Session 9

13/11 – Friday

Afternoon

14:30 - 14:40 Motivation of the SWOT mission for hydrology (S. Cherchali)

14:40 - 15:00 SWOT science objectives for lakes and reservoirs (Jean-françois Cretaux)

Authors
  1. Jean-françois Cretaux (.)
Abstract

Large lakes affect climate on a regional scale through albedo and evaporation. In some regions highly ephemeral lakes provide information on extreme events such as severe droughts or floods. On the other hand endorheic basin lakes are sensitive to changes in regional water balance. In a given region covered by a group of lakes, if the records of their level variations are long enough, they could reveal the recurrence of trends in a very reliable and accurate manner. Lakes are thought to have enough inertia to be considered as an excellent proxy for climate change. Moreover, during last century thousands of dams have been constructed along the big rivers worldwide, leading to the appearance of large reservoirs. This has several impacts on the basins concerned by those constructions, as well as effects on global sea level rise. The response of water levels to regional hydrology is particularly marked for lakes and inland seas of semi-arid regions.

Accurate and continuous monitoring of lakes and inlands seas is possible thanks to the success of satellite altimetry missions since 1993. Global processing of the data of these satellites can provide time series of lakes surface height over the entire Earth at different temporal and spatial scales with a sub-decimetre precision. Yet, many small or even medium size lakes and reservoirs are still missed by the current nadir-looking altimeters. Only a swath altimeter like SWOT which will provide altogether the changes in height and extent of these water bodies will be able to provide a complete view of their evolution, either it has a climate or an anthropic origin.

15:00 - 15:20Sciences objectives of the SWOT mission for rivers (Sylvain Biancamaria)

Authors
  1. Sylvain Biancamaria (LEGOS, 14 avenue Edouard Belin, 31400 Toulouse, France)
  2. Tamlin Pavelsky (University of North Carolina)
  3. Jean-françois Cretaux (LEGOS-CNES)
Abstract

The future Surface Water and Ocean Topography mission is currently under development at NASA, CNES, CSA and UKSA for a planned launch in 2020. It will provide elevation maps of the top of all water bodies over the continent and of all oceans, in between 78°S and 78°N. This presentation will provide a review of the SWOT mission, science objectives of the mission for rivers, expected measurements and products and current studies done by an international team of scientists aiming to estimates benefits of SWOT data for better understanding the continental part of the Earth water cycle.

15:20 - 15:40 The SWOT mission: a wide swath altimeter for observing ocean surface topography and surface water (Thierry Lafon)

Authors
  1. Thierry Lafon (CNES)
Abstract

NASA, CNES, the Canadian Space Agency and the UK Space Agency are collaborating on a new space mission to build and operate the Surface Water and Ocean Topography (SWOT) satellite, based on a new technical concept: wide-swath interferometric altimetry. The partnership is based on 25 years of CNES/NASA cooperation in the field of oceanographic altimetry with Topex/Poseidon and the Jason satellite series (Jason-1, Jason-2 and the 2015-planned launch of Jason-3). SWOT is the first satellite to join both land hydrology and oceanography communities into a single satellite mission. With a launch date scheduled for late 2020, SWOT will allow mapping surface water extent and elevation of rivers wider than 100 m, lakes larger than ~0.06 km2, and will resolve ocean and near coastal eddies to scales around 10 km. SWOT will be considered as a revolution in hydrology science by providing the first global survey of Earth’s surface water. SWOT mission is now in Phase B. A new Mission Science Team will be formed to start in 2016.
The key instrument payload is a Ka-band radar interferometer (KaRIN) capable of making high-resolution wide swath altimetry measurements with antennae at opposite ends of a 10 m boom. Because the elevation error increases with the look angle, KaRIN has a near nadir look angle from 1° to 4° leading to a swath of 50 km each side of the nadir (Figure 1). The 200 MHz bandwidth achieves cross-track ground resolutions varying from about 10 m in the far swath to about 60 m in the near swath. A resolution of about 2 m in the along track direction is derived by means of synthetic aperture processing. SWOT will provide a nearly global coverage of the earth between ±77.6° latitude due to its inclination and its repeat period of 21 days. A one-day repeat phase for initial Cal/Val will last minimum 60 days up to 90 days before the 21 day repeat science phase.
However, radar interferometer measurement is sensitive to the stability of the spacecraft and is prone to large-scale errors. To address this issue, the SWOT payload includes a conventional Jason-class nadir altimeter for calibration and validation of the interferometer measurement for its large-scale performance. This will also allow filling the ± 10km gap at the nadir track. Thus this approach would enable the demonstration of the transition of profile altimeter to swath altimeter for observing ocean surface topography over a wide range of scales.
SWOT promises high-precision measurement over both land surface water and the ocean. The sampling capability of SWOT will enable the measurement of ocean surface topography at scales not resolvable by current altimeters. Over the deep oceans, the SWOT planned resolution is 1×1 km with an error at2.4 cm/km². Over land, SWOT will work in a high-resolution mode turned on by a mask. It will resolve 100 m wide rivers (baseline, with a goal of 50 m) and lakes (of areas greater than 250 m²), wetlands, and reservoirs. The water level elevations will have an accuracy of

