Doug Alsdorf, U.S. WatER PI; Ohio State Univ., Geological Sciences, Mendenhall Lab.; Columbus OH 43210

Drs. Anthes, Charo, and Moore NRC Decadal Survey Earth Science and Applications from Space 500 Fifth Street, NW Washington, DC 20001 Tel: 202 334-3477, Fax at 202 334-3701 email: acharo@nas.edu; rfi@nas.edu May 13, 2005 Re: WatER response to NRC RFI Dear Drs. Anthes, Charo, and Moore On behalf of the participants in WatER and the Earth science community in general, thank you for your time and efforts to organize and lead the NRC Decadal Survey. The enclosed document is a collective effort of international researchers who recognize our scientific and societal needs for spaceborne measurements of terrestrial surface water fluxes. WatER is a international, cooperative mission with participants from 14 countries on four continents. It will be jointly proposed to the European Space Agency and to NASA.

Other groups are planning submissions to this NRC RFI and such efforts are viewed by all as collegial. Planned submissions on scientific concepts that complement WatER include long-term observations of all components of the global water cycle [Hildebrand et al.], measurements necessary to better understand freshwater ecology [Hess, McDonald, et al.], and hydrologic mass from temporally repeated gravity measurements [Watkins et al.]. Submissions on technologies that complement WatER include a radar altimetry method to measure the ocean surface at a high resolution [Smith, Raney, Sandwell, et al.] and an interferometer of design similar to the WatER technology [Fu et al.]. All groups encourage, and will help support, the Decadal Panel toward a systematic and integrated plan for NASA’s Earth observing science.

Sincerely,

Doug Alsdorf 614-247-6908 alsdorf.1@osu.edu

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WatER: The Water Elevation Recovery Satellite Mission

Response to the National Research Council Decadal Survey Request for Information Doug Alsdorf1, Ernesto Rodriguez2, Dennis Lettenmaier3, and Jay Famiglietti4

1Ohio State Univ., 2NASA JPL, 3Univ. of Washington, and 4Univ. of California, Irvine

Preface:

Fresh water is essential for life, yet we have surprisingly poor knowledge of the spatial and temporal dynamics of surface water storage and discharge globally. WatER is a swath based altimetry mission designed to acquire elevations of inland water surfaces (and hence the capability to derive surface water storage and river discharge) at spatial and temporal scales necessary for answering key water cycle and water management questions of global importance. This document is a synthesis of a larger, more encompassing manuscript found on the WatER mission web page (link below). WatER is an international effort with participants from 14 countries (as of 19 April 2005). The concept mission is, in part, an outgrowth of a NASA Surface Water Working Group formed in 1999 subsequent to NASA’s Easton post-2002 planning effort.

Other groups are planning submissions to this NRC RFI and such efforts are viewed by

all as collegial. Planned submissions on scientific concepts that complement WatER include long-term observations of all components of the global water cycle [Hildebrand et al.], measurements necessary to better understand freshwater ecology [Hess, McDonald, et al.], and hydrologic mass from temporally repeated gravity measurements [Watkins et al.]. Submissions on technologies that complement WatER include a radar altimetry method to measure the ocean surface at a high resolution [Smith, Raney, Sandwell, et al.] and an interferometer of design similar to the WatER technology [Fu et al.]. All groups encourage, and will help support, the Decadal Panel toward a systematic and integrated plan for NASA’s Earth observing science. Thank You, Doug Alsdorf 13 May 2005 alsdorf.1@osu.edu WatER Homepage http://www.geology.ohio-state.edu/water Current Participant List (please join us!): http://www.geology.ohio-state.edu/water/participants.php First mission document: http://www.geology.ohio-state.edu/water/WatER_Document.pdf The NASA Surface Water Working Group http://www.geology.ohio-state.edu/swwg

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WatER: The Water Elevation Recovery Satellite Mission

Response to the National Research Council Decadal Survey Request for Information Doug Alsdorf1, Ernesto Rodriguez2, Dennis Lettenmaier3, and Jay Famiglietti4

1Ohio State Univ., 2NASA JPL, 3Univ. of Washington, and 4Univ. of California, Irvine

Table of Contents Preface i Table of Contents ii Summary 1 Introduction 2 A. Difficulties with In-Situ Measurements 3 B. Science and Applications Questions Motivating WaTER 3

B.1 The Global Water and Climate Cycles 3 B.2 Flow Hydraulics 4 B.3 Aquatic Ecosystem Dynamics 5 B.4 Flooding Hazards, Water Resources, and Management 5

C. Measurements Required to Address Science Themes 6 D. Problems With Existing Spaceborne Methods 6 E. Why Images of Water Surface Elevations Are Required 7 F. The Virtual Mission: Sampling Resolutions and the Discharge Question 7 G. The WatER Mission 8

G.1 Instrument Description 8 G.2 Data Processing and Instrument Data Products 10 G.3 Instrument Performance and Calibration 11

H. Potential Secondary Science Targets 12 Conclusions 13 References Cited 13 Appendix A: OMB and OSTP Memo 19 Appendix B: United Nations Resolution 58/217 24 Appendix C: Participants in WatER 26

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WatER: The Water Elevation Recovery Satellite Mission Response to the National Research Council Decadal Survey Request for Information

Summary Surface fresh water is essential for life, yet we have surprisingly poor knowledge of the

spatial and temporal dynamics of surface water storage and discharge globally. For instance, 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 the continents?” Furthermore, key societal issues, such as the susceptibility of life to natural hazards such as drought and floods, cannot be answered with the current global networks designed to observe river discharge but not flood events. WatER is a swath altimeter tailored to inland water conditions that would respond to this need by acquiring elevations of water surfaces at spatial (o(10-100m)) and temporal (o(1 week)) scales necessary for answering key water cycle and water management questions of global importance. Scientific Objectives: WatER will contribute to a fundamental understanding of the Earth system by providing global measurements of surface water elevations that will allow derivation of terrestrial surface water storage changes and discharge, which are critical for understanding the land surface water balance. Societal Objectives: WatER will facilitate societal needs by (1) improving our understanding of flood hazards and the ability to forecast floods by measuring water surface elevations in large rivers and floodplains, which are critical for hydrodynamic models; (2) mapping space-time variations in water bodies that contribute to disease vectors (e.g., malaria); and (3) provide freely available data in near-real time on the storage of water available for potable and other human uses in lakes, rivers, and wetlands in support of water management decision making, particularly in trans-boundary river basins. Measurements Required: WatER will provide repeated (at time intervals of ~3 to ~16 days, depending on location) measurements of spatial fields of water surface elevations (h) for wetlands, rivers, lakes, and reservoirs. Each successive h measurement will allow computation of both spatial variations (water surface slope, ∂h/∂x) and temporal changes in elevation ∂h/∂t, hence allowing computation of both storage changes, and hydraulic gradients which are a primary determinant of river discharge. Technology Description: WatER is 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), which is becoming the basis for most global digital topographic maps, and (3) the development effort of the Wide Swath Ocean Altimeter (WSOA). It is a near-nadir viewing, 120 km wide, swath based instrument that will use two Ka-band synthetic aperture radar (SAR) antennae at opposite ends of a 10 m boom to measure the highly reflective water surface. Interferometric SAR processing of the returned pulses yields 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 repeat cycle will be 16 days thus yielding a global h map every 8 days. Estimated cost, including launch vehicle, bus, interferometer, downlinking, and ground segments is about $270M. Criteria Met: WatER will meet high priority targets identified by President Bush’s Cabinet. The Offices of Science & Technology Policy (OSTP) and Management & Budget (OMB) have both called for a U.S. focus on our “ability to measure, monitor, and forecast U.S. and global supplies of fresh water.” It will contribute strongly to ESAS Panel Themes 5 (Water resources and the global hydrologic cycle), 3 (Weather), 4 (Climate), 2 (Ecosystems), 6 (Human Health),

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and 1 (Societal needs). WatER will contribute to the following IWGEO themes: “protect and monitor water resources”, “understand, assess, predict, mitigate, and adapt to climate variability and change”, “improve weather forecasts”, and “reducing loss of life and property from disasters”. The mission is an affordable ESSP class design, with all components already being space tested. WatER is an international effort with a large support community.

Introduction Understanding the movement of water at and near the land surface requires

measurements of the temporal and spatial variations in water stored in rivers, lakes, and wetlands. Such measurements are now provided almost exclusively by in situ networks, but ability to provide this fundamental information varies greatly by country. In situ networks are generally best in those parts of the industrialized world that are most densely settled, and are worst in sparsely settled areas (e.g., high latitudes), and the underdeveloped world. 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.

An August 12, 2004 memorandum from the President’s Offices of Science and Technology Policy and Management and Budget (Appendix A) called for a focus among U.S. federal agencies on an “ability to measure, monitor, and forecast U.S. and global supplies of fresh water”. This memorandum places the basic needs for hydrologic information at the same level of importance as research on superconductors, molecular electronics, and novel atomic systems. The international community represented by the United Nations similarly has expressed a strong interest in freshwater resource issues. They state in resolution A/RES/58/217 (Appendix B) “that the goals of the Decade should be a greater focus on water related issues at all levels and on the implementation of water-related programmes and projects…”

Despite these priorities, knowledge of the changes in the volume of water stored and flowing in rivers, lakes, and wetlands is poor (details provided in Sections A and B). Furthermore, the spatial extent and variability of wetlands, lakes, reservoirs, and other water bodies are poorly known, even though they strongly affect biogeochemical and trace gas fluxes between the land and atmosphere, and transport to the oceans [e.g., Richey et al., 2002]. Lacking spatial and temporal measurements of water storage changes, changes in discharge, and the variations of inundated area, leads to several basic questions (Section B). Perhaps the most fundamental question we face is, what is the spatial and temporal variability in terrestrial surface water storage and how can we predict these variations more accurately?

Water surfaces are highly reflective in the electromagnetic spectrum, thus nadir viewing radar altimeters have very successfully measured the elevation of oceanic waters [e.g., Cabanes et al., 2001; Cazenave et al., 1999; 2001]. 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 [e.g., Section D; Birkett, 1995, 1998; Birkett et al., 2002; Cazenave et al., 1997; Maheu et al., 2003; Koblinksi et al., 1993]. It is now feasible to produce space-time measurements of surface water elevations using an interferometric radar altimeter (Section E), built upon the heritage of previous altimeters, as well as upon the highly successful Space Shuttle Radar Topography Mission (SRTM), and the extensive development already completed for the Wide Swath Ocean Altimeter (WSOA).

Recognizing the potential scientific and applications benefits of satellite-based surface water observations, NASA’s Terrestrial Hydrology Program (THP) formed a Surface Water Working Group (SWWG, www.geology.ohio-state.edu/swwg) in 1999. The SWWG has encouraged the development of spaceborne technologies capable of collecting global surface

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water measurements that would help fill these voids in global surface water observations [Alsdorf et al., 2003; Alsdorf and Lettenmaier, 2003]. Similarly, a community of researchers throughout Europe met at the “Hydrology from Space” workshop at the French Space Agency (Centre National d’Etudes Spatiales, CNES) in Toulouse on September 2003 (gos.legos.free.fr), and initiated actions intended to lead to a spaceborne platform capable of measuring surface water hydrology.

