The IGCP 565 Project
Geodetic Observations of the Global Water Cycle
For the development of an observational strategy, the water cycle can be divided into slow and fast branches. The 'fast branch' consists of precipitation (liquid and solid), evapotranspiration, clouds and water vapor. The dynamics of the elements in this branch can vary significantly within a day. The 'slow branch' consists of changes in soil moisture, ground water, snow and ice, freeze-thaw states, ocean dynamics, salinity and volume, and river discharge. The elements in this branch change on much longer time scales. From a perspective of an observing system with two corresponding branches, it is the slow branch where geodetic observations can best contribute (Lawford et al., 2004).
At time scales from sub-daily to decades, the largest mass redistributions on the surface of the solid Earth mainly occur in the water stored in the atmosphere, in the ocean, on land, and in glaciers and ice sheets. Exchanges of mass between these major reservoirs of the water cycle are linked to each other through a conservation-of-mass equation (Clarke et al., 2005). These mass movements load and deform the solid Earth. Any of these mass movements changes the Earth's gravitational field primarily due to the mass redistribution, and, secondarily, due to deformations of the solid Earth. Through associated changes in the angular momentum and the moments of inertia of atmosphere, ocean, terrestrial hydrosphere, and solid Earth, the redistribution of mass in the fluid envelope also affects the rotation of the Earth. Any of these changes will in turn impact the mass distribution in the ocean and thus create additional loads and variations in the geodetic parameters. Therefore, the geodetic loading signals of atmosphere, ocean, and terrestrial hydrosphere are inherently linked together (Blewitt & Clarke, 2003) and an integrated gravitationally consistent modeling approach is required in order to predict these geodetic signals with high accuracy. Moreover, variations in groundwater level lead to surface displacements and local gravity changes.
The current gravity mission GRACE is now producing the best ever estimates of sub-continental-scale variation in terrestrial hydrology over several years (e.g., Tapley et al., 2004; Rowlands et al, 2005; Crowley et al., 2006), and is also providing the best available estimates of present-day changes in the large ice sheets (e.g., Velicogna & Wahr, 2006). However, the integration of the three areas of geodesy is still in an initial stage, with the main focus on a combination of GPS and GRACE observations for the inversion of surface mass changes or associated surface deformations (e.g., Davis, et al., 2004; Kusche & Schrