sunshine belt” of the Southwest is changing the balance of supply and demand, and the western states have been more aggressively pursuing management options, including drought plans. Basic sources of hydrologic data that allow “nowcasting” and forecasting of drought evolution that are required for effective drought response are incomplete. Among the key deficiencies is information about the space-time distribution of soil moisture and snow-water storage—information that is nearly impossible to obtain from in situ sensors but would be produced by the SMAP and SCLP mission concepts proposed in the section “Prioritized Observation Needs.” FIGURE 11.1.3 Rio Conchos discharge, 1955–2001. The 1993–2001 discharge was less than half that of 1955–1992 and included the 3 lowest discharge years on record, but precipitation over the same period was only about 10 percent below the long-term mean. SOURCE: After Vigerstol (2002). Courtesy of Kari Vigerstol.
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FIGURE 11.3 Soil moisture exerts substantial control on evapotranspiration in terrestrial ecosystems. Field measurements of soil moisture by a truck-mounted L-band radiometer are plotted with normalized evapotranspiration flux in a California agricultural field. As the soil becomes drier, the flux is reduced. Evapotranspiration is the key flux that links the water, energy, and carbon cycles in terrestrial ecosystems. SOURCE: Dara Entekhabi, Massachusetts Institute of Technology, after Cahill et al. (1999). role in ecosystems (Figure 11.3) and society is also required. Better water-cycle observations, especially on the continental and global scales, will be essential. Water-cycle predictions need to be readily available globally to reduce loss of life and property caused by water-related natural hazards, notably floods (see Box 11.2) and droughts. The panel envisions a future in which surface, subsurface, and atmospheric water will be tracked continuously in time and space over the entire globe and at resolutions useful for timely inclusion into models for prediction and decision support related to use of water for agriculture, human health, energy generation, and hazard mitigation. Space-based observations and supporting infrastructure can help to make that vision a reality for the next generation. Such predictions will have enormous social and economic value for the management of water, food security, energy production, navigation, and a range of other water uses.
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SCIENCE AND APPLICATIONS NEEDS AND REQUIREMENTS The previous section offers a rationale for the importance of understanding the global water cycle as a major feature both of the Earth system and of human society. This section presents a strategic overview of planned and new water-cycle missions and mission concepts that the Panel on Water Resources and the Global Hydrologic Cycle believes should constitute the U.S. water-cycle observing system from space over the decade 2010–2020. It also reviews the status and heritage of planned missions and programs that are the underpinnings of the new mission concepts described in the section “Prioritized Observation Needs” below. The primary focus in this respect is on the Global Precipitation Measurement (GPM) mission and the National Polar-orbiting Operational Environmental Satellite System (NPOESS), for which the panel offers recommendations on the basis of issues of immediate urgency to both programs. Observing the Global Water Cycle: A Strategic View Precipitation arguably is the most important part of the global water cycle. It dominates the land-surface branch of the water cycle and is, in terms of magnitude, second only to evaporation over the oceans. Furthermore, because the fraction of Earth covered by oceans is so large, even relatively small changes in the net of oceanic evaporation minus precipitation can lead to large changes in precipitation over adjacent land areas, and so, indirectly, ocean precipitation strongly affects land conditions. Over the last decade, the ability to observe and thereby understand the dynamics of tropical precipitation has advanced immensely. Much of the advancement is attributable to the launch of the Tropical Rainfall Monitoring Mission (TRMM) in 1997 and the continuing data stream it has provided for over 9 years. The improved understanding that has been gained by flying active and passive microwave sensors on the same platform has been instrumental in better characterizing precipitation not only from the TRMM sensors but also from operational sensors, such as the Special Sensor Microwave Imager (SSM/I). Those improvements have come from a better understanding and interpretation of brightness temperature (Tb) information at wavelengths that are most sensitive to precipitation. The improved understanding has also translated into better precipitation products from the Advanced Microwave Scanning Radiometer-EOS (AMSR-E) sensor on Aqua and forms the basis of the approved GPM mission. The success of precipitation measurements from space forms a blueprint for strategic thinking about observation of “fast” storage terms in the global hydrologic cycle, such as moisture storage in soil, in rivers, lakes, reservoirs, and wetlands, and as ephemeral snow. While estimates of soil moisture are routinely produced from the AMSR-E sensor, their quality is at best experimental (the wavelength is too short to produce good soil-moisture estimates for all but sparsely vegetated areas), and the AMSR-E soil-moisture product is insufficient to constrain the surface hydrologic models in any useful way. The same is true for snow-water equivalent, especially in mountainous terrain, which is critical for the water resources of many parts of the globe, such as the western United States. Here, the issue has to do primarily with spatial resolution. Aside from very large inland water bodies, which are captured by such ocean altimeters as Ocean Topography Experiment (TOPEX)/Poseidon and Jason, surface-water variations are not captured by current sensors. Estimation of river discharge from space remains an elusive goal. Having high-quality estimates of those variables, coupled with measures of surface-water storage and transport, would substantially improve the ability to model and understand the amounts and flows of surface water and in turn to provide an integrated understanding of the water cycle globally. The four highest-ranked water-cycle missions (listed in rank order) would contribute to that goal as follows:
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The approved GPM mission will provide estimates of precipitation at a sampling interval (3–4 h) sufficient to resolve the diurnal cycle and at a spatial resolution sufficient to resolve major spatial variations over the continents and oceans. A soil moisture mission would provide estimates of a key part of the land-surface water balance, which controls land-atmosphere fluxes of heat and water over many parts of the globe (in particular, recycling of moisture from the land to the atmosphere) and is a key variable that affects the nonlinear response of runoff to precipitation. A surface-water and ocean-topography mission (see the section “Prioritized Observation Needs”) would provide observations of the amount and variability of water stored in lakes, reservoirs, wetlands, and river channels and would support derived estimates of river discharge. It would also provide critical information necessary for water management, particularly in international rivers. A cold-season mission would estimate the water storage of snowpacks, especially in spatially heterogeneous mountainous regions that are the source of many of the world’s most important rivers. Taken together, those four missions, described in some detail in the section “Prioritized Observation Needs,” would form the basis of a coordinated effort to observe most components of the surface water cycle globally. They also would provide critical information about precipitation over the world’s oceans and the basis for prediction of circulation in coastal areas that is not possible with current sensors. In addition to measurements that would be made by these four missions, several measurements that would benefit analyses of the water cycle were highly rated by the water-cycle panel but with somewhat lower priority. They include missions that would estimate water vapor transport, sea ice and glacier mass balance, groundwater and ocean mass, and inland and coastal water quality (see Table 11.1). Those measurements and water-cycle issues are discussed in the section “Other High-Priority Water-Cycle Observations.” As discussed in that section, all the measurements have direct relevance to the measurement needs identified by other panels, and that synergy was considered in the selection of the integrated missions recommended in Chapter 3. Summary of Existing and Planned Missions and Products As noted in this chapter’s “Overview” section above, the queue of approved U.S. Earth science missions is sparse, especially those relevant to the global water cycle. It consists of CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), launched in April 2006; the GPM mission, which was further delayed for more than 2 years by NASA in spring 2006 despite the decadal survey committee’s recommendation against further delays (NRC, 2005). Aquarius, which will measure ocean salinity and facilitate estimation of E—P (evaporation minus precipitation) over the oceans, is scheduled for launch in 2009. On the operational side, the staggering cost growth in NPOESS has resulted in cancellation or descoping of instruments that are central to water and climate science, including cancellation of the ocean altimeter and cancellation of the Conical-Scanning Microwave Imager/Sounder (CMIS). This section does not discuss the implications of CloudSat/CALIPSO, in light of its recent launch; however, it does address the necessity of and urgency for GPM and for measurement of certain key water-cycle variables that will be observed by NPOESS. Global Precipitation Measurement Mission Precipitation is the central component of the global water cycle. It regulates the global energy balance through coupling to clouds and water vapor (the primary greenhouse gas) and shapes global winds and
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atmospheric transport through release of latent heat. Precipitation is also the primary source of freshwater in a world that is facing an ever more severe freshwater crisis. Accurate and timely knowledge of global precipitation is essential for improving the ability to manage freshwater resources and for predicting high-impact weather events, such as floods, droughts, and landslides. The objective of GPM is to provide a reference standard for unifying a constellation of dedicated and operational microwave radiometers to provide accurate and frequent measurements of global precipitation for basic research and applications (Smith et al., 2006). The GPM core spacecraft will carry a first-ever, dualfrequency precipitation radar and a multifrequency microwave radiometric imager with high-frequency capabilities to serve as a precipitation physics laboratory (with detailed microphysical measurements) and a calibration standard for constellation radiometers in terms of brightness temperature measurements and precipitation retrievals. In addition, NASA will provide a constellation radiometer to be flown in an orbit that optimizes the sampling and coverage of global precipitation. GPM is thus the key to providing a uniform global-precipitation data product leveraging all available satellites capable of precipitation measurement. By extending the success of TRMM to the entire globe with new capabilities to measure rain, snow, and precipitation microphysics, GPM is poised to improve the understanding of the water cycle and the modeling and prediction of weather, climate, and hydrologic systems. GPM is in formulation at NASA and the Japan Aerospace Exploration Agency (JAXA), with potential participation by other international space agencies. This is a complex international partnership, and any further delay in launching the GPM core spacecraft jeopardizes the mission by increasing the total cost and creating development problems for all partners. The viability of GPM will depend critically on NASA’s commitment to a firm launch schedule, thus providing a solid basis for securing international partnership. The president’s FY 2007–2008 budget supports a GPM launch in mid-2013 (rather than 2012, as suggested in NASA documents), which may further jeopardize the NASA-JAXA partnership. Maintaining the viability of the JAXA partnership adds another compelling reason for not delaying the GPM launch to 2013. As noted above, the decadal survey committee strongly recommended that GPM be launched without further delay (NRC, 2005), and the water resources panel repeats that recommendation here: The panel recommends that GPM be launched in a timely manner, without further delay. NPOESS NPOESS was originally intended to include several measurements that are of key importance to understanding climate and the global water cycle. They included snow-covered area, which would be measured at high spatial resolution by the Visible/Infrared Imaging Radiometer Suite (VIIRS), similar to the Moderate Resolution Imaging Spectroradiometer (MODIS) product, and at lower resolution (but all-weather, or nearly so) by CMIS; snow-water equivalent, by CMIS (similar to AMSR-E); soil moisture, by CMIS (6-GHz channel, assuming that AMSR-E radio interference problems at this frequency could be resolved, and otherwise at 10 GHz); ocean-surface height, by a nadir-pointing radar altimeter; and precipitation, by CMIS. In addition, NPOESS was to include the capability to measure ocean wind speed and direction (needed for estimation of water vapor transport) and all-weather sea-surface temperature (needed for evaporation estimation), both by CMIS. The CMIS instruments on all three NPOESS platforms were also intended to act as “constellation” satellites for GPM. The highest frequency (183 GHz) would facilitate retrievals of falling snow, not possible with AMSR-E or Defense Meteorological Satellite Program (DMSP) satellites. The recent cancellation of CMIS and problems with VIIRS call into question whether many of those observations will be made by NPOESS. The extent of the problem is difficult to determine until the nature of a downscaled CMIS replacement is known. However, it appears likely that the lowest-frequency channel or channels will be lost. That would eliminate all soil-moisture information that may have been available from CMIS.