We propose that WatER (www.geology.ohio-state.edu/water) be an international effort with strong foci on both scientific and applications needs for surface water data. WatER will be particularly valuable in providing globally comprehensive data on the land surface branch of the water cycle, and will be especially useful in an applications context in sparsely populated, and underdeveloped parts of the globe where in situ networks are currently unable to fulfill the needs for basic hydrologic data. Participants in WatER are listed in Appendix C.

A. Difficulties with In-Situ Measurements In situ gauge measurements are the backbone of much of our understanding of surface

water dynamics globally. The have helped to quantify the movement of water (discharge) in river channels, but provide comparatively little information about floodplain flows and the dynamics of wetlands. In-situ methods essentially 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. This complexity is fundamentally a three-dimensional process varying in space and time, which cannot be adequately sampled with one-dimensional observation protocols. Observations of river elevations have been collected across many of the world’s densely inhabited basins for over a century, however a similar effort in the non-industrialized nations is greatly lacking. For example, over 20% of the freshwater discharge to the Arctic Ocean is ungauged, and over two-thirds of the gauges that once provided rudimentary estimates of discharge over Africa have been discontinued over the last two decades [IAHS, 2001; Stokstad, 1999; Shiklomanov et al., 2002]. Therefore, our “ability to measure, monitor, and forecast U.S. and global supplies of fresh water” [OSTP & OMB Memo, Appendix A] using in-situ methods is essentially impossible because of (1) the two-decade long decline in gauge numbers worldwide (including industrialized and underdeveloped countries), (2) economic and infrastructure problems in non-industrialized nations that preclude a global view of land surface hydrologic dynamics, and (3) the physics of water flow in low relief areas where surface water storage movement cannot be captured using in situ observations. Given that rivers and wetlands cover over 4% of the earth’s surface [Prigent et al., 2001; Matthews and Fung, 1987; Matthews, 1993; Mitchell, 1990] and up to 20% of humid river basins such as the Amazon [Hess et al., 2003; Richey et al., 2002], the implications of the problem are global.

B. Science and Applications Questions Motivating WatER WatER would provide a unique capability to address the following questions in ways that

are not possible with existing (or any reasonably feasible expansion of) in situ gage networks.

B.1 The Global Water and Climate Cycles 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

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modeling results showing that precipitation predicted by weather forecast models is often inconsistent with observed discharge. For example, Roads et al. [2003] performed a comprehensive assessment of global weather forecast models, using their predicted precipitation over the continental U.S. and routing the runoff to locations at which stream discharge is observed. They found that the resulting model predictions of streamflow 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. 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 provide water surface area for precipitation and evaporation [e.g., Coe, 2000]. Coupled ice-ocean modeling indicates that fresh water inputs of precipitation and runoff are required to maintain the stable arctic halocline, and therefore river runoff can strongly influence the ocean-climate system [Weatherly and Walsh, 1996]. Therefore, while global Earth system models continue to improve through incorporation of better soils, topography, and land-use land cover maps, their representations of the surface water balance are still greatly in error, in large part due to the absence of an adequate observational basis for quantifying river discharge and surface water storage. These issues are summarized in the following questions, which WatER would enable us to address: What is the spatial and temporal variability in the world’s terrestrial surface water storage? What is the global distribution of freshwater runoff delivered to the oceans and what is its inter-seasonal and inter-annual variability?

B.2. Flow Hydraulics Floodplains are marked by a rich variety of water sources including overbank flows

(regional contributions) as well as groundwater, hyporheic water, local tributary water, and direct precipitation (local contributions) [Mertes, 1997]. Floodplain flow is equally complex and includes diffusive transport across broad, flat pans, temporary storage in lakes of varying

morphologies, and slow drainage through a maze of channels of various widths, depths, degrees of boundary definition, and vegetation densities (Figure 1). This complexity impacts water balance and wetland ecologies. For example, based on Muskingum modeling, Richey et al. [1989] estimate that the main Amazon River alone exchanges about 25% of its average annual flow with its adjacent floodplain. Although this percentage is greater than twice the discharge of the Mississippi River, the estimate is not constrained by any in-situ floodplain gauges. In fact, Alsdorf [2003] used spaceborne synthetic aperture radar (SAR) measurements of the floodplain to demonstrate the possibility of significant errors. Given that the Amazon Basin contains about 750,000 km2 of annually inundated area [Melack and Forsberg, 2001], the impacts

Figure 1. Inundated floodplain of the Amazon River. Singular gauges are incapable of measuring the flow conditions and related storage changes implied by this photo whereas complete gauge networks are cost prohibitive. The ideal solution is a spatial measurement of water heights from a remote platform. (photo courtesy L. Hess)

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likely extend far beyond the mainstem. Similarly, the flow of water through braided rivers is nearly impossible to measure from a singular gauging point because braided rivers contain dynamic channels that increase in number, widen, and shift in response to changes in discharge (Figure 2). Arid, glacierized, and high-latitude basins all typically contain braided rivers, yet their geomorphic complexity limits in-situ efforts to measure flow variations related to the observed retreat of many of the world’s alpine glaciers [Meier, 1984; Haeberli et al., 1989]. Unfortunately, nearly all of the world’s wetlands lack in-situ measurements of storage and flow, while the remoteness and morphology of many Arctic braided rivers limits gauging methods. Lacking observations, we cannot answer key scientific questions such as: How much water is stored on a floodplain and subsequently exchanged with its main channel? What are the local and continental-scale responses of braided rivers to climate induced changes in glacier mass-balances?

