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Innovative processing method for altimetry data allows for monitoring water level variations in wetlands

Pantanal by Marcelo Mendes de Oliveira / CC BY

Over the last years satellite altimetry has proven its potential to monitor water level variations not only over the oceans but also over inland water bodies. DGFI-TUM provides altimetry-derived time series of water stage variations of various globally distributed rivers and lakes via its web service "Database for Hydrological Time Series over Inland Waters" (DAHITI; see below).

Now, scientists of DGFI-TUM have developed an innovative processing method for monitoring and analyzing water level variations in wetlands and flooded areas. The approach is based on automated altimeter data selection by waveform classification and an optimized waveform retracking. It is described in the article Potential of ENVISAT Radar Altimetry for Water Level Monitoring in the Pantanal Wetland (Remote Sensing, 2016, available via open-access).

Using the example of the Pantanal wetland in South America, this study demonstrates the capability and limitations of the ENVISAT radar altimeter for monitoring water levels in inundation areas. The accuracy of water stages varies between 30 and 50 cm (RMSE) and is in the same order of magnitude as reported for smaller rivers. Most areas of the Pantanal show clear annual water level variations with maximum water stages between January and June. The amplitudes can reach up to about 1.5 m for larger rivers and their floodplains. However, some areas of the wetland show water level variations of less than a few decimeters, which is below the accuracy of the method. These areas cannot reliably be monitored by ENVISAT. Further investigations will show if the usage of Delay-Doppler altimeter data (such as measured by the recently launched Sentinel-3 mission) might improve the results there.

IAG adopts a new conventional value for the reference gravity potential W0 of the geoid

The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) promotes through its Focus Area 1 Unified Height System the definition and realization of a global vertical reference system with homogeneous consistency and long-term stability. For the term 2011-2015 DGFI-TUM coordinated the Working Group Vertical Datum Standardization, which main purpose was to determine an updated value for the gravity potential W0 of the geoid to be introduced as the conventional reference level for the realization of a global height system.

The derived value was officially adopted by the IAG in its Resolution No. 1, July 2015, as the conventional W0 value for the definition and realization of the International Height Reference System. A detailed description about the DGFI-TUM computation strategy of W0, applied models, conventions and standards, as well as results is presented in the recent publication A conventional value for the geoid reference potential W0 (Journal of Geodesy, 2016, DOI: 10.1007/s00190-016-0913-x).

Read more about the background and DGFI-TUM's activities related to the determination of the new W0 value here.

Data portal DAHITI: Water level time series of rivers and lakes from multi-mission satellite altimetry

Scientists of DGFI-TUM have developed a new approach for the automated estimation of water levels of inland water bodies based on satellite observations from multi-mission altimetry. Time series of water stage variations of various globally distributed rivers and lakes are made available through DGFI-TUM's web service “Database for Hydrological Time Series over Inland Waters” (DAHITI).

The approach is described in the publication DAHITI – an innovative approach for estimating water level time series over inland waters using multi-mission satellite altimetry (Hydrology and Earth System Sciences, 2015, available via open-access).

The new method is based on an extended outlier rejection and a Kalman filter incorporating cross-calibrated multi-mission altimeter data from Envisat, ERS-2, Jason-1,Jason-2, TOPEX/Poseidon, and SARAL/AltiKa, including their uncertainties. The paper presents water level time series for a variety of lakes and rivers in North and South America. A comprehensive validation is performed by comparisons with in situ gauge data and results from external inland altimeter databases. The new approach yields RMS differences with respect to in situ data between 4 and 36 cm for lakes and 8 and 114 cm for rivers. For most study cases, more accurate height information than from other available altimeter databases can be achieved.

DGFI-TUM contributes to the implementation of an UN Resolution for a Global Geodetic Reference Frame

A highly precise and long-term stable global geodetic reference frame is an indispensable requirement for a reliable determination of global sea level rise over many decades.

In February 2015, the UN General Assembly adopted its first geospatial resolution „A Global Geodetic Reference Frame for Sustainable Development“. This resolution recognizes the importance of geodesy for many societal and economic benefit areas, including navigation and transport, construction and monitoring of infrastructure, process control, surveying and mapping, and the growing demand for precisely observing our planet's changes in space and time. The resolution stresses the significance of the global reference frame for accomplishing these tasks, for natural disaster management, and to provide reliable information for decision-makers.

The United Nations Global Geospatial Information Management (UN-GGIM) Working Group on the Global Geodetic Reference Frame (GGRF) has the task for drafting a roadmap for the enhancement of the GGRF under UN mandate.

Based on its competence in the realization of reference frames DGFI-TUM is involved in this activity by contributing to the compilation of a concept paper in the frame of the International Association of Geodesy (IAG). The main purpose of this paper is to provide a common understanding for the definition of the GGRF and the scientific basis for the preparation of the roadmap to be accomplished by the UN-GGIM Working Group on the GGRF. [more]

Antarctic outlet glacier mass change resolved at basin scale from satellite gravity gradiometry

Dramatic ice mass loss has been observed in West Antarctica using space gravimetry from the Gravity Recovery and Climate Experiment (GRACE) satellite mission, satellite altimeter data, and input-output methods (IOM). This was reported by DGFI scientists in their article Antarctic outlet glacier mass change resolved at basin scale from satellite gravity gradiometry (Geophysical Research Letters, 2014, DOI:10.1002/2014GL060637).

GRACE, satellite altimetry and IOM complement one another: GRACE provides a direct estimate of total mass change at fairly low resolution while the other two methods provide ice height variation or flux variation (IOM) at much higher spatial resolution. For satellite altimetry, an array of assumptions, with possible biases, may corrupt the conversion of ice height to mass change. Time-varying GRACE mass mapping is limited by the fact that it cannot provide the small-scale resolution that satellite altimetry provides, and it is difficult to isolate the exact location of a mass anomaly without introducing constraints. [more]

Cross-calibration for 20 years satellite altimetry

Climate studies require long data records extending the lifetime of a single remote sensing satellite mission. Precise satellite altimetry exploring global and regional evolution of the sea level has now completed a two decade data record. A consistent long-term data record can only be constructed from a sequence of different, partly overlapping altimeter systems by means of a careful cross-calibration.

At DGFI, satellite altimeter cross-calibrations are routinely performed with all available altimeter systems including mission phases from geodetic and drifting orbits. The methodology of this calibration and latest results are described in the article Multi-Mission Cross-Calibration of Satellite Altimeters: Constructing a Long-Term Data Record for Global and Regional Sea Level Change Studies (Remote Sensing, 2014, available via open-access).

The paper compiles – for the first time – a complete list of range biases between all relevant altimeter systems of the last 20 years, from ERS-1 to SARAL including less prominent missions such as GFO, ICESat, or HY-2A (cf. Table). Moreover, the computations provide time series of radial errors and allow to estimate empirical auto-covariance functions, systematic variations in the geo-centering of altimeter satellite orbits, as well as geographically correlated mean errors for all altimeter systems.