15:40 - 16:00 Learning river properties and infering river discharge from SWOT-like data time-series (Pierre-André Garambois)

Authors
  1. Pierre-André Garambois (INSA de Strasbourg)
  2. et al. (.)
Abstract

Authors : Pierre-André Garambois, Hélène Roux, Jérôme Monnier

Abstract :
New generations of satellites and sensors offer promising possibilities to overcome the lack of in situ data for hydrological sciences, with increasing spatio-temporal coverage and accuracy. Nevertheless, inverse problems in hydraulics such as the estimation of river discharges from space are still open questions. Remotely sensed measurements of hydrosystems can provide valuable information but adequate methods are still required to take maximum advantage of it. Lots of studies have shown the possibility of retrieving discharge given the river bathymetry or roughness and/or in situ time series. The new challenge is to use SWOT-type data (that is to say water surface elevation, free surface slope and top width) to inverse the triplet formed by the roughness, the bathymetry and the discharge (A 0 , K, Q) ([?]). We show that the most complete shallow-water like model allowing to separate the roughness and bathymetry terms is the so-called low Froude model. The few inverse models elaborated for inferring (A 0 , K, Q) are analyzed in two contexts: 1) only remotely sensed observations of the water surface (surface elevation, width and slope) are available ; 2) one additional water depth measurement (or estimate) is available. Results of hydraulic parameters inversions will be presented for a large dataset of rivers with contrasted properties, in the context of the PEPSI challenge that is an
intercomparison project of several discharge inversion methods using SWOT-like data. Considering effective hydraulic parameterizations, depending on observation scale, several perspectives are discussed for data assimilation into 1D and 2D hydraulic models. The temporal sampling of a mission such as SWOT will offer new possibilities in terms of hydraulic visibility for describing and learning river reaches, floodplains, and hydrosystems behaviours.

17:00 - 17:15 ALTIMETRY OF THE SOUTH AMERICA RIVERS (Joecila Santos da Silva)

Authors
  1. Joecila Santos da Silva (Universidade do Estado do Amazonas)
  2. stephane calmant (Institut de Recherche pour le Développement – IRD, UMR ESPACE-DEV)
  3. Frédérique Seyler (Institut de Recherche pour le Développement – IRD, UMR ESPACE-DEV)
  4. Rhasa Team (Universidade do Estado do Amazonas)
Abstract

Altimetry of rivers all along their course is major information in hydrology, whatever it is for running hydrological model, determine the amount of surface water stored, and predict the consequences of extreme events. Satellite altimetry can be used in many ways to retrieve consistent stage information throughout the course of rivers. Now, it is well known as a useful tool to retrieve the space and time variations of the water surface. Besides this basic use of satellite altimetry, it can also be used to: 1/ level gauges , so making a consistent dataset merging high temporal sampling from gauges and dense sampling from the crossings between satellite tracks and river network; 2/ densify climatic series (mean value per month of the year) all along the river course when series are long enough and, by comparison with time series, evidence extreme events; 3/ detail the altitudinal changes of the river course which, when compared to a DTM, inform over the basin hypsometry; 4/ level bathymetric profiles in order to obtain altitudinal changes of the river bed; 4/check for errors in the gauge series or in the metadata information related to a gauge; 5/ compute rating curves when combined with discharge series. We present examples of such applications of satellite altimetry for the major contributors of the South America Rivers. These examples are based on series from the ERS2 – ENVISAT – SARAL missions in the one hand (1995-2015) and from the T/P & JASON2 missions in the other hand (1992-2002 / 2008-). All series have been carefully checked manually and we present statistics of comparison with ground-truth, i.e. water levels from GPS-leveled gauges. Rivers of very different widths have been sampled, ranging from several km wide to less than 100m wide. For many of them, satelite altimetry is the only possibility to get stage and slope information since these rivers are devoid of in-situ measurement or the existing measurements are not distributed.