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BOX 11.2 FLOODS IN LARGE RIVERS—THE POTENTIAL FOR GLOBAL FLOOD FORECASTING Floods are among the most destructive of natural disasters. From a monetary standpoint, flood damages in the United States averaged around $5 billion per year in the 1990s in 1995 dollars (Table 3.1 in Pielke et al., 2002). Outside the United States, the impact is even more striking; flood losses globally increased 10-fold (inflation-corrected) over the second half of the 20th century to a total of around $300 billion in the decade of the 1990s (Kabat and van Schaik, 2003). Aside from the economic costs, the social consequences of flooding can be staggering. The Mississippi River flood of 1927 displaced over 700,000 people and had impacts on the social structure of the lower Mississippi River valley that persist to this day (Barry, 1997). Both the number of floods (Figure 11.2.1) and flood damages (in constant dollars) have been increasing in recent decades (UNDP, 2004). Although it is not clear whether climate change or increased economic development is playing a greater role in those changes (Pielke, 2005), the trend is of concern to both governments and the insurance industry. The magnitude of flood losses is generally greatest in the developed world (losses from the 1993 Mississippi River flood were estimated to be about $15 billion, and from the 2003 Elbe River flood about 9 billion Euros, or about $11 billion), but the impact—in terms of loss of life and economics—is greatest in the developing world. For instance, flooding associated with Hurricane Mitch caused an estimated $3 billion to $4 billion (U.S. dollars) in damages in Honduras, which was almost 70 percent of that country’s gross domestic product (GDP) (UNDP, 2004). In comparison, the 1993 Mississippi River flood damages represented less than 0.3 percent of the U.S. GDP. Most of the developed world has reasonably sophisticated flood-forecasting systems. They are based on a combination of precipitation-gauge and radar precipitation observations, river-stage observations, and hydrologic models coupled with quantitative precipitation forecasts derived from weather prediction models. However, the forecast systems are almost all regional. For instance, in Europe, each country has an agency (generally affiliated with the weather services) that is responsible for flood forecasts in that country. In the United States, flood-forecasting responsibilities lie with the National Weather Service River Forecast Centers, of which there are 13 (generally partitioned according to major river basins). On a global basis, there is no coherent flood-forecasting capability as there is for global weather (Lettenmaier et al., 2006). The absence of a global flood-forecasting capability affects the developing world especially. In the Mozambique flood of 2000, for instance (Figure 11.2.2), there were only a handful of precipitation stations reporting on the Global Telecommunications System, and the precipitation radar systems that are a key element of flood-forecasting systems in the developed world were nonexistent. But the capability for global flood forecasting clearly exists, particularly for large river floods, which are responsible for most loss of life and economic damages (Webster et al., 2006). Accurate flood forecasting requires good knowledge of the initial state of the land system (primarily soil moisture and, where relevant, snow-water storage) and river levels, an accurate forecast of the space-time dis FIGURE 11.2.1 Number of major flood disasters globally, 1975–2001. SOURCE: Reprinted from UNDP (2004). Copyright 2004 United Nations, with the permission of the United Nations.
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model products. However, the use of those model products eliminates the possibility of comparing the resulting evaporation fields as independent data sources. Given the inability to measure evaporation directly over large areas (with in situ or remote sensing methods), it is likely that the most important progress in this area will be in combination with improvements in assimilation into models that have improved boundary-layer physics. The panel believes that progress in this area should be a primary focus of the community over the next decade. Although it is not possible at present to define a satellite mission that would address the key science questions in this area, several planned satellite missions will have a central role and should be supported. They include VIIRS aboard NPOESS, which will provide functionality in estimating land and sea-surface temperature under clear skies and vegetation information over land, similar to what currently can be derived with the Terra and Aqua MODIS sensors. A high-resolution thermal infrared sensor equivalent to those previously flown on Landsat satellites would also be useful. Nonetheless, a more focused effort to address the complex problem of combining observations and modeling to produce consistent estimates of ocean and land evaporation is a pressing need, progress on which is essential before observation requirements can be fully specified. Coordinated Observing Systems The panel recognized that the current paradigm is for missions that focus on single primary measurements designed to address a primary science question, perhaps with so-called secondary science, but it also recognizes an alternative paradigm that attempts to address measurements of the water cycle in a more coordinated fashion—for example, by focusing on a broader set of issues and attempting to realize synergies associated with multiple, coordinated observations. Two such strategies are outlined here: one addresses coordinated measurement of global water-cycle variables, and the other is a cloud-aerosolprecipitation initiative. Integrated Water-Cycle Observing System Society’s welfare, progress, and sustainable economic growth—and life itself—depend on the abundance and vigorous cycling and replenishing of water throughout the global environment. The water cycle operates on a continuum of time and space scales and exchanges large amounts of energy as water undergoes phase changes and is moved from one part of the Earth system to another. A central challenge of a future water-cycle observation strategy is to progress from single-variable water-cycle instruments to multivariable integrated water-cycle instruments, probably in electromagneticband families. Experience has shown that the microwave range in the electromagnetic spectrum is ideally suited for sensing the state and abundance of water because of water’s dielectric properties. Until now, limits on antenna technology have stymied the harvesting of the synergy that would be afforded by simultaneous multichannel active and passive microwave measurements. The removal of that roadblock is now possible. A coordinated water-cycle observation strategy will require innovative technology in large microwave antennas that probably will come after the time frame of this decadal review. However, it is an essential element of the technology development needed to support advanced multivariate retrieval methods that can exploit the totality of the microwave spectral information and will facilitate next-generation watercycle observing systems. It is possible to see how existing technology and extensions thereof would support a multidisciplinary water-cycle measurement strategy—for example, through use of rotating-antenna technology.
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A cross-disciplinary multichannel active and passive microwave concept would provide coincident and improved measurements of many key oceanic, atmospheric, terrestrial, and cryospheric dimensions. One possibility would be for active and passive microwave channels from 1.26 to 37 GHz to share a large (6-m) rotating parabolic deployable mesh antenna. That appears to be feasible as a result of recent advances in antenna technology and would have the effect of enhancing the spatial resolutions of many measures and providing the coincident measurements needed to optimize the retrieval of geophysical information. It would also allow characterization of the multiscale and nonlinear interaction of the turbulent atmosphere and ocean. The mission would be a water-cycle and terrestrial-biomass observatory. The simultaneous multichannel active and passive microwave measurements would allow improved-accuracy retrievals of dimensions that were the focus of several Explorer-class mission concepts. To be concise, that means that the multiple instruments are not just sharing a spacecraft. Their simultaneous measurements lead to retrievals that are not possible with isolated measurements. Furthermore, the simultaneous monitoring of several of the land, atmospheric, oceanic, and cryospheric states brings synergies that will substantially enhance understanding of the global water cycle as a system. A flagship mission based on that concept would combine the following measurements that in the present paradigm constitute individual missions (specific missions referred to elsewhere in this chapter are also indicated): Precipitation measurement (GPM follow-on), Ocean wind, Soil moisture and land freeze-thaw mission (Hydros), Cold-land processes Pathfinder (CLPP), Biomass monitoring, and Very-low-frequency subcanopy and subsurface observations (see, for example, discussion of the MOSS RFI response (70) in the section “Soil Moisture and Freeze-Thaw State”). A shared-antenna subsystem would allow many of the requirements outlined in Table 11.2 to be met. It would accommodate the following measurements: scatterometry at P-, L-, C-, and Ku-bands and radiometry at L-, C-, and X-bands. The measurements would be simultaneous and would have the advantage of a common and steady look angle. The rotating antenna would facilitate a wide swath for 2- to 3-day repeat coverage. Because the antenna subsystem would be shared, the instrument cost would not scale with the number of scatterometer and radiometer channels. Total mission cost would probably be around $700 million to $1 billion, which, although considerably larger than the cost for any of the individual component measurements, would represent a substantial savings relative to the sum of the costs for stand-alone missions for each of the elements. The cost is end-to-end with 30 percent reserves and for 5 years of operation. The instruments would share a single 6-m lightweight deployable mesh reflector capable of supporting multiple frequencies up to Ku-band. The sharing of elements of the antenna and digital subsystems results in substantial cost savings. To reap the benefits of the synergy, some tradeoffs need to be made with respect to the constitutive Explorer-class mission. The principal one is to trade some spatial resolution loss for gains in revisit time and multichannel observations. That trade particularly affects biomass, snow, and deep-soil moisture (P-band). However, because carbon biomass, deep-soil moisture, and snowpack vary slowly, it may be possible to regain the resolution by combining multitemporal passes. On the other hand, the multichannel approach affords advantages to some constituent retrievals—for instance, simultaneous retrieval of vegetation biomass
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TABLE 11.2 Elements of an Integrated Water-Cycle Observing System Mission Science Objective Science Requirement SMAP-extended (includes P-band) Monitor processes that link water, energy, carbon cycles Monitor vegetation and water relationships over land Extend capability of climate and weather prediction Near-surface soil moisture: 4 percent of volumetric content RMSE in top 2–5 cm soil for vegetation cover <5 kg/m2 Root-zone soil moisture: top 50 cm of soil for vegetation cover <20 kg/m2 Land freeze-thaw state: detect state transition to within 1–2 days Biomass monitoring Monitor aboveground forest biomass and terrestrial stock Estimate changes in terrestrial carbon sources and sinks Aboveground woody biomass: 20 percent relative accuracy or 1 kg/m2 SCLP Support operational weather and water-resources applications Study cause and effects of changes in water cycle Develop freshwater inventory Snow water equivalent: 2 cm RMSE in snowpacks <20 cm 10 percent relative in snowpacks >20 cm Ocean-surface monitoring Improve weather prediction with high-resolution ocean wind speed and direction in all-weather conditions Monitor heat content of oceans and improve air-sea interaction modeling and climate prediction Improve weather prediction and characterization of moist processes in models Monitor coastal and open-ocean climate variability and water cycle Extend capabilities for climate and weather prediction Ocean wind speed and direction: 1 m/s and 20° Sea-surface temperature: 0.5°C Cloud water: 2 mm (land), 0.1 mm (ocean) Rain rate: 5 mm/hr Snow water equivalent: 3 cm Sea-surface salinity: 0.2 practical salinity unit would improve soil-moisture retrieval by avoiding the need for auxiliary vegetation information. It should be noted that because altimetry, although based on radar, uses SAR rather than scatterometry, such a system would not monitor surface-water stage and surface area (hence volume) or river slopes. In addition, one shortcoming of shared-antenna systems is that an entire system is susceptible to complications in any of the individual instruments; that is, the cost savings are achieved by accepting the risk of multi-instrument “slippage,” as has been the case with NPOESS. Cloud-Aerosol-Precipitation Initiative One of the key uncertainties in weather, climate, and freshwater supply remains the processes that govern the interaction among aerosols, clouds, and precipitation (see Figure 11.9). The need to understand those processes better has been articulated by a number of studies, including the Intergovernmental Panel on Climate Change report Climate Change 2001: The Scientific Basis (IPCC, 2001), the Strategic Plan for the U.S. Integrated Earth Observation System (IWGEO, 2005), and Our Changing Planet: U.S. Climate Change Science Program for Fiscal Year 2006 (CCSP, 2005). Because aerosols serve as nuclei for cloud
