This chapter is divided into four main sections. First, current and emerging sensor networking technologies are described in detail, with a focus on embedded sensor networks, which will provide a platform or ground-truthing for many of the other methods. Second, recent and emerging biogeochemical sensor technologies, many of which can be integrated into emerging in-situ embedded sensor networks to broaden the parameters measured at low cost, are described. Third, current capabilities and potential for Earth observations from airborne platforms are discussed. Finally, current capabilities and potential for Earth observations from spaceborne platforms are reviewed.

Although the chapter is organized in this manner, the different technologies tend to work best when they are well integrated across scales, using information from one scale to help refine strategies at another scale and purposely focusing efforts for data collection at different scales on specific locations of interest, with appropriate time and density of sampling. Temperature is a key example. It has numerous applications, such as industrial water management, aquatic habitat mapping, and crop evapotranspiration estimates. It is also a quantity that can be measured using in-situ, airborne, and spaceborne platforms.

IN-SITU SENSOR AND SENSOR NETWORKING TECHNOLOGIES

Hydrologic science has used networks of physical and, in many cases, chemical sensors for decades. Examples include the U.S. Geological Survey (USGS) stream gaging network, the Natural Resources Conservation Service (NRCS) snow telemetry network and more recently their soil moisture Soil Climate Analysis Network (SCAN), and the National Oceanic and Atmospheric Administration (NOAA) cooperative weather station network. Besides these operational networks, there is a wide range of state, local, and research networks that include state departments of transportation that monitor weather impacts on highways, state environmental and agricultural services, and research networks such as the Ameriflux network for monitoring carbon and water fluxes. The hydrologic research community has just begun to take advantage of recent developments in sensor technologies, wireless communications, and cyberinfrastructure to develop increasingly sophisticated sensor networks allowing for sampling at greater spatiotemporal resolution and for more comprehensive ‘sensor-to-scientist’ operation (e.g., Barrenetxea, 2006; Cayan et al., 2003; Hanson et al., 2003; Hamilton et al., 2007; Harmon et al., 2007; Seders et al., 2007).

State-of-the-art sensing capabilities for environmental observatories reflect the co-evolution of sensors (NSF, 2005), communication technologies (Porter et al., 2005), and cyberinfrastructure (Estrin et al., 2003). There are both promising opportunities and major challenges associated with effectively linking space-based and ground-based environmental observations. This section contains (1) a



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