17:15 - 17:30 State of the art of Altimetry for Lakes in South America (R. Abarca Del Rio)

Authors
  1. R. Abarca Del Rio (UNC, Chile)
Abstract

Generally speaking (that is worldwide) great lakes are heavily utilized by their bordering countries for water supply (for drinking, agriculture, industry, and hydropower production), transportation, fish production, waste disposal, recreation, and tourism and are so under considerable pressure from a variety of human activities. Therefore nourished academic research is carried out in all aspects of its characteristics as these are very relevant to the daily life of the surrounding society. In contrasting way, except in some very specific cases (such as the case of Lake Titicaca), studies on the physical, biological, morphological characteristics of South American great lakes are quite rare not to say the least. In modern times this is quite surprising, as although it is true that it has not been possible to cover all those lakes of in situ instruments to follow all their characteristic variability through times, today several observation satellite space missions exist that allow shoveling this lack of in situ measurements.
As an example, we will only mention one of its physical characteristics, perhaps the most important, which is the variability of the water body, in direct relation to the hydrological basin that feeds it. Thus, the volume variations of water stored within Lakes and are indicators of the combined impact of climate change and water cycle, hence able to correctly estimate the anthropocentric effect, be it remote or direct. The overall lake water volume depends on the balance between the water inputs and outputs. The inputs are the sum of direct rainfall over the lake, surface runoff from the drainage basin area, and underground seepage (which can be neglected). The outputs are the sum of direct evaporation from the lake, river outflow, and groundwater seepage. Some of these components can be remotely sensed (e.g. Rainfall, lake level, water mass), while others can be estimated by a combination of remote sensing, and in situ data + global and regional atmospheric and hydrological models (e.g. Evaporation and evapotranspiration, soil moisture,water fluxes) or other solely from the later (e.g. Surface temperature).
Thus, Accurate and continuous monitoring of lakes and inland sea level has been possible since 1993 thanks to the success of satellite altimeter missions: TOPEX/POSEIDON (T/P), GFO, JASON-1, and ENVISAT. Global processing of the data from these satellites can provide time series of lake surface heights over the entire Earth at different temporal and spatial scales with a subdecimeter precision. Instead, Satellite gravimetry, particularly the Gravity Recovery and Climate Experiment (GRACE) mission, has been widely used for estimating global and regional water storage variations since 2002. In addition, the combination of different kinds of remote sensing data, for example, combined radar/laser altimeter (with Laser ranging altimeter ICESat) and optical imagery (LANSAT, MODIS etc.) have been used to monitor the areal extent and storage variations in lakes and reservoirs, in some cases even providing with bathimetry construction. In this presentation we will review the most relevant aspects of these methods (particularly altimetry) when have been applied to lakes of South America. We’ll also contrast these results

17:30 - 17:45 Assimilation of altimetry data in hydrological models ( Rodrigo Paiva)

Authors
  1. Rodrigo Paiva (UFRGS)
  2. et al. (.)
Abstract

We present recent advances of assimilation of altimetry data into hydrological models. Hydrologic models are one of the main tools used for predictions of water cycle with several potential applications as flood hazards, flood and drought forecasting, water management, water interactions with climate and environment, among others. On the other hand, there are already several past and present satellite missions that can be used as a complement to monitor surface waters, as satellite altimeters and imagery satellites and the planned Surface Water and Ocean Topography (SWOT) mission. These satellite observations have a great potential to improve the knowledge about water systems and improve the hydrologic modelling tools that are being used to address water related problems. The examples focus on recent advances on the integration of altimetric data into the process based MGB-IPH model, a simulation tool that is being extensively used for studying South American river basins. We present results on the validation of the MGB-IPH for the Amazon basin using a database of from several altimetry stations derived from the ENVISAT satellite. It is also presented experiments with a data framework of radar altimetry and water levels applied these large scale hydrologic models, showing that it greatly improves model predictions. These examples encourage the potential altimetry to improve hydrology predictions at large rivers and/or poorly monitored regions by combining models and remote-sensing information.