B.3 Aquatic Ecosystem Dynamics Recent efforts are demonstrating that direct water surface-to-atmosphere carbon evasion

(outgassing) may be equivalent or significantly greater than fluvial export [Cole and Caraco, 2001; Richey et al., 2002]. The combined export and respiration of CO2 from fluvial and wetland environments globally may be on the order of a Gt C per year [Richey et al., 1980]

which is comparable with the net oceanic uptake of anthropogenic CO2 [Sarmiento and Sundquist, 1992]. Calculations of organic carbon require knowledge of the spatial distributions of aquatic ecosystem habitats such as open water, herbaceous macrophytes, and flooded forests [Melack and Forsberg, 2001] whereas estimates of carbon evasion require measurements of the spatial and temporal variations in the extents of inundation [Richey et al., 2002]. Thus, water levels in rivers and wetlands exert a significant control on biogeochemical fluxes (Figure 3). Lacking these observations, the following questions arise: What are the temporal and spatial

relationships in biogeochemical fluxes from fluvial and wetland environments and how do they contribute to global budgets of carbon and nutrients?

B.4 Flood Hazards, Water Resources, and Management The impact of water availability on mankind is obviously great. Thousands of people

perish each year in floods and over a billion are without adequate supplies of drinking water [Gleick, 2003]. Indeed, population growth by 2025 is expected to impact water availability

Figure 2. Arctic braided river. Braided rivers frequently shift channels during floods, thus the reinstallation of gauges can be required. Instead, a spatial measurement of flow from a remote platform is preferred.

Figure 3. Water level changes between high stage and dry periods include anaerobic conditions facilitating methane production. (photo courtesy L. Hess)

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much more greatly than will greenhouse warming [Vörösmarty et al., 2000] However, lacking measurements of surface water storage changes and fluxes limits predictive capabilities regarding future water availabilities as well as our present ability to predict flood hazards [e.g., Bates et al., 1997; 2000]. Furthermore, many major rivers cross international boundaries, but information about water storage, discharge, and diversions in one country that affect the availability of water in its downstream neighbors is often not freely available (e.g., the Nile, Jordan, Indus, Mekong, Tigris, and Euphrates). In fact, hydrological observations that have implications for water management often are closely guarded, and are only released, if ever, many years after any practical utility has passed. Major health issues are also linked to fresh water storage and discharge. Disease vectors such as malaria are a function of mosquito habitats, which in turn, are directly related to water surface areas. Yet, there is no source for either archival or real-time observations of these highly dynamic and sometimes ephemeral water bodies. The lack of available data related to spatial and temporal changes in flooded area, water storage, and discharge limits efforts to characterize flood hazards, resource availability, trans-boundary management issues, and health issues. Associated societal questions include: What are the water surface elevations across a flood wave in urbanized and natural environments and what are the corresponding extents of inundation? What are the policy implications that freely available water storage data would have for water management? Can health issues related to waterborne diseases be predicted through better mappings?

C. Measurements Required to Address Science and Societal Themes Alsdorf and Lettenmaier [2003] have considered the questions posed in Section B and

determined that answering them will require fundamentally different approaches to measuring surface water storage and rates of change. Hydraulic measurements that are central to the fluid equations of motion include elevations of the water surface (h), temporal changes in water levels (∂h/∂t), water surface slope (∂h/∂x), and inundated area. Essentially, temporally repeated measurements of h provide a basis for estimating ∂h/∂t which summed over an inundated area is a measure of the volume of water lost or gained over a time interval. Water storage is an essential variable in continuity based estimates of mass balance whereas h is a state variable in hydrodynamic models that predict flow hydraulics through channels and wetlands. To provide useful estimates of surface slope, ∂h/∂x, h needs to be measured with accuracy as high as several cm for slope in low gradient rivers (e.g., the Amazon River approaches 1.0 cm/km). Measurements need to extend from about 75º N to 60º S to ensure coverage of the global land areas. Temporal sampling needs to include both smooth and regular hydrographs (e.g., the Amazon) as well as sharp, melt driven pulse-like hydrographs (e.g., the Arctic). As detailed in Section F, we are working toward a careful, model based resolution of the required samplings. At a minimum, we recognize that a monthly equatorial sampling and a sub-monthly Arctic sampling are required to characterize the respective hydrologies.

D. Problems With Existing Spaceborne Methods By themselves, none of the presently operating satellite technologies supply the surface

water measurements needed to accurately model the water cycle and to guide water management [Alsdorf et al., 2003; Alsdorf and Lettenmaier, 2003]. Problems range from poor spatial or temporal resolutions to inability to penetrate clouds or smoke. Coarse spatial resolutions are associated with the Gravity Recovery and Climate Experiment mission (GRACE) and all profiling altimeters. Conventional radar and lidar altimetry is nadir viewing and misses water bodies between orbital tracks (see Section E). Presently operating radar altimeters are not

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designed to sample small freshwater bodies, compared to the surface of the open ocean, thus height and along track resolutions are not ideal [e.g., height resolutions are ~10 cm, at best, and are more typically ~50 cm; Birkett et al., 2002; Birkett 1995, 1998; Maheu et al., 2003; Koblinsky et al., 1993]. Although SRTM produced a high spatial resolution image of land surface topography, the errors over water surfaces are quite large [e.g., ±5.51 m, Hendricks and Alsdorf, 2004]. Poor temporal resolutions are associated with interferometric SAR (repeat orbits are usually monthly at best, which greatly limits the ability to estimate storage change). Interferometric SAR will not work over open-water, instead it requires special hydro-geomorphologies of flooded vegetation [Alsdorf et al., 2000; 2001a, 2001b; Lu et al., 2005; Kim et al., 2005]. Optical sensors cannot penetrate the canopy of inundated vegetation and typically fail to image water surfaces when clouds or smoke are present [e.g., Smith, 1997]. The prevalent vegetation and atmospheric conditions in the tropics lead to very reduced performances for technologies operating in and near the optical spectrum.