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FIGURE 11.9 Saharan dust storm of July 24, 2003, showing dust cloud over the Atlantic Ocean and Canary Islands off north-west Africa, as captured by NASA’s MODIS instrument on the Terra satellite. Earlier, USSR cosmonaut Vladimir Kovalyonok had observed, “As an orange cloud formed as a result of a dust storm over the Sahara and, caught up by air currents, reached the Philippines and settled there with rain, I understood that we are all sailing in the same boat.” SOURCE: Image courtesy of NASA. particles and affect their growth to precipitation-size particles—as well as influencing the opacity of clouds to sunlight—the close interaction among these processes is evident. The main objective of an integrated cloud-aerosol-precipitation mission would be to provide a more quantitative basis for predicting changes in the planet’s hydrologic cycle and energy balance as a step toward prediction of severe weather, climate, and climate change with much higher confidence than now exists. The cloud-aerosol-precipitation climate problem is complex and progress will probably require a
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coordinated combination of observation and theoretical techniques, platforms and vantage points, and strategies that explicitly plan for integration of the components. The rewards, however, are also extremely high and could include advances in issues of air pollution and human health, availability of freshwater, prediction of weather and extreme events, aerosol effects on climate, and cloud influences on climate. Portions of the space-based component of a coordinated observation plan along those lines were articulated in various RFI responses dealing with cloud-aerosol, aerosol-precipitation, and cloud-aerosol-precipitation relationships. This mission concept and package might involve the addition of instruments to approved missions (e.g., GPM and NPOESS). Additional assets, such as a new high-spectral-resolution lidar for aerosol detection and analysis combined with multifrequency Doppler radar for cloud content and vertical motions, are required. Tropospheric wind observations would also be needed to help to separate the effects of atmospheric motions from the effects of aerosol concentrations. That will be addressed in part by the Aerosol-Cloud-Ecosystem (ACE) mission concept (Parts I and II) but is referred to here as an initiative because the complexity of the problem requires careful coordination of the ACE mission with other proposed missions, such as Wind Lidar (3D-Winds in Parts I in II) and potential international contributions (e.g., EarthCARE and potential follow-ons). A coordinated cloud-aerosol-precipitation initiative requires close coordination among the weather, climate, and water communities. International cooperation, specifically with Japanese and European scientists and agencies, would be needed to bring such an initiative to fruition. A working group to plan and coordinate the addition of targeted observations to future missions should be established immediately to bring this grand challenge in Earth sciences to bear in the 2015–2025 time frame. The initiative—which would involve multifrequency Doppler radars, high-spectral-resolution lidars, wind lidars, and radiometers—would be expensive. It should not be envisioned as a stand-alone mission, but rather as an initiative that would enable systematic planning among national and international agencies to bring the measurement concept to fruition. By systematic leveraging of assets approved for other missions, the cloud-aerosol-precipitation initiative would focus mostly on optimizing (and coordinating) planned missions rather than on new launches. The complexity of the problem, and the great wealth of potential assets in the form of planned missions globally, require that this effort be undertaken by a body more formal than an ad hoc group of interested scientists. End-to-End Information-System Needs Managing the next generation of satellite data will be more challenging, and user requirements much greater, than today. Global water-cycle information must be synthesized from a wide variety of sensors—optical, thermal, passive and active microwave, polar orbiting, geostationary, and so on. Some of the data must be delivered in real time, especially for weather forecasting and flood warnings. Other data must be archived stably to allow retrieval for analysis of climate trends over many decades. Additionally, critical in situ information—such as stream flow, snowpack, and lake and reservoir stage data—must be integrated with the satellite data for optimal interpretation and policy analysis. Scientifically, it is most valuable to have water-cycle data harmonized and accessible from one (possibly virtual) location and at multiple time and space resolutions. For instance, one cannot understand or forecast runoff trends, including floods, without first knowing about precipitation. Lead responsibility for observing various aspects of the water cycle crosses NASA, NOAA, USGS, and USDA. Building and sustaining integrated hydrologic data sets for the United States will require coordination among those agencies that, although technologically feasible, does not yet exist—notwithstanding efforts such as those of the Corporation of Universities for the Advancement of Hydrologic Sciences Hydrologic Information System “WaterOneFlow” Web services enterprise.
