Earth observation (EO) satellites provide a wealth of data for scientific, operational and commercial exploitation. But, observations of the redistribution of environmental mass in the Earth system (hydrology, ice, ocean and atmosphere) are not part of the inventory of EO data products to date, although they constitute a new and unique remote sensing opportunity. These observations provide fundamental insights into the global water cycle, polar and mountain ice mass loss, changes in ocean surface currents, sea level rise as well as many other processes. The largest and arguably the most important mass redistribution is that from changes in continental water storage (liquid water or ice) that controls the regional water budget and can, in extreme cases, result in floods and droughts that often claim a high toll on infrastructure, economy and human lives. The aim of this proposal is to demonstrate that observations of the redistribution of water and ice mass as derived from satellite gravity data provide critical and complementary information to more traditional, i.e. optical and radar, EO products and open the door for innovative approaches to flood and drought monitoring and forecasting.
Up until the end of the last century, monitoring temporal changes in the distribution of environmental mass had been quite challenging. However, since 2002, the German-American Gravity Recovery and Climate Experiment (GRACE, Tapley et al. 2004), a satellite gravity mission, has demonstrated the potential and worth for globally observing mass variations. The future German-American GRACE-FO (Follow-on) mission is already in its implementation phase and is due for launch in 2017 (Flechtner et al. 2014). GRACE-FO will already be equipped with an innovative Laser Ranging Interferometer (LRI) to improve measurement precision compared to GRACE. A subsequent mission, which offers another leap in accuracy and temporal/spatial resolution, is already being studied at ESA and NASA.
GRACE data have resulted in a number of scientific insights as evidenced by the nearly 2000 scientific publications related to the mission. Meanwhile, GRACE observations of climate-related issues contribute to the reports of the Intergovernmental Panel on Climate Change (IPCC). However, the potential for turning that scientific windfall into a benefit for society has been alluded to but has not been fully realised. Today the situation can be summarised as follows:
- A number of Analysis Centres (ACs) inside and outside the GRACE Science Data System (SDS) perform only the first level data processing and do not adopt uniform processing standards. This processing consists of providing monthly gravity fields in a spherical harmonic (SH) representation to the scientific user community. This format seriously limits the accessibility of these valuable data because the end user is left with the difficult choice of deciding (1) which results from which AC to use and (2) what actions to perform for converting spherical harmonics into gridded mass values appropriate for their study.
- Currently, it takes approximately two months from the time that the actual observation is taken to the time when scientists can access and examine the data. Temporal sampling is at best 7-10 days but most reliably one month. Both of these time constraints limit the potential of using the results in time-critical monitoring applications. This applies in particular for early-warning and forecasting systems of extreme hydrological events. Flood forecast models need, e.g. near-real time information to estimate the probable development of the event in terms of flood stage or river discharge with typical lead times of a few days for larger river basins. Also, the usefulness of high-resolution follow-up observations such as optical and radar EO data for emergency management is strongly influenced by the time-span from alert reception, satellite programming, satellite acquisition and data reception.
- At present the spatial resolution of gravity data is often smaller than the resolution area of interest for hydrological management and early-warning purposes. This applies in particular for Europe where the relevant size of river basins or sub-catchments is in the order of 100 000 km² or smaller, while the current available resolution of GRACE products is at best between 70 000 and 100 000 km².
Given these limitations, the efforts of this project can be summarised into three key objectives. We aim to
1) Deliver the best gravity products for applications in Earth and environmental science research. The unification of the knowledge of the entire European GRACE community will pave the way for a long awaited standardisation of gravity-derived products. Combining the results obtained from different ACs of the EGSIEM consortium, each of which will perform independent analysis methods but will employ consistent processing standards, will significantly increase the quality, robustness and reliability of these data;
2) Reduce the latency and increase the temporal resolution of the gravity and therefore mass redistribution products. An increased temporal resolution from one month to one day and the provision of gravity field information within five days (essentially near real-time) will translate into tremendous added value for warning and forecasting the onset of natural hazards; and
3) Develop gravity-based indicators for extreme hydrological events and demonstrate their value for flood and drought forecasting and monitoring services . Adequate data products and indicators will be provided to support operational satellite-based flood information services. The applicability and added value of these indicators will be exploited within the framework of the DLR’s (Deutsches Zentrum für Luft- und Raumfahrt) Center for Satellite Based Crisis Information ZKI and international initiatives such as the Copernicus Emergency Management Service and the International Charter “Space and Major Disasters”.
A careful assessment of the performance of our products as early warning indicators is mandatory and will be performed by a combination of modelling and comparison with complementary ground-based and satellite data, e.g. Synthetic Aperture Radar (SAR), altimetry, Global Navigation Satellite System (GNSS) data, gauge data (water level and river discharge), and in particular with the post-processed state-of-the-art mass redistribution products derived from the combined knowledge of the entire European GRACE community unified in our consortium.
The objectives will be achieved by setting up three dedicated services:
1) A scientific combination service;
2) A near real-time (NRT) / regional service; and
3) A hydrological/early warning service.
All of these services will be tailored to the various needs of the respective communities. The products of all services are of such value for the scientific user community that we will release them within the framework of dedicated and user-oriented data centres, e.g. the International Center for Gravity Earth Models (ICGEM). For the second and third service a notable effort is needed to increase the temporal resolution and to reduce the time delay to make the gravity-derived products usable by decision makers. Nonetheless, the prospect of acquiring reliable early warning indicators from near real-time mass redistribution data demands prompt action in order to preserve European competitiveness within this particular space sector.
To further enlarge the user community from the purely scientific users to the general public at large, the service products will also be transformed into user-friendly and easy-to-interpret data sets. Significant effort will be devoted to the development of visualisation tools that will demonstrate the unique value of mass redistribution products.