17:45 - 18:00 Potential contribution of SWOT satellite in the estuaries (Benoit Laignel)

Authors
  1. Benoit Laignel (Université de Rouen)
  2. et al. (.)
Abstract

Estuaries are complex ecosystems which can include wetlands and mangrove swamps along their intertidal shores, areas of saltmarsh, fen or peatlands, tidal floodplains, and/or island areas with complex channel networks. They serve as a meeting point between land and sea, exhibit high primary productivity, and occupy ecological niches of considerable importance. Estuarine hydrodynamics are very complex because there are interactions between different water bodies: sea with different phenomena (tide, wave, surges, sea level rise), main river and tributaries and groundwater.
In-situ measurements provided by tide gauges are important to improve our knowledge of estuarine water elevations and then monitor the water quality and the possible changes in zonation (movement of the salinity fronts and high turbidity zones within the estuary…). The number of gauges needed to survey an estuary varies in relation to its size, but only a few estuaries in the world have enough gauges to allow this calibration/validation. Due to the complex physical processes occurring in an estuary, numerical models have been increasingly used to predict flow, water quality and sediment and contaminant transport processes.
The interest of using satellite data for estuaries is to provide information on the hydrodynamic spatial variability. Until now, within estuaries, the contribution of satellites has mainly focused on the monitoring of water color and its relation to suspended sediment and chlorophyll by measuring the amount of solar radiation at different wavelengths correlated to water quality parameters (e.g. total suspended solids, TSS). In terms of water level, satellite radar altimetry has had limited applications in estuaries due to the land perturbations within the radar footprint.
To overcome these problems, the SWOT satellite by interferometry radar (Surface Water and Ocean Topography; NASA and CNES program, launch in 2020), will provide higher spatial resolution (1 km for the oceans and 50-100 m for continental waters), which will be fundamental in order to improve our knowledge of the complexity of the physical processes in these systems, validate and calibrate our models and improve the data assimilation by coupling physical models to altimetry measurements.
To answer to the question of the use and contribution of SWOT in the estuaries, we use two approachs in the Seine and Gironde estuaries. The first approach is to simulate SWOT data without error and with error (white noise) according to the SWOT orbit to study the effect of the number of passages of SWOT (from 2 to 7 per cycle of 22 days) to this capacity to record the hydrological temporal variability. We use statistical and signal processing methods (wavelets) to compare the in situ water level and the simulated SWOT water level. The second approach is the modeling of the spatial and temporal variability of water level by the T-UGO model and the use of model data as input data of SWOT simulator HR (High Resolution).
The results of the first approach show SWOT reproduces well the main hydrological variability patterns in the river part and in the estuary upstream (2y = NAO mode, 1y = hydrological cycle, 1,5-3 months

18:00 - 18:15 Improvements in Space-borne Operational Flood Forecasting and Reservoir Outflow Estimation through Multi-Mission Interactions (Faisal Hossain )

Authors
  1. Faisal Hossain (Department of Civil and Environmental Engineering, University of Washington)
  2. et al. (.)
Abstract

We present an overview of how well we are able to forecast Monsoonal flooding using observations of river height from multiple nadir altimetry missions for operational applications. In addition, we also overview the current state of the art in estimation of transboundary reservoir outflow through mission interactions of satellite altimetry (for reservoir storage change) and multi-sensor precipitation estimation (for reservoir inflow estimation). Our goal of this presentation is to show the community 1) real world application of operational flood forecasting in large Monsoonal river systems and the expected benefit of additional missions such as wide swath altimetry (SWOT) and other nadir altimetry missions (Sentinels, ICESat-2); 2) demonstrate the level to which reservoir outflow can be estimated through joint use of precipitation and surface water mission of SWOT.

18:15 - 18:30 SWOT perspectives (Stéphane Calmant )