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NASA may have responsibility only for delivering the satellite-based data stream, but the agencies named above collectively have responsibility for building the coordinated data system that an integrated hydrologic forecast model requires. Furthermore, although some of the measurements needed to understand and predict water-cycle changes are included in the observation systems that support global weather forecasts (e.g., NPOESS), the measurements that have demonstrated potential for research and applications will need to be sustained (see the section “Prioritized Observation Needs”) to monitor trends and to allow the development of prediction schemes. One or more of the above agencies will need to be responsible for sustaining the observations beyond the individual proposed missions. Global hydrologic information presents an even greater challenge, particularly because in situ data sets are the property of individual nations and are generally less openly available than is the case in the United States (IAHS, 2001). Because water is an economic commodity, cross-border water jurisdiction issues require international data sets. Satellite data represent the only unbiased repeatable measurements available from some countries and so are exceptionally valuable for global hydrologic studies. Given that U.S. scientists must rely more on foreign satellites for data, sharing global data sets will be essential for scientific progress. SUMMARY Water is central to life on Earth, but there are substantial gaps in understanding of the location of stored water and the processes that control its movement. Better understanding of the water cycle not only would have important science benefits but also would benefit society by facilitating more effective management of this renewable resource. That better understanding will require new and more comprehensive measurements, which are feasible only through a combination of remote sensing and in situ observations. The imperative for future water-cycle missions is the ability to address both scientific and societal challenges. The scientific challenge is to integrate in situ and space-based observations to quantify the key water-cycle state variables and fluxes. The centerpiece of this vision will be a series of Earth observation missions that will measure the states, stocks, flows, and residence times of water on regional to global scales, followed by a series of coordinated missions that will address the processes, on a global scale, that underlie changes in the state parameters. The accompanying societal challenge is to make better use of water data produced by in situ and remote-sensing missions to manage water resources more effectively. The four highest-ranked water-cycle missions proposed in this chapter would contribute greatly to the science and societal goals associated with water. The approved GPM mission is recommended for launch without further delay. It will provide diurnal estimates of precipitation at a spatial resolution sufficient to resolve major spatial variations over land and sea. A soil moisture mission (Soil Moisture Active/Passive, or SMAP in Parts I and II) would provide estimates of soil moisture over most of the globe. Soil moisture is a key term in the land surface water balance that controls land-atmosphere fluxes over many parts of the globe (in particular, recycling of moisture from land to atmosphere); it is a key variable that affects the response of runoff to precipitation and hence is critical for flood and drought prediction. A surface water mission (a generalization of which is SWOT, Surface Water and Ocean Topography, in Parts I and II) would provide observations of the variability of water stored in lakes, reservoirs, wetlands, and river channels and would support estimates of river discharge. It would also provide information necessary for water management, particularly in international rivers. And a snow and cold lands mission would estimate the water storage of snowpacks, especially in spatially heterogeneous mountainous regions that are the source areas for many of the world’s most important rivers. Taken together and in coordination with in situ and airborne sensors, these four missions would form the basis of a coordinated effort to observe the terrestrial surface water cycle globally. However, building
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and sustaining integrated hydrologic data sets for the United States will require close coordination among many federal agencies and a commitment to sustaining the observations beyond the individual proposed missions. In addition to the four missions, several that would benefit analyses of the water cycle were highly rated by the water resources panel, albeit less highly than the four identified above. They include missions that would estimate water vapor transport, sea ice thickness and glacier mass balance, groundwater storage and ocean mass, and inland and coastal water quality (see Table 11.1). Those measurements and watercycle issues are discussed in the section “Other High-Priority Water-Cycle Observations” above. Some of the measurements have direct relevance to the needs of other panels, and that synergy was considered in the selection of the integrated missions recommended in Parts I and II. The panel identified several next-generation observation and estimation challenges that must be addressed but need additional time for technology development. They included development of the capability to monitor evaporation directly from space and creation of coordinated observing systems. Two examples of the latter might be a cloud-aerosol-precipitation initiative and a global water-cycle system to “simultaneously” measure precipitation, ocean wind, soil moisture and land freeze-thaw, snow-water equivalent, biomass, and the subsurface. BIBLIOGRAPHY Adam, J.C., E.A.Clark, D.P.Lettenmaier, and E.F.Wood, 2006. Correction of global precipitation products for orographic effects. J. Climate 19(1):15–38. Alsdorf, D.E., and D.P.Lettenmaier. 