Measuring surface water from space

Abstract

Surface fresh water is essential for life, yet we have surprisingly poor knowledge of the spatial and temporal dynamics of surface freshwater discharge and changes in storage globally. For example, we are unable to answer such basic questions as “What is the spatial and temporal variability of water stored on and near the surface of all continents?” Furthermore, key societal issues, such as the susceptibility of life to flood hazards, cannot be answered with the current global, in situ networks designed to observe river discharge at points but not flood events. The measurements required to answer these hydrologic questions are surface water area, the elevation of the water surface (h), its slope (∂h/∂x), and temporal change (∂h/∂t). Advances in remote sensing hydrology, particularly over the past 10 years and even more recently, have demonstrated that these hydraulic variables can be measured reliably from orbiting platforms. Measurements of inundated area have been used to varying degrees of accuracy as proxies for discharge but are successful only when in situ data are available for calibration; they fail to indicate the dynamic topography of water surfaces. Radar altimeters have a rich, multidecadal history of successfully measuring elevations of the ocean surface and are now also accepted as capable tools for measuring h along orbital profiles crossing freshwater bodies. However, altimeters are profiling tools, which, because of their orbital spacings, miss too many freshwater bodies to be useful hydrologically. High spatial resolution images of ∂h/∂t have been observed with interferometric synthetic aperture radar, but the method requires emergent vegetation to scatter radar pulses back to the receiving antenna. Essentially, existing spaceborne methods have been used to measure components of surface water hydraulics, but none of the technologies can singularly supply the water volume and hydraulic measurements that are needed to accurately model the water cycle and to guide water management practices. Instead, a combined imaging and elevation‐measuring approach is ideal as demonstrated by the Shuttle Radar Topography Mission (SRTM), which collected images of h at a high spatial resolution (∼90 m) thus permitting the calculation of ∂h/∂x. We suggest that a future satellite concept, the Water and Terrestrial Elevation Recovery mission, will improve upon the SRTM design to permit multitemporal mappings of h across the world's wetlands, floodplains, lakes, reservoirs, and rivers.

Technical note: Bathymetry observations of inland water bodies using a tethered single-beam sonar controlled by an unmanned aerial vehicle

“…Detailed bathymetry maps of inland water bodies are essential for simulating flow dynamics and forecasting flood hazard (Conner and Tonina, 2014;Gichamo et al, 2012;Schäppi et al, 2010), predicting sediment transport and streambed morphological evolution (Manley and Singer, 2008;Nitsche et al, 2007;Rovira et al, 2005;Snellen et al, 2011), and monitoring instream habitats (Brown and Blondel, 2009;Powers et al, 2015;Strayer et al, 2006;Walker and Alford, 2016). Whereas exposed floodplain areas can be directly monitored from aerial surveys, riverbed topography is not directly observable from airborne or space-borne methods (Alsdorf et al, 2007). Thus, there is a widespread global deficiency in bathymetry measurements of rivers and lakes.…”

Three decades of Landsat-derived spring surface water dynamics in an agricultural wetland mosaic; Implications for migratory shorebirds

“…Despite this small areal extent, there are substantial gaps in our knowledge of the distribution and variability of terrestrial surface water resources over time (Alsdorf et al, 2007;Turpie et al, 2015). Freshwater is poorly monitored and managed across the globe, especially with regard to wetlands and water applied to agricultural lands (Alsdorf et al, 2007;Pimentel et al, 2004).…”

“…Despite this small areal extent, there are substantial gaps in our knowledge of the distribution and variability of terrestrial surface water resources over time (Alsdorf et al, 2007;Turpie et al, 2015). Freshwater is poorly monitored and managed across the globe, especially with regard to wetlands and water applied to agricultural lands (Alsdorf et al, 2007;Pimentel et al, 2004). Even in California, which has one of the most highly engineered and intensively managed water systems in the U.S., water rights in some areas represent 1000% of the surface water supply (Grantham and Viers, 2014;Jenkins et al, 2004).…”

“…Satellite remote sensing is key to understanding surface water resources and how they are responding to intensified appropriation and modification by humans (Alsdorf et al, 2007;Turpie et al, 2015). In recent years, several global inundation analyses have been released that have moved from static, coarse scale (>1 km) products (e.g., Bontemps et al, 2011;Lehner and Döll, 2004;Verpoorter et al, 2014) to more dynamic offerings, such as the Global Inundation Extent from Multi-Satellites (Fluet-Chouinard et al, 2015) and the MODIS water mask (Carroll et al 2009).…”

“…Landsat Thematic Mapper imagery can be used to map open water at 30-m resolution (Alsdorf et al, 2007;Baker et al, 2006) appropriate for resolving smaller, shallow wetlands that provide important migratory shorebird habitat (Strum et al, 2013). There is great potential for tracking long-term surface water dynamics given Landsat's <16-day revisit period from 1983 to the present (e.g., Feng et al, 2016;Mueller et al, 2016;Tulbure et al, 2016).…”