E. Why Images of Water Surface Elevations Are Required Rodriguez and Moller [2004] and Rodriguez [2004] have considered the measurement

requirements (Section C) in a study comparing orbits of various existing altimeters to the geographic distributions of rivers and lakes. In particular, they evaluated the capabilities of swath and profiling (nadir-viewing) instruments. They concluded that an interferometric altimeter collecting height samples throughout a 120 km wide swath would sample all, or nearly all, of the world’s rivers and lakes for any typical sun-synchronous orbit. On the other hand, pulse limited altimeters which collect samples only along a profile would severely under-sample rivers and lakes. For example, using a profiling instrument and a 16-day orbital repeat cycle, like that of Terra, misses ~30% of the rivers and ~70% of the lakes in the data bases. Restricting the study to the largest rivers and lakes provides better coverage, but significant water bodies are still missed. Furthermore, the rivers which are covered can have only a few visits per cycle, leading to problems with slope calculations. A 120 km swath instrument misses very few lakes or rivers: ~1% for 16-day repeat and ~7% for 10-day repeat.

F. The Virtual Mission: Sampling Resolutions and the Discharge Question The core problem for spaceborne methods of measuring surface waters is to determine

the value added science and related costs that can be attained from various spatial (i.e., pixel size) and temporal samplings of surface water storage and movement. NASA’s THP has provided funding for the first stage of a “virtual mission” (VM Stage-I), which is designed to emulate samplings of surface water area and fluxes that a real mission would produce. Stage-I of the VM consists of two elements [e.g., Bates and Wilson, 2004; Goteti et al., 2004; Rodriguez and Moller, 2004; Clark et al., 2004]. (1) A macroscale water and energy balance model (Variable Infiltration Capacity, VIC, Liang et al, 1994) implemented at the continental scale and which simulates over large river basins evapotranspiration, soil moisture, snow accumulation and ablation, runoff and streamflow, and surface area and storage variations in lakes and wetlands. (2) A flow hydraulics model (LISFLOOD-FP; Bates and De Roo, 2000; Bates et al., 1997, 2000; Horritt and Bates, 2001) that routes runoff generated by the macroscale hydrology model through various channel and floodplain morphologies (defined by the SRTM DEM). Essentially, the VIC model simulates the water cycle with predictable discharge at a coarse resolution, but provides the essential inflow and outflow boundary conditions for the high resolution modeling in LISFLOOD-FP. LISFLOOD-FP derived inundation area and hydraulics are then synthetically sampled at various resolutions to identify science and cost trade-offs. A preliminary result of

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VM Stage-I is that profiling instruments poorly sample hydraulic regimes. Flow hydraulics are complex across the modeled floodplains such that singular profiles with orbital spacings of 10s of km provide scant information on the spatial distribution of water surfaces. Determination of science and cost trade-offs of these sampling regimes is in-progress, but it is increasingly apparent that spatial samplings need to approach sub-100 m scales.

The discharge question results from the impracticality of measuring flow velocities and channel cross-sections from space. A key issue is that no velocity measuring technique has been proven in space. Instead, Stage-II of the virtual mission will address the discharge question using data assimilation techniques. Hydrodynamic models, such as LISFLOOD-FP, predict discharge and can be tested against remotely sensed hydraulic parameters, (e.g., Bates et al., 1997 have already tested against inundated area). Rather than testing the output of a flow model, the VM Stage-II will focus on ingesting images of h, ∂h/∂x, and ∂h/∂t to update model predictions of discharge and storage change which can then be compared to in-situ observations of river discharge.

G. The WatER Mission G.1 Instrument Description

Conventional altimeters use ranging measurements, which require additional a priori assumptions (e.g., the first return is from the nadir direction) in order to obtain heights and location measurements. In addition, because altimeters are nadir looking instruments, they can only achieve limited across-track spatial resolution, at best on the order of ~1 km. These limitations of radar altimetry can be overcome by the introduction of a second antenna to achieve triangulation by measuring the phase difference between the radar channels [Rodriguez and Martin, 1992; Rosen et al., 2000], a technique called synthetic aperture radar interferometry (IFSAR). This technique is quite mature and has been demonstrated from airborne platforms, and most notably from space by the SRTM, where two IFSARs (at C and X-bands) produced global data with an accuracy of a few meters.

In order to achieve centimeter accuracies over water bodies, a few changes to the SRTM design are required. The major contributor to height errors is the lack of knowledge of the interferometric baseline roll angle: an estimation error of δθ will result in a height error δh = xδθ, where x is the cross track distance. Clearly, the error will be reduced if the swath cross-track distance, or, equivalently, the radar look angles are reduced. In SRTM, the look angle varied from about 20º to about 60º. We propose to limit our maximum look angle to about 4.3º, which will reduce the outer swath error by about 14 times, compared to the SRTM outer swath attitude error. A similar reduction applies to errors due to phase, since the two errors have similar angular signatures [Rodriguez and Martin, 1992].

The reduction in look angles entails a reduction in swath, from 220 km for SRTM, to about 50 km (from 10 km to 60 km in cross-track distance), for the WatER instrument. In order to mitigate this loss in coverage, the instrument looks to both sides of the nadir track to achieve a total swath of 120 km. The isolation between the two swaths is accomplished by means of offset feed reflectarray antennas which produce beams of orthogonal polarizations for each swath. This technology was developed for WSOA, and the antennas have been prototyped and their performance demonstrated.

The reduction in incidence angles has a great benefit for a surface water mapping mission. Radar scattering from water is predominantly specular (i.e., scattering from facets pointing towards the radar), which means that at large off-nadir angles, water will be much

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darker than land [e.g., Hess et al., 1995] and exhibit predominantly diffuse scattering. For larger radar frequencies (e.g., X-band and above), there is sufficient scattering from water bodies at large angles to form an interferogram, but the reduced signal to noise ratio (SNR) leads to noisy returns (i.e., the SRTM h values over water, Hendricks and Alsdorf, 2004). For near-nadir incidence (typically <10º), the situation is reversed: water bodies scatter much more strongly than land. This well known fact has enabled conventional altimeters to be used to measure the height of water bodies, even when a significant fraction of the illuminated area consists of land [e.g., Birkett 1995, 1998; Birkett et al., 2002; Cazenave et al., 1997; Maheu et al., 2003; Koblinksi et al., 1993].

The height noise of the instrument is proportional to the ratio between the electromagnetic wavelength (λ) and the interferometric baseline (B). For SRTM, a 63 m baseline was required to achieve the desired height accuracy using a wavelength of 5.6 cm (λ/B ~ 8.9x10-4). Such a large structure entails large costs. In order to reduce the instrument size, WatER uses a smaller wavelength (Ka-band, λ=0.86 cm), and reduces the interferometric mast size to 10 m (λ/B ~ 8.6x10-4). The technology for a 10 m interferometric mast capable of meeting the stringent mechanical stability required for centimetric measurements has been developed by Able Engineering (SRTM mast manufacturer) in support of the WSOA technology development.

Height noise can be reduced by averaging neighboring image pixels. SRTM averaged about two pixels in order to achieve a 30 m spatial resolution with a meter level height noise. To achieve centimetric height noise, and also to produce images of the water bodies, an increase in

the intrinsic range resolution of the instrument is required. Using a 200 MHz bandwidth system (0.75 m range resolution) achieves ground resolutions varying from about 10 m in the far swath to about 70 m in the near swath. A resolution of about 5 m (after onboard data reduction) in the along track direction is derived by means of synthetic aperture processing. Noise reduction is achieved by averaging over the water body, as described below. The increase in bandwidth requires an appropriate amount of power to be available: building on heritage from the CloudSat mission EIK technology achieves an appropriate SNR for centimetric accuracies.

Figure 4 shows a conceptual implementation of the instrument. The system parameters are summarized in the table on the next page. Note that to achieve the desired resolution, SAR processing must be performed. Current processing technology is not yet able to accommodate the required computations onboard. Therefore, after passing through a data

Figure 4. Conceptual view of the interferometric altimeter. Maximum incidence angle is 4.3º, thus the instrument operates very near nadir where water surfaces are very bright. At Ka band, the interferometer will easily penetrate clouds and relies on subtle canopy openings to penetrate to any underlying water surfaces (openings of only 20% are sufficient). Spatial sampling resolutions are noted in the figure. Height accuracies will be ±50 cm for individual “pixels” thus centimetric accuracies are achieved through polynomial averaging schemes.

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reduction presuming filter, the raw data are stored and subsequently downlinked to the ground. The data

downlink requirements can be met with four 300Mbit/sec X-band receiving stations. The interferometric concept will not work all the way to nadir incidence, leaving a gap in

the nadir coverage. This gap can be filled by making sure that the interferometric antennae include nadir in their illumination pattern. The nadir data can then be processed in conventional SAR mode to implement a full synthetic aperture altimeter whose along-track resolution would be on the order of 5 m (single-look), which would increase the along-track resolution capability of conventional altimeters by more than two orders of magnitude. The CyroSat SIRAL instrument will implement a multi-looking unfocused SAR altimeter, with a resolution on the order of hundreds of meters. Full SAR processing builds on this heritage to implement a next generation nadir altimeter capabilities.

G.2 Data Processing and Instrument Data Products After IFSAR processing, the instrument data products will consist of a radar

interferogram: a complex image in range and along-track direction whose amplitude is proportional to the scene scattering strength, and whose phase can be used to determine the 3-dimensional geolocation of each pixel imaged [Rodriguez and Martin, 1992]. Figure 5 shows an example of simulated returns for the WatER instrument over small tributaries to the Amazon. The image brightness represents backscatter strength, and the rivers and flooded areas can be clearly differentiated. The image color represents the interferometric phase, which can be used to retrieve the look direction.

After interferogram formation, an image classifier will be run over the brightness image to retrieve a water mask. This water mask will then be used to average the water and land data separately (to avoid height contamination), thus reducing speckle noise. Finally the data will be

Parameter Units Value

Mass kg 150 Frequency GHz 35 Peak Transmit Power

W 1500

Duty Cycle % 2.65 RF Operating Power W 790 Operating Time/Orbit % 40 Average Power/Orbit W 316 Raw Data Rate Mbits/s 504 Data Volume/Orbit Gbytes 76 Antenna Length m 4 Antenna Width m 0.2 Boresight Look angle deg 3.5 Baseline Length m 10 Orbit Height km 800 One-Sided Swath km 50 Number of Swaths 2 Range Resolution m 0.75 Azimuth Resolution m 5

Figure 5. Simulated Interferometer return. The interferometer return signal contains both radar brightness (for water boundary delineation) and phase (color) for height estimation. Image geolocation accuracy given by timing accuracy, not platform attitude, unlike an optical imager.

Ran

ge

Along-track Position

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geolocated and calibrated (see below). The level 2 data products will thus consist of topographic snapshots of the water surface together with a co-registered water mask. The topographic data can then be used to estimate h and ∂h/∂x over the water body, while the multi-temporal data collected over multiple revisits can be used to estimate ∂h/∂t. In addition, away from mountains in regions of low relief, it will be possible to derive topography for land surfaces. This will allow the WatER instrument to obtain unprecedented topographic mapping of flood plains by averaging the height data over the mission lifetime.

G.3 Instrument Performance and Calibration IFSAR errors can be separated into

random errors, which are independent from pixel to pixel, and systematic errors, which consist of residual tilts over the entire swath. In addition to these instrumental errors, additional range errors can be caused by media delays due to wet and dry tropospheric delays and ionospheric delays (although these last are small at Ka-band).

Random errors are due to thermal noise in the system, and cannot be compensated. For the WatER system, their magnitude will depend on wind speed over the water surface, but, as an order of magnitude, they are about 0.5 m for 10 m pixels. These errors can, however, be reduced by averaging and typically they will decrease linearly with the surface area used for the averaging. Thus, an option to decrease the effect of random errors is to fit the water surface with a polynomial as a function of downstream distance for rivers [e.g., Hendricks and Alsdorf, 2004], or direct averaging over the water area for standing bodies of water.

As an illustration of the precision of the WatER instrument, Figure 6 presents the expected height and slope performance of the WatER instrument over rivers of various widths and for a range of water surface RMS slope (which governs water surface brightness; RMS = root mean squared). It was assumed for this figure that at least 10 km of the river reach could be imaged for polynomial fitting. Detailed coverage studies show that this situation will occur with a 95% probability. It is clear that the proposed instrument precision exceeds the science needs outlined in Sections C, E, and F.

Residual surface tilts due to lack of knowledge in the baseline roll angle will be responsible for the bulk of the systematic errors. Current star-tracker technology is capable of ~1 arcsec accuracy (e.g., Ball CT-602 used in ICESat). This will result in a residual cross-track height error ranging from about 5 cm in the near swath to about 30 cm in the far swath (or, equivalently, a slope bias of about 5µrad (0.5cm/km) which should be compared to the 10µrad (1cm/km) characteristic of the smallest slope rivers, such as the Amazon. Below, we outline a calibration method for removing these errors.

Media delays due to the dry troposphere and the ionosphere can be compensated to a ~1 cm accuracy by using GPS ionospheric maps and metereological pressure fields. Wet

Figure 6. Interferometer height and slope precision. Height and slope estimates are made by using the radar image to isolate water bodies and fitting a linear height change over the swath. Precision depends on water brightness and the length and width of the imaged water body.

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tropospheric delays, which can be corrected with a 3-frequency radiometer for ocean altimetry, cannot be compensated over land due the intrinsic variability of surface emmisivity. The magnitude of these delays can be as large as 30 cm, but the RMS variability is typically in the range between 3 cm and 5 cm. The typical scale length of these delays, as observed by the TOPEX and Jason altimeters, is on the order of ~100km, so for the WatER instrument the main impact will be a range error of < 5cm, with a residual slope of a few centimeters.

The two errors discussed above can be characterized as height biases with long wavelength slopes. These residual errors can be corrected using multiple calibration techniques. First, data collected by ascending and descending passes can be required to be consistent over the land surface. Second, control points can be generated by the nadir altimeter at river and lake crossings, since the nadir altimeter is very insensitive to roll errors. Finally, a global adjustment can be required to match a topographic map. This topographic map could be the SRTM DEM, which is available for all latitudes smaller than 60º, or the topographic map which will be obtained by averaging the data collected by the WatER instrument over land. Since roll and troposphere are uncorrelated from pass to pass, the average error will be reduced by an order of magnitude after about 100 passes, or after about 2 years of data collection. All of these techniques have been used in the SRTM mission whereas cross-over minimization is routine for altimetric missions such as TOPEX, Jason, or ICESat. We have performed a simulation of the residual long wavelength errors after calibration, assuming that only a pre-existing topographic map with a posting of 90 m and a random height error of 2 m is used for calibration (consistent with SRTM data quality), and found that the residual errors will be sub-centimetric if a calibration region of 60 km x 60 km is used. The calibrated data will suffer from the same long-wavelength biases as the calibration map. For the SRTM data, these biases consist of ~5m level height errors over scales of ~thousands of kilometers, or residual slopes on the order of 5µrad (0.5cm/km). Most importantly, these errors will be consistent from pass to pass, so that the surface topography time series will be consistent and the relative river stage variations can be recovered to centimetric precision.

As a final comment, we note that, due to the near nadir incidence angles of the WatER instrument, the penetration into vegetation canopies will be no worse than that of optical instruments, such as the ICESat laser altimeter, which have demonstrated penetration even for tropical canopies [Harding, 2004]. Since the water surface is much brighter than the vegetation, the requirement that inundation extent be detectable under vegetation canopies will be possible as long as the fraction of canopy gaps times the ratio of water to land brightness be greater than about 2. This leads to a requirement that the canopy have a fraction of gaps greater than about 20% for typical scattering situations. This situation is true for all but the heaviest canopies.

H. Potential Secondary Science Targets Additional science goals that essentially are no-cost benefits of the mission include

lowland topographic mapping and near shore ocean surface elevation mapping. The hydrologic science goals of Section B are directly related to goals of improving topographic mapping of lowland areas, particularly floodplains. Many floodplains experience dry conditions when their topography is exposed. The interferometric altimeter will repeatedly sample these locations with decimeter-scale sized pixels, so that when temporally averaged over the lifespan of the mission, centimeter-scale height accuracies will be yielded. This information is of sufficient resolution for estimating of channel roughness, which is a key parameter in hydrodynamic models. Although the near-nadir viewing geometry of the interferometric altimeter will not provide measurements for accurately constructing DEMs of high relief terrains, this multi-temporal

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averaging approach will provide high spatial and vertical resolutions for topographic regions of low relief. Given that the mission will extend to 75º N, it will also help supplement the SRTM data base which extends to 60º N. In fact, the WatER acronym is easily expanded to WATER, Water And Terrestrial Elevation Recovery.

Near shore ocean surface elevations are poorly known because existing oceanic altimeters lack the required between track spacing. The interferometric altimeter will need these sea level measurements to provide a datum for constructing accurate elevations, thus near shore environments will be included as part of the baseline mission.