2003. Tracking fresh water from space. Science 301(12):1491-1494. Alsdorf, D.E., E.Rodriguez, and D.P.Lettenmaier. 2007. Measuring surface water from space. Rev. Geophys. 45:RG2002, doi:10.1029/ 2006RG000197. Barnett, T.P., J.C.Adam, and D.P.Lettenmaier. 2005. Potential impacts of a warming climate on water availability in snow-dominated regions. Nature 438:303–309, doi:10.1038/nature04141. Barry, J.M. 1997. Rising Tide: The Great Mississippi flood of 1927 and How It Changed America. Simon and Schuster, New York. Cahill, A., M.Parlange, T.Jackson, P.O’Neill, and T.Schmugge, 1999. Evaporation from non-vegetated surfaces: Surface aridity methods and passive microwave remote sensing. J. Appl. Meteorol. 38:1346–1351. CCSP (Climate Change Science Program). 2005. Our Changing Planet: The U.S. Climate Change Science Program for Fiscal Year 2006. CCSP, Washington, D.C. Chen, F., T.T.Warner, and K.Manning. 2001. Sensitivity of orographic moist convection to landscape variability: A study of the Buffalo Creek, Colorado, flash flood case of 1996. J. Atmos. Sci. 58(21):3204–3223, doi:10.11 75/1520–0469(2001)058. Curry, J.A., A.Bentamy, M.A.Bourassa, D.Bourras, E.F.Bradley, M.Brunke, S.Castro, S.H.Chou, C.A.Clayson, W.J.Emery, L. Eymard, C.W.Fairall, M.Kubota, B.Lin, W.Perrie, R.R.Reeder, I.A.Renfrew, W.B.Rossow, J.Schulz, S.R.Smith, P.J.Webster, G.A.Wick, and X.Zeng. 2004. SEAFLUX. Bull. Am. Meteorol. Soc. 85:409–424. du Plessis, L.A. 2002. A review of effective flood forecasting, warning and response system for application in South Africa. Water SA 28:129–137. Entekhabi, D., E.Njoku, P.Houser, M.Spencer, T.Doiron, J.Smith, R.Girard, S.Belair, W.Crow, T.Jackson, Y.Kerr, J.Kimball, R.Koster, K.McDonald, P.O’Neill, T.Pultz, S.Running, J.C.Shi, E.Wood, and J.van Zyl. 2004. The Hydrosphere State (HYDROS) mission concept: An Earth system pathfinder for global mapping of soil moisture and land freeze/thaw. IEEE Trans. Geosci. Remote Sens. 42(10):2184–2195. Fu, L-L., and A.Cazenave, eds. 2001. Satellite Altimetry and Earth Sciences: A Handbook of Techniques and Applications. International Geophysics Series Vol. 69. Academic Press, New York. Goni, G., and J.Trinanes. 2003. Ocean thermal structure monitoring could aid in the intensity forecast of tropical cyclones. EOS 84:573–580. Hong, S.-Y., and E.Kalnay. 2000. Role of sea surface temperature and soil-moisture feedback in the 1998 Oklahoma-Texas drought. Nature 408:842–844. Hossain, F., and N.Katiyar. 2006. Improving flood forecasting in international river basins EOS 87(5):49–50. Huang, J., H.M.van den Dool, and K.P.Georgakakos. 1996. Analysis of model-calculated soil moisture over the United States (1931– 1993) and applications to long-range temperature forecasts. J. Climate 9:1350–1362. IAHS (International Association of Hydrological Sciences). 2001. Global water data: A newly endangered species. EOS 82(5):54, 56, 58. IMF (International Monetary Fund). 2002. The World Economic Outlook (WEO) Database. IMF, Washington, D.C.
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Drought planning in Mexico’s Rio Bravo basin, MS Thesis, Department of Civil and Environmental Engineering, University of Washington. Webster, P.J., T.Hopson, C.Hoyos, A.Subbiah, H.-R.Chang, and R.Grossman. 2006. A three-tier overlapping prediction scheme: Tools for strategic and tactical decisions in the developing world. Pp. 645–673 in Predictability of Weather and Climate (T. Palmer and R.Hagedorn, eds.). Cambridge University Press, Cambridge, U.K. ATTACHMENT Table 11.A.1 lists 47 responses to the decadal survey committee’s RFI (see Appendixes D and E) that were considered by the Panel on Water Resources and the Global Hydrologic Cycle for possible relevance to the water cycle. The panel’s use of the RFI responses is discussed in the section “Prioritized Observation Needs” earlier in this chapter.
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TABLE 11.A.1 Water-Cycle-Relevant RFI Responses Examined by the Panel RFI Response Number Response Title Comment 2 SIRICE Submillimeter Infrared Radiometer Ice Cloud Experiment (SIRICE): Daily Global Measurements of Upper Tropospheric Ice Water Path and Ice Crystal Size 5 ATOMMS Active Temperature, Ozone, and Moisture Microwave Spectrometer: Constellation of small satellites to provide high vertical resolution moisture,ozone,temperature and pressure measurements in troposphere and middle atmosphere 9 ARIES Atmospheric Remote-Sensing and Imaging Emission Spectrometer: Observe the infrared spectrum from 3.6 to 15.4 µm at high spatial resolution ≥ 1×1 km globally; both of these features are critical for the study of the hydrology cycle and for understanding the water vapor feedback 13 CHARMS Cloud Height and Altitude-Resolved Motion Stereo-imager 14 CHASM Cloud Hydrology and Albedo Synthesis Mission: Mission to measure the water content of clouds, concurrently with their albedo and cloud-top height 17 Climate Scope Reanalysis Mission Concept A mission to produce, validate, and disseminate physically consistent climate research quality data sets from separate missions and satellite platforms 19 CLPP Cold Land Processes Pathfinder: Advanced Space-based Observation of Fresh Water Stored in Snow 21 COCOA Coastal Ocean Carbon Observations and Applications: Integrated observations (hyperspectal from GEO) and models to discriminate and quantify particulate and dissolved carbon species in coastal waters, as well as the exchanges of carbon between the land, atmosphere, and ocean 23 C-CAN Continuous Coastal Awareness Network will measure sea surface height,coastal currents and winds and sea spectral reflectance from different Earth vantage points at high spatial and/or temporal resolution 25 Daedalus Daedalus: Earth-Sun Observations from L1: Simultaneously observe key solar emission/space weather parameters and spectrally resolved radiances over the