IGCP 565 Project Workshops

IGCP 565 Workshop 4: Integration of geodetic observations and products in models of the hydrological cycle Support for water management through hydrological models and data assimilation November 21-22, 2011 Johannesburg, South Africa

IGCP 565: The workshop series and recommendations

Hans-Peter Plag
Nevada Bureau of Mines and Geology, University of Nevada, Reno, USA

Geodetic observations of the Earth's gravity field, shape, and rotation and their changes in time (the three fundamental areas of geodesy) capture the signals of variation in the entire fluid envelope of the solid Earth, including the terrestrial water storage. Therefore, the geodetic techniques have the capability to monitor mass transport particularly in the global water cycle. The IGCP 565 Project aims to utilize this potential and to develop the globally available geodetic observaing infrastructure into a monitoring system for the hydrological cycle on global to regional scales. Key scientific issues addressed are: (1) Development of an integrated dynamic model for the predictions of the geodetic signals of daily to interannual surface mass changes; (2) Inversion algorithms for integrated geodetic observations for surface mass changes; (3) Assimilation of observed surface mass changes in hydrological models; (4) Development of products relevant for regional water management. The project supports capacity building in space-geodetic data processing, modeling of the hydrological cycle, and interpretation of the observations in terms of terrestrial water storage. A focus is on products for regional water management, particularly in developing countries.

Coordination of the research and capacity building is provided through a series of five annual workshops. The first three workshops addressed in sequence the topics: (1) Science of geodetic monitoring of the hydrological cycle; (2) Towards a Roadmap for Future Satellite Gravity Missions; (3) Separating Hydrological and Tectonic Signals in Geodetic Observations. The current workshop has the theme: Integration of geodetic observations and products in models of the hydrological cycle - Support for water management through hydrological models and data assimilation. The final workshop to be held in 2012 will focus on improving regional water management in Africa and Asia on the basis of geodetic water cycle monitoring.

Each of the workshops agreed on a number of recommendations, which provided guidance for the project. The first workshop identified as main gaps in the hydrological budget the deep groundwater and evaporation. Seasonal predictions were seen as an important problem in water management, emphasizing the need for models with predictive capability. The assimilation of geodetic observations into hydrological models was considered the preferred approach to utilize geodetic observations. For addressing the hydrological questions, a hybrid approach of local implementation and global observations and models was considered necessary. The workshop concluded that geodetic observations are valuable on all scales, and that the best way to get the products to the users was by demonstrating to operational agencies what can be done with geodesy. It was agreed that the IGCP 565 Project would focus on regional applications in Africa.

The second workshop produced a roadmap towards future gravity satellite missions that emphasized the importance of these missions for addressing major challenges in our understanding and quantitative knowledge of the water cycle and for the monitoring of the water cycle as it changes under global warming. This roadmap was later distributed to decision makers and may well have contributed to decisions in the USA and Germany to implement a GRACE follow-on mission much earlier than originally planned.

The third workshop focused on the separation of hydrologic and tectonic signals in geodetic observations of time variable Earth's gravity field, surface displacements, and rotation. In summary, the presentations and discussion at the workshop made very clear that there is considerable added value in applying geodesy to support hydrologic cycle modeling and monitoring, especially for terrestrial water storage. However, there remain challenges in the uncertainties of observations, data analysis, forward modeling, inversion, and synthesis of geodetic products, which need to be addressed in the development of hydrogeodesy as a science field with a high potential to help address the world's water problems. Questions on the suitability of geodesy signals for hydrologic applications hinge on the variability of the signal within different timescales from months, seasons, to decades. Geodetic forward modeling requires quantification of the hydrologic loading, which can be modeled as a global signal. The determination of the global mass balance, however, remains an issue, especially if local hydrologic variations are to be determined. Model errors will continue to propagate through such calculations, requiring new methods of error reduction for improved predictive capability. Tectonic challenges include exploring different rheological models and quantifying the error budget.

The two primary recommendations of the 3rd workshop are: (1) Capacity building with application of geodetic products that water resource decision makers are able to readily access and easily use, and (2) development of a demonstration project in California that merges geodetic information with hydrologic modeling via assimilation and leads to realistic technology transfer to African nations through a similar project in the Nile Basin.

During the 3rd Workshop, it was proposed that an initial pilot demonstration projects in California will demonstrate the utility of hydrogeodesy, as it is a region rich in groundwater and surface water observations. There is already a collaboration with the California Department of Water Resources. GRACE-derived terrestrial water storage variations for the period 2002-2009 have been calculated for the California Central Valley, a region where more than half of the US fruits and vegetables are grown, and there is currently State agency (CWDR) interest in GRACE and hydrogeodesy applications. A key question is whether the approach applied to California will also apply in regions of Africa, and whether such a pilot can bring together the ongoing activities in Africa needed to build a pilot project that links science to water resource decision makers in Africa. The advantage of the Nile River basin is that there are no significant signals from postglacial rebound that would impact the geodetic signal analysis. Such a pilot should have region-specific issues that bring stakeholders and water managers into such an activity. It is suggested that the Africa pilot should involve the World Bank, USAID, UN Habitat, UNESCO IHP, WaterNet, NASA, and other groups already working in the Nile basin. The science contribution will require development of tools for end users, provide advice for infrastructure, and capacity building for operational activities. This will require international participation from African countries within the Nile basin and data sharing through a common decision support framework.