The SWOT Mission and Its Capabilities for Land Hydrology [chapter]

Sylvain Biancamaria, Dennis P. Lettenmaier, Tamlin M. Pavelsky
2016 Space Sciences Series of ISSI  
28 Surface water storage and fluxes in rivers, lakes, reservoirs and wetlands are currently poorly 29 observed at the global scale, even though they represent major components of the water cycle and 30 deeply impact human societies. In situ networks are heterogeneously distributed in space, and many 31 river basins and most lakesespecially in the developing world and in sparsely populated regions -32 remain unmonitored. Satellite remote sensing has provided useful complementary observations,
more » ... 33 no past or current satellite mission has yet been specifically designed to observe, at the global scale, 34 surface water storage change and fluxes. This is the purpose of the planned Surface Water and 35 Ocean Topography (SWOT) satellite mission. SWOT is a collaboration among the (U.S.) National 36 Aeronautics and Space Administration (NASA), Centre National d"Études Spatiales (CNES, the 37 French Spatial Agency), the Canadian Space Agency (CSA), and the United-Kingdom Space 38 Agency (UKSA), with launch planned in late 2020. SWOT is both a continental hydrology and 39 oceanography mission. However, only the hydrology capabilities of SWOT are discussed here. 40 After a description of the SWOT mission requirements and measurement capabilities, we review the 41 SWOT-related studies concerning land hydrology published to date. Beginning in 2007, studies 42 demonstrated the benefits of SWOT data for river hydrology, both through discharge estimation 43 directly from SWOT measurements and through assimilation of SWOT data into hydrodynamic and 44 hydrology models. A smaller number of studies have also addressed methods for computation of 45 lake and reservoir storage change or have quantified improvements expected from SWOT compared 46 to current knowledge of lake water storage variability. We also briefly review other land hydrology 47 capabilities of SWOT, including those related to transboundary river basins, human water 48 withdrawals, and wetland environments. Finally, we discuss additional studies needed before and 49 after the launch of the mission, along with perspectives on a potential successor to SWOT. 50 51 Keywords: Surface Water and Ocean Topography (SWOT) satellite mission; continental surface 52 waters; lakes; reservoirs; rivers 53 54 3 1. SWOT mission overview 55 56 1.1. The needs for a global water surface mission and its requirements 57 58 In the late 1990s and early 2000s, the crucial need for more quantitative data on spatiotemporal 59 dynamics of surface waters at a global scale became clear in context of a declining in situ gage 60 network and increasing need to observe and model the global water cycle (Alsdorf et al. 2003). To 61 address this challenge, Alsdorf and Lettenmaier (2003) advocated development of a "topographic 62 imager" satellite mission with ~100 m spatial resolution (to observe main channels, floodplains and 63 lakes), temporal resolution on the order of a few days (to sample flood waves and river dynamic at 64 basin scale), and capability to measure height changes that characterize variations in river discharge 65 and lake water storage. Alsdorf et al. (2007) provided a more in-depth study showing that "spatial 66 and temporal dynamics of surface freshwater discharge and changes in storage globally" are poorly 67 known because: 68 -in situ networks are very heterogeneous (some countries have dense networks, whereas others 69 have a few measurements points), 70 -these data are not always shared at the international level, 71 -current satellite missions do not provide measurements adequate to observe global spatio-temporal 72 dynamics of continental water surface. 73 For that reason, Alsdorf et al. (2007) proposed a new satellite mission based on synthetic aperture 74 radar (SAR) interferometry, called Water and Terrestrial Elevation Recovery (WATER). The 75 concept of this satellite mission is built on the legacy of the Shuttle Radar Topography Mission 76 (SRTM) and the Wide Swath Ocean Altimeter (WSOA). SRTM (Farr et al. 2007) was a SAR 77 interferometer in C-and X-bands that flew in February 2000 on the NASA Space Shuttle 78 Endeavour. SRTM provided a near-global Digital Elevation Model (DEM) at 90 m spatial 79 resolution between 60°S and 60°N, but because of the specular returns characteristic of its oblique 80 look angles (between 30° and 60°) it provided poor measurements of surface water. Because the 81 two interferometric antennas were separated by a 60 m mast, construction of an SRTM-like system 82 on a satellite platform would be problematic. A similar concept, WSOA, was envisioned as an 83 additional payload to the altimetry Jason-2 satellite mission with the aim of imaging ocean 84 topography. The distance between the two Ku-band antennas was set to 6.4 m to facilitate inclusion 85 on a satellite platform (resulting in kilometric pixel resolution), and a near-nadir look angle was 86 chosen to better observe the ocean surface (Fu and Rodríguez 2004). WSOA was definitely 87 withdrawn in 2004 and never flown. To adapt this concept to the needs of continental water surface 88 observation, Alsdorf et al. (2007) proposed to use Ka-band instead of Ku-band, allowing better 89 4 spatial resolution (see section 1.2). In 2007, in its Decadal Survey (NRC 2007), the National 90 Research Council recommended to NASA this new satellite mission, under the name Surface Water 91 and Ocean Topography (SWOT, https://swot.jpl.nasa.gov/), to measure both the ocean and land 92 water surface topography. Since then, SWOT has been collaboratively developed by NASA, the 93 Centre National d"Etudes Spatiales (CNES, the French space agency) and more recently the 94 Canadian Space Agency (CSA/ASC) and the United-Kingdom Space Agency (UKSA). Currently, 95 SWOT is planned for launch in late 2020. It will observe the whole continental waters-estuaries-96 ocean continuum and therefore link the ocean and hydrology scientific communities. However, in 97 this paper, the ocean component of the mission will not be addressed. 98 Figure 1 gives an overview of the main spatiotemporal physical processes related to the land 99 hydrological cycle and the SWOT observation window. SWOT is designed to observe a large 100 fraction of rivers and lakes globally and will provide robust observations of their seasonal cycles. 101 However, at least by itself, it is not conceived to observe climate-scale variability (and especially 102 climate change) and will not be able (except on rare occasions) to monitor flash floods. As stated by 103 Rodríguez (2015) , SWOT aims to address the following hydrologic science questions: 104 -What are the temporal and spatial scales of the hydrologic processes controlling surface water 105 storage and transport across the world's continents? 106 -What are the spatially distributed impacts of humans on surface water, for example through water 107 impoundment behind dams, withdrawals and releases to rivers and lakes, trans-boundary water 108 sharing agreements, diversions, levees, and other structures? 109 -What are the regional-to global-scale sensitivities of surface water storages and transport to 110 climate, antecedent floodplain conditions, land cover, extreme droughts, and the cryosphere? 111 -Can regional and global extents of floodable land be quantified through combining remotely 112 sensed river surface heights, widths, slopes, and inundation edge with coordinated flood modeling? 113 -What are the hydraulic geometries and three-dimensional spatial structures of rivers globally, 114 knowledge of which will improve our understanding of water flow? 115 The scientific rationales for these questions and the measurement needs are presented in the 116 SWOT Mission Science Document (Fu et al. 2012) . Based on these needs, the SWOT Science 117 Requirements (Rodríguez 2015, summed up in Table 1 ) have been derived to design the SWOT 118
doi:10.1007/978-3-319-32449-4_6 fatcat:id24d3wd6ffcbhkll5ijf7oyou