1. A cold season mission that would estimate the water storage of snow packs, especially in spatially heterogeneous mountainous regions that are the source areas for many of the world’s most important rivers.

Taken together, these four missions would form the basis for a coordinated effort to observe most components of the surface-water cycle globally.

In addition to these four, other missions recommended in NRC (2007) primarily for other areas of geoscience, but with direct application to water science and applications, included missions that would estimate water vapor transport, sea ice and glacier mass balance, groundwater and ocean mass, and inland and coastal water quality. Details of these missions are given in NRC (2007).

These recommended missions would clearly contribute greatly to the kinds of scientific investigations illustrated by the case studies in Chapter 4 and proposed for hydrologic, environmental engineering, and ecological observatories. For example, the improved estimates of light rain and snow would benefit virtually all of the case studies and proposed observatories. Since soil moisture (and its freeze/thaw state) is the key variable that links the water, energy, and biogeochemical cycles, and is a key determinant of evapotranspiration (NRC, 2007), the same could be said for the soil moisture mission. The cold season mission would be directly applicable to snowy regions of highly variable topography, such as that illustrated in “Mountain Hydrology in the Western United States.” The surface-water mission, based on a radar altimeter, would be able to capture the spatial dynamics of many periodically flooded areas, thus assisting in studies of water-related disease, for example, “Water and Malaria in Sub-Saharan Africa.” It might also help capture the slight variations in water elevations that drive the ecology and nutrient cycling described in “Monitoring the Hydrology of the Everglades in South Florida.”

The other four missions cited in the water cycle chapter of the decadal report would also have obvious impacts on studies that attempt to integrate observations at different scales. For example, the groundwater and ocean mass mission, known as the Gravity Recovery and Climate Experiment (GRACE) follow-on mission, could lead toward better evapotranspiration estimates by improving the terrestrial water storage change, as suggested in the Southern High Plains case study. And the inland and coastal water quality mission would have a steerable 250 m resolution spectrometer designed to quantify the response of marine ecosystems to short-term physical events—one of the primary topics of the Neuse River Basin and Estuary Study. Details of these missions are given in NRC (2007).

The water-related satellite missions recommended by the decadal study are consistent with the vision, findings, and recommendations of this study. The measurements and retrievals from these missions should contribute to the vision offered in this report of ground-to-space integrated observations systems feeding into decision-support systems, if they are strategically combined with a multiagency, ground-based measurement strategy.

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