The contribution of satellite radar altimetry to river monitoring is well-established, with data forming valuable inputs to river models.Surface soil moisture can also be determined from altimetry using DRy EArth Models (DREAMs) which model the response of a completely dry surface to Ku band nadir illumination. New DREAMs over Africa now cover more than 70% of the continent, encompassing more than 30 river basins including the Congo, Niger, Okavango, Zambezi and Volta. It was decided to fly multi-mission altimetry over these river DREAMs to assess the potential of this technique to contribute to studies in river basins. As a detailed DREAM exists for the Amazon basin, this was also included. Envisat, ERS-1/2, Jason-1/2, CryoSat-2 and Sentinel-3A altimeter data were utilised in this study, together with a database of over 86000 graded altimeter River and Lake height time series. Soil moisture estimates were generated and validated.Summative conclusions: the highest data retrieval rate over river DREAMs is found over ‘river’ and ‘wetland’ pixels, with lower percentages over ‘soil’ pixels where soil moisture estimates can be generated. This is an expected outcome, as targeting ‘soil’ pixels will select for rougher topography. Within the constraints of satellite orbit and repeat period, data can be successfully gathered over the majority of these overflown DREAM surfaces. It is also clear that very detailed DREAM models, at least 10 arc seconds resolution, are required to capture the intricate structure in river basins. It is noted that many tributaries are below the current 10 arc seconds spatial resolution of the DREAMs, and are classified with their surrounding terrain as wetland pixels.ERS-2 and Envisat performed best; Sentinel-3A OLTC mask is found to preclude monitoring of almost all ‘soil’ pixels, except those adjacent to the largest rivers.The ability of nadir-pointing altimeters to penetrate vegetation canopy gives a unique perspective in rainforest areas. Amazon soil moisture time series in the lower Amazon are seen to correlate to river height variations: in the upper Amazon basin the annual rainfall signature is dominant.Over much of the river DREAMs, along-track time series of soil moisture can be generated at the spatial resolution of the underlying DREAM, currently 10 arc seconds. The major constraint, as with altimeter height measurements, is the spatio-temporal sampling, so use is envisaged in combination with other remote sensed and in-situ data. However, DREAMing provides a valuable independent dataset which can be used to validate soil moisture estimates from other techniques.

This report summarises the main results, conclusions and recommendations of the “HYDROSPACE-GEOGLOWS 2021” Workshop organised by the European Space Agency (ESA), in collaboration with the French Space Agency (CNES) and the GEO Global Water Sustainability Initiative (GEOGloWS) (Fig. 1). This Workshop is a sequel to the ones held in Toulouse (F) in 2003, in Geneva (CH) in 2007 and in Frascati (I) in 2015. Nearly 300 scientists, engineers and managers registered to this virtual event from 41 countries from all time zones worldwide, submitting 123 papers with more than 500 co-authors. The inclusion in the programme of large time slots for discussion and the advance preparation of “Seed Questions” offered the opportunity to have a community discussion focused on the future challenges of Inland Water monitoring and prediction and the future observational requirements. A “Manifesto” was drawn-up from the discussion nourished by the participants. This report presents this “Manifesto”, highlights the state of the art presented in the sessions, summarises the discussions and provides recommendations and guidance for future mission design, research activities for enhancing processing algorithms and developing new ones, calibration and validation, sustainable data exploitation, dissemination, outreach, capacity building and co-designing applications and operational services.

The unique contribution of satellite radar altimetry to river monitoring is well understood, with ‘ altimeter virtual gauge’ heights increasingly ingested into river basin models. However, altimeters gather a wealth of additional information. Waveform shapes reflect underlying topographic variation, surface composition and roughness, and distribution of surface water within the footprint. Backscatter measurements allow soil surface moisture under the satellite track to be determined, using DRy EArth ModelS (DREAMS) crafted from multi-mission altimeter data and ground truth. Initially developed over desert areas, DREAMs are now being built over river basins to extend the scope of altimeter soil moisture measurement.This paper investigates the potential contribution of these additional data to river basin analysis and modelling.The following key questions are addressed.1) How useful are the data encoded in complex waveform shapes?2) Can altimeter soil moisture estimates contribute to modelling in river basins?A series of example river basins were chosen in different topographic and climate situations, including the Amazon, Orinoco, Nile, Niger and Congo basins, and wetlands including the Okavango delta.This paper presents outcomes from analysis of multi-mission altimetry, with ERS-1/2, Envisat, Topex, Jason-1/2, Cryosat-2 and Sentinel-3A/B, plus a database of over 86,000 river and lake timeseries.The analysis outcomes demonstrate the value of altimeter soil surface moisture estimates, both as co-temporal and co-spatial data with inland water height measurements, and as an independent validation dataset to assess soil moisture estimates derived from other remote sensing techniques. The precise backscatter cross-calibration of altimeters on successive missions allows derivation of long soil moisture time series. The ability of nadir-pointing altimeters to penetrate vegetation canopy gives a unique perspective in rainforest areas, with information on underlying water height and extent as well as surface soil moisture. Waveform shape classification allows diverse information to be gleaned, particularly at the higher pulse repetition frequencies of the new generation of SAR Altimeters. In conclusion, satellite radar altimeters collect a wealth of information over river basins; this valuable resource is not yet fully exploited.

(02/11/2019) LOTUS— Preparing Sentinel-3 SAR Altimetry Processing for Ocean and Land The Sentinel-3 satellite mission with its SRAL instrumentation contains new features compared to the conventional radar altimeter mission that form the basis for new innovative scientific analyses of both ocean and inland water levels. To utilize the full potential of the new data source, new methods and processing chains need to be developed. Subsequently, new potential Copernicus products should be developed that utilize the improved alongtrack resolution over both the oceans and over land. The main objective of the LOTUS project is to prepare the scientific and operational use of data from Sentinels 3. Then new operational processing, validation and delivery mechanisms need to be developed and implemented for generating the new dynamic products. Finally, the take-up of the new Copernicus products by the valueadding sectors needs to be stimulated and demonstrated to ensure that they will be used for commercial activities. LOTUS will develop processing scheme for extracting high-resolution sea surface heights, wave heights and wind speeds from SAR mode data. Over land, the LOTUS will develop processing scheme for extracting high-resolution river and lake heights, soil moisture, and snow water equivalents. This presentation shows results based on analyses using CRYOSAT data and available S-3 data. Furthermore, new DEMO data sets are presented and examples of scientific impact demonstrated.

Measurements of river and lake water levels from space-borne radar altimeters (past missions include ERS, Envisat, Jason, Topex) are useful for calibration and validation of large-scale hydrological models in poorly gauged river basins. Altimetry data availability over the downstream reaches of the Brahmaputra is excellent (17 high-quality virtual stations from ERS-2, 6 from Topex and 10 from Envisat are available for the Brahmaputra). In this study, altimetry data are used to update a large-scale Budyko-type hydrological model of the Brahmaputra river basin in real time. Altimetry measurements are converted to discharge using rating curves of simulated discharge versus observed altimetry. This approach makes it possible to use altimetry data from river cross sections where both in-situ rating curves and accurate river cross section geometry are not available. Model updating based on radar altimetry improved model performance considerably. The Nash–Sutcliffe model efficiency increased from 0.77 to 0.83. Real-time river basin modelling using radar altimetry has the potential to improve the predictive capability of large-scale hydrological models elsewhere on the planet.

CRUCIAL: Cryosat-2 Success over Inland Water and Land CRUCIAL is an ESA/STSE funded project investigating innovative land and inland water applications from Cryosat-2 with a forward-look component to the future Sentinel-3 mission. The fact that the Earth’s land surface is, in general, a relatively poor reflector of Ku band energy, with the exceptions of inland water, salar and ice surfaces has enabled Earth-orbiting satellite radar altimeters to be used for land surface applications including mapping and measurement of river and lake systems. Research with EnviSat Burst Echoes has shown that substantial high frequency information content is present at short spatial scales with a small bright reflecting patch at nadir, such as over inland water, able to dominate the returned echo. Onboard echo averaging of the previous generation of satellite radar altimeters therefore causes loss of significant amounts of information. The high along-track sampling of Cryosat-2 altimeter in SAR mode (I8 KHz) offers the opportunity to recover high frequency signals over much of the Earth’s land surface, enhancing the inland water height retrieval capability. Constraining this application is the limited availability of SAR Full Bit Rate (FBR) data from Cryosat-2 over these land surfaces; however, for Sentinel-3 the SAR mode will be deployed widely over land. The Cryosat-2 CRUCIAL project will not only provide valuable data, but, as precursor of the Sentinel-3 SAR mode data, gives a valuable first look at this new measurement capability. This paper will summarise the CRUCIAL aims and objectives and showcase first results from retracking Cryosat-2 SAR and LRM waveforms over multiple inland water targets.

A methodology is developed to estimate daily river discharge at an ungauged site using remote sensing data. Use is made of ERS-2 and ENVISAT satellite altimetry to provide a time series of river channel stage levels and longitudinal channel slope and Landsat satellite imagery to provide a range of channel widths over a 50km reach of river. The data are substituted into the Bjerklie et al. () equation, which is based on the Manning's resistance equation and has been developed using a global database of channel hydraulic information and discharge measurements. Our methodology has been applied at three locations on the Mekong and Ob Rivers and validated against daily in situ discharge measurements. The results show Nash-Sutcliffe efficiency values of 0.90 at Nakhon Phanom and 0.86 at Vientiane on the Mekong, and 0.86 at Kalpashevo on the Ob. Copyright (c) 2012 John Wiley & Sons, Ltd.

The series of satellite radar altimeter missions has provided a huge database of altimeter waveforms over the earth's inland water surfaces. This paper outlines the current measurement capability. A comprehensive global analysis shows that waveforms from 22,223 targets have been identified from ERS2; just under 50% of these targets are currently producing useable time series. For EnviSat, 25,363 targets yield 59% of useable time series. This is attributed in part to the dynamic mode-switching capability of the RA-2. The 10-day repeat cycles of TOPEX, Jason1 and Jason2 produce lower numbers; reasons for these retrieval differences are identified and discussed. Using data from CryoSat-2, and 1800Hz 'burst echo' data from EnviSat, the additional potential for inland water measurement from Sentinel-3 is explored. The results demonstrate that the high along track sampling rate of Sentinel-3 SAR FBR waveforms can transforms the measurement technique to a true global monitoring capability for entire river systems.