Science targets that might be added with only a modest cost increase include open ocean topography mapping and the measurement of sea ice freeboard. Meso-scale ocean circulation patterns are poorly known but the swath mode of the interferometric altimeter permits a dense sampling that can be used to infer these patterns. Costs incurred by this additional science goal are mostly related to a requirement for more data downlink stations. Sea ice freeboard could also be mapped by comparing the elevations across the top of floating ice to the surrounding ocean elevations. Additional costs here are related only to changes in orbital inclination and the expansion of the sampling regime beyond continental edges.

Conclusions Current technology facilitates a satellite mission to map the elevations of surface waters

across the globe and to derive storage changes in lakes, reservoirs, and wetlands, and the discharge of major rivers. The science, technology, societal, and governmental supports for such a mission are now in place. Such mappings are essential for answering outstanding hydrologic questions of global significance as well as freely providing all countries with essential measurements necessary for understanding critical water management issues. The technology is an evolutionary, not revolutionary, step forward with nearly all components already space proven. However, the resulting measurements will be revolutionary in their ability to spatially map the three-dimensional nature of water flow across the globe. A thriving, international community of researchers and engineers has developed the surface water mission concepts and are committed to its success.

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Appendix A: OSTP & OMB Memo www.whitehouse.gov/omb/memoranda/fy04/m04-23.pdf (memo is included on the next 5 pages)

Appendix B: United Nations Resolution 58/217 http://daccessdds.un.org/doc/UNDOC/GEN/N03/507/54/PDF/N0350754.pdf?OpenElement (memo is included on the 2 pages following the OSTP & OMB memo)

Appendix C: WatER Participants http://www.geology.ohio-state.edu/water/participants.php (participant list is included after the UN resolution)

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Appendix C: Participants in WatER Participants and their organizations are presented in alphabetical order. This list is dated 19

April 2005 and includes researchers from 14 countries on 4 continents.

ESA and NASA have both made explicit that the missions they choose to fund must be the highest priority of a vocal science and user community. We are inviting you to be part of that critical support network. A “participant” differs from a satellite mission team member. Participants are part of a broad category of researchers and users (e.g., decision support persons) who are in favor of WatER and its science and societal goals. We welcome insights from our participants that will help the mission be more successful. For example, thoughts on your requirements for measurements and unanswered science questions are most welcomed. Mission team members, on the other hand, are responsible for very specific and rather tedious tasks should WatER eventually become a fully funded satellite mission. Mission team members have not been decided, and final selection is dependent upon ESA and NASA regulations. We will truly make every effort to ensure that participants are listed on all WatER proposals to ESA and NASA. However, the space agencies have individual formatting rules and may object. Still, we continue to push to fully recognize everyone who wants to be a part of WatER – this is not an exclusive effort, rather a mission for everyone!

BRL Ingénierie Laurent Tocqueville

CNES & LEGOS Georges Balmino Stéphane Calmant Anny Cazenave Jean-François Crétaux Bruno Cugny Bruno Lazard Yves Ménard Nelly Mognard Eric Thouvenot

CEMAGREF Pascal Kosuth

Collecte Localisation Satellites Ouan-Zan Zanife

China Ministry of Water Resources Jianyun Zhang

Delft University of Technology Nick van de Giesen

DLR German Aerospace Center Peter Gege Andreas Neumann

EU Joint Research Center Stefan Niemayer

Geophysical Institute, Univ. of Alaska Fairbanks

Claude Duguay

George Mason University Paul Houser

Global Runoff Data Centre Thomas Maurer

Institut de Physique du Globe de Paris Stephane Jacquemoud

Institute for Oceanography, Hamburg Detlef Stammer

INPA-CPEC Bruce Forsberg

ITC – The Netherlands Bob Su Remco Dost Wim Timmermans

JAXA Ake Rosenquist

Lake Superior State University John Lenters

LMTG Frédérique Seyler Jean-Loup Guyot

NASA Goddard Space Flight Center Peter Hildebrand Mike Jasinski Christa Peters-Lidard

NASA Jet Propulsion Laboratory Jeff Booth Richard Gross

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Yunjin Kim Kyle McDonald Delwyn Moller Eni G Njoku Ernesto Rodriguez James Smith

National Academy of Sciences of the Kyrgyz Republic

Dushen Mamatkanov National Hydrology Research Centre

Al Pietroniro Ohio State University

Doug Alsdorf Mike Bevis Natalie Johnson Hahn Chul Jung Peter Luk Brian Kiel Frank Schwartz C.K. Shum Kevin Toomey

Politecnico di Milano Marco Mancini

Princeton University Eric Wood

Russian Academy of Sciences Andrey Kostianoy

San Diego State University Ed Beighley

State University of New York Theodore Endreny

Technische Universitaet Dresden Reinhard Dietrich Martin Horwath Claudia Walter

United States Bureau of Reclamation Daniel O'Connell

Universidad de Concepcion, Chile Rodrigo Abarca del Rio

University of Buffalo Matt Becker

University of Bologna Alberto Montanari

University of Bristol Paul Bates Matt Horritt

University of Calgary Alex Braun

University of California Berkeley Elizabeth Boyer Xu Liang

University of California Irvine Jay Famiglietti

University of California Los Angeles Larry Smith

University of California Santa Barbara John Melack

University of De Montford Philippa Berry

University of Exeter Matt Wilson

University of Lancaster Keith Beven

Universit of New Hampshire Charles Vörösmarty Balazs Fekete

University of Maryland Charon Birkett

University of South Carolina Venkat Lakshmi

University of Texas at Austin Daene McKinney

University of Washington Stephen Burges Dennis Lettenmaier Bill Plant

Vrije Universiteit Brussel Okke Batelaan

Wageningen Univ. & Research Ctr. Peter Troch

Western Michigan University Philip Micklin

Woods Hole Research Ctr. Michael Coe Josef Kellndorfer