entire illuminated Earth to characterize the direct influence of solar variability on the Earth system 27 WOWS Water and Ocean Wind Sensor using active and passive microwave concepts 36 Emory CU Surge GPS to measure ocean wind speed/direction, sea surface height and land surface soil moisture 38 FLORA Global, high spatial resolution measurements of vegetation composition, ecosystem processes and productivity controls, and their integrated responses to climate 42 Grace Follow-on Mission GRACE follow-on 44 GISMO Glaciers and Ice Sheets Mapping Orbiter 46 Global Water Resources Mission An international effort consisting of about two dozen satellite systems, each of which is comparable with current operational GEO and LEO satellites 49 GPS-HOT High resolution/high temporal revisit oceanography mission for mesoscale process characterization, will also yield data suitable for global tsunami warning 50 H2S Ocean Emissions H2S emitting from ocean surface 55 Human-Induced Land Degradation Detecting Human Induced Land Degradation Impact on Semi-Arid Tropical Rainfall Variability. Uses satellite-derived precipitation data, satellite-derived vegetation index data (no apparent observation program proposed) 56 Hydros Hydrosphere Mapper: Radar interferometry system to make high-resolution measurement of the surface of the ocean and water bodies on land 61 CAMEO Composition of the Atmosphere from Mid-Earth Orbit
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RFI Response Number Response Title Comment 62 OOLM Operational Ocean and Land Mission: Wide swath ocean altimeter, and dual frequency (C- or X-band and L-band) SAR, plus Visible/Infrared Imaging Spectrometer on two satellites,for various (primarily ocean/coastal) needs 66 CLAIM 3-D Mission Satellite mission to advance understanding of cloud and precipitation development by measuring vertically resolved cloud microphysical parameters in combination with state of the art aerosol measurements 67 MATH Monitoring Atmosphere Turbulence and Humidity 70 MOSS The Microwave Observatory of Subcanopy and Subsurface is a synthetic aperture radar (SAR) operating at the two low frequencies of 137 MHz (VHF) and 435 MHz (UHF) with the primary objective of providing measurements for estimation of global soil moisture under substantial vegetation canopies (200 tons/ha or more of biomass) and at useful soil depths (2–5 meters) 71 GEOCarb Explorer GEOCarb mission will provide continent-wide measurements of ecosystem carbon and water dynamics with multiple observations per day 72 Multiplatform InSAR Forest Subcanopy Topography and Soil Moisture 74 Suborbital Earth System Surveillance UAVs to be used for synoptic weather, hurricanes, air quality, stratospheric ozone, ozone depleting substances, greenhouse gases, ice sheets, forest fires, droughts, and storm damage 76 Far IR Far-Infrared for understanding natural greenhouse effect, atmospheric cooling by water vapor, and the role of cirrus clouds in climate 79 Integrated Water Cycle Observations Coordinated water cycle observations from space 80 Low-Cost Multispectral Earth Observing System Global land observation system that enhances Landsat and OLI with stereo multispectral imaging, greater coverage, revisit (eight days and better), and higher resolution 82 Surface Uncertainty Surface Shortwave and Longwave Broadband Network Observation Uncertainty for Climate Change Research 83 InSAR InSAR from orbital platform, in particular to produce spatially continuous maps of ground displacements at fine spatial resolution, for natural hazards science and applications 86 OCEaNS Ocean Carbon, Ecosystem and Near-Shore Mission designed to advance quantification of ocean primary production, understanding of carbon cycling,and capacity for predicting ecosystem responses to climate variability 87 OLOM Ocean and Land Operational Mission: Similar to OOLM except that second satellite would carry a 2 frequency Delay-Doppler Altimeter and a Water Vapor Radiometer rather than WSOA 88 Our Vital Skies Program to address scientific questions at the interface between aerosol, cloud and precipitation research using combination of in situ and space-based observations 90 Polar Polar Environmental Monitoring, Communications, and Space Weather from Pole Sitter Orbit 91 ABYSS Radar altimeter for bathymetry, geodesy, oceanography 92 GPSRO Contributions of Radio Occultation Observations to the Integrated Earth Observation System 97 SAVII Spaceborne Advanced Visible Infrared Imager Concept: Hyperspectral measurements in vis-near IR; multispectral measurements in short wave infrared, and multispectral measurements in thermal infrared for vegetation studies, changes in surface cover and composition,and thermal monitoring
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RFI Response Number Response Title Comment 99 SH2OUT Sensing of H2O in the Upper Troposphere 100 GPM Global Precipitation Mission 103 Surface Observatories in Support of Observations of Aerosols and Clouds Surface observations of water vapor, temperature, and winds, plus surface radiative fluxes and cloud and aerosol properties 104 Terra-Luna Earth-Moon science mission that would provide Earth measurements over a relatively short period during Earth-orbiting phase, revisited at intervals of a decade or so, including boreal and tropical forest land cover and biomass mapping, global ocean eddies, coastal currents and tides, and land cover and canopy height 107 Water Vapor Monitoring Missions 108 WatER The Water Elevation Recovery Satellite Mission 110 Climate-Quality Observations from Satellite Lidar Lidar measurements to address the themes of climate variability and change, weather, and water resources and the global hydrologic cycle 111 Advanced ICESat Ice Cloud and land Elevation Satellite NOTE: A complete list of RFI responses is provided in Appendix E. Full-text versions of the responses are included on the compact disk that contains this report.
Representative terms from entire chapter: