The SWOT Satellite Mission ConceptSurface 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/waterLee 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/

• 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 Solution4. 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

1. The Problem1. The Problem100% Inundated! 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.

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.

Funding provided by CNES, JPL, NASA’s Physical Oceanography and Terrestrial Hydrology Programs, and the Ohio State University Climate, Water, & Carbon Program

2. Science Questions & 2. Science Questions & Societal ApplicationsSocietal 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.

3. Measurements Required3. Measurements Required Two dimensional mappings of h, which yield dh/dt and dh/dx

dh/dx

h

from SRTM

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.

0 5 10 15

0 5 10 15

SRTM DEM

1993 Beruri

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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

11 Apr: 618 cm

26 Feb: 375 cm

dh = 243 cm

1993 Itapeua

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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

11 Apr: 561 cm

26 Feb: 372 cm

dh = 189 cm

1993 Manacapuru

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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

11 Apr: 772 cm

26 Feb: 532 cm

dh = 240 cm

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.

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-HM 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. WATER 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.

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).

As the heritage for WATER HM, 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.