1. The SWOT Satellite Mission Concept Surface Water and Ocean Topography This poster describes the surface water portion of the joint concept: “Where is water stored on Earth’s land surfaces, and how does this storage vary in space and time?” Doug Alsdorf, U.S. Hydrology alsdorf.1@osu.edu bprc.osu.edu/water Lee Fu, JPL Oceanography llf@pacific.jpl.nasa.gov sealevel.jpl.nasa.gov Nelly Mognard, CNES Hydrology nelly.mognard@cnes.fr www.legos.obs-mip.fr/recherches/missions/water/ 1. The Problem 3. Measurements Required Two dimensional mappings of h, which yield dh/dt and dh/dx Measurements required to answer the science and applications questions require multi-dimensional sampling protocols distributed globally – essentially a space based solution. Water surfaces are strongly reflective in the electromagnetic spectrum, thus nadir viewing radar altimeters have been highly successful in measuring the elevation of the world’s oceans. Expansion of this technology to inland waters, which have much smaller spatial dimensions than the oceans, has met with some success despite the construction of existing radar altimeters for ocean applications which are designed to average over relatively large areas, and hence are problematic for surface water applications where the lateral extent is comparatively limited. 051015 051015 SRTM DEM 1993 Manacapuru 0 100 200 300 400 500 600 700 800 900 1000 JanFebMarAprMayJunJulAugSepOctNovDec 11 Apr: 772 cm 26 Feb: 532 cm dh = 240 cm Coverage from a pulse limited altimeter severely under samples rivers and especially lakes. For example, a 16-day repeat cycle (i.e., Terra) coverage misses ~30% of rivers and ~70% of lakes in the data bases (CIA-2; UNH; UH) whereas a 120 km swath instrument misses very few lakes or rivers (~1% for 16-day repeat and ~7% for 10-day repeat) Water flow across wetlands and floodplains is complex as demonstrated in the plot of “Actual dh/dt” acquired with repeat-pass interferometric SAR. Because this method has a far off-nadir look angle, it requires flooded vegetation to return the radar pulse to the antenna after it is specularly reflected from the water surface. The repeat pass also limits the method to dh/dt measurements, only (not h, not dh/dx). Global models of weather and climate could be constrained spatially and temporally by stream discharge and surface storage measurements. Yet this constraint is rarely applied, despite modeling results showing that precipitation predicted by weather forecast models is often inconsistent with observed discharge. For example, Roads et al. (2003) found that the predictions of runoff by numerical weather prediction and climate models were often in error by 50%, and even 100% mismatches with observations were not uncommon. Coe (2000) found similar results for climate model predictions of the discharge of many of the world’s large rivers. The inter-seasonal and inter-annual variations in surface water storage volumes as well as their impact on balancing regional differences between precipitation, evaporation, infiltration and runoff are not well known. Recent efforts have demonstrated that direct water surface-to-atmosphere carbon evasion are an important component of the carbon cycle. Calculation of organic carbon fluxes requires knowledge of the spatial distributions of aquatic ecosystem habitats, such as herbaceous macrophytes and flooded forests, and estimates of carbon evasion require measurements of the spatial and temporal variations in the extents of inundation. 2. Science Questions & Societal Applications Lacking spatial measurements of wetland locations and sizes, hydrologic models often do not properly represent the effects of surface storage on river discharge. Errors can exceed 100% because wetlands moderate runoff through temporary storage and change the surface area available for direct interception of precipitation and free evaporation. While earth system models continue to improve through incorporation of better soils, topography, and land-use land- cover information, their representations of the surface water balance are still greatly in error, in part due to the absence of an adequate observational basis for quantifying river discharge and surface water storage. Ka-band SAR interferometric system with 2 swaths, each 50 km in width Produces water and land heights and co-registered all weather amplitude imagery 200 MHz bandwidth (0.75 cm range resolution) Use near-nadir returns for SAR altimeter angle of arrival mode (e.g. Cryosat SIRAL mode) to fill swath No data compression onboard: data down-linked to ground stations 4. The Solution 4. The Solution KaRIN: Ka-band Radar Interferometer. SRTM, WSOA heritage. Images of h globally every 8 days. ~250 Participants from 30+ Countries on 5 Continents (and growing!) visit on the web at: bprc.osu.edu/water Designed by Natalie Johnson and Jonathan Partsch, the Ohio State University Funding provided by CNES, JPL, NASA’s Physical Oceanography and Terrestrial Hydrology Programs, and the Ohio State University Climate, Water, & Carbon Program dh/dx h from SRTM SWOT will be an interferometric altimeter which has a rich heritage based on (1) the many highly successful ocean observing radar altimeters, (2) the Shuttle Radar Topography Mission (SRTM), and (3) a development effort for a Wide Swath Ocean Altimeter. SWOT will provide surface elevation data in a 120 km wide swath using two Ka-band synthetic aperture radar (SAR) antennae at opposite ends of a 10 m boom. Interferometric SAR processing of the returned pulses will yield a 5m azimuth and 10m to 70m range resolution, with elevation accuracy of ± 50 cm. Polynomial based averaging increases the height accuracy to about ± 3 cm. The orbital repeat cycle is designed to permit a global sampling of all surface water bodies about three times a month, or near weekly. As the heritage for SWOT, SRTM is already providing measurements of water surfaces around the world. The top panel shows h measurements on the Amazon River and the derived dh/dx curve. Discharge estimated from these data is within 8% of the gauged discharge. The bottom panel shows h measurements above and below the Hoover Dam in Columbus Ohio; the height difference matches the dam. Although in situ gauge measurements are the backbone of much of our understanding of surface water dynamics globally, these gauge networks provide essentially no information about floodplain flows and the dynamics of wetlands. In situ networks are generally best in the industrialized world and are worse in sparsely settled areas (e.g., high latitudes and tropics). For instance, the network of stream gauges in the Potomac River (expressed in number of gages per unit drainage area) is about two orders of magnitude greater than in the Amazon River basin. In-situ methods provide a one- dimensional, point-based view of water surfaces in situations where a well defined channel boundary confines the flow. In practice, though, water flow and storage changes in many riverine environments are not simple, and involve the spatially complex movement of water over wetlands and floodplains and include both diffusive flows and narrow confined (channel) hydraulics. Wetlands and floodplains are governed by the dynamics of water movement, and as described next, are vital to ecology and to climate and weather. 100% Inundated!