More Than Just Surface Water Topography: Phenomenology Studies for the Surface Water and Ocean Topography (SWOT) Mission

The Synthetic Aperture Radar (SAR) frequency on the Surface Water and Ocean Topography (SWOT) Mission is Ka-band: 35.75 GHz, which equates to an approximately 8mm wavelength. This is a relatively high-frequency system compared with the C-band on the now-ubiquitous Sentinel-1 or the L-band from UAVSAR and soon-to-launch NISAR Mission. Lower frequencies are the preferred method for most land surface studies because they have all-weather capabilities, and they can penetrate vegetation to reveal sub-canopy ground deformations, they make water detection easy as water surfaces are uniform; lower frequencies make these observations easier, higher frequencies make these observations harder. As a high-frequency system, SWOT was never designed to penetrate canopies or examine ground deformations. Rather, the high frequency from SWOT was selected for its potential to produce very high-resolution observations and have strong sensitivities to surface water, with the primary goals of measuring water surface elevations and water surface extents with high accuracy. Knowing that the high-frequency system would be sensitive to noise from vegetative land components, the mission requirements firmly asserted that the aim was to observe purely open water environments with high accuracy; vegetated water surfaces were to be assessed independently from the official SWOT open water algorithm goals. Recent studies being published have demonstrated that, as promised, SWOT can produce high-accuracy water surface elevations and water surface extents for the majority of cases, including in some sparsely vegetated wetlands. While there are still many studies to be conducted on SWOT open water, this presentation examines other phenomenological attributes of SWOT observations. As a swath mapper, SWOT's observations are acquired for 64km x 64km tiles covering both land and water. As a high-frequency system, with sensitivities to <1cm features, SWOT is not expected to penetrate the canopy-- it should observe it; SWOT is not expected to make water surfaces appear uniform-- it should highlight water surface roughness. Prior studies have demonstrated SWOT sensitivities related to 1) wind-driven water surface roughness, 2) vegetation structure, and 3) sub-canopy ponding and soil moisture. This presentation highlights progress in examining SWOT observations for More Than Just Surface Water Topography in support of improving SWOT discharge algorithms and other critical water cycle algorithms, such as for evaporation, transpiration, and canopy interception, for further reaching improvements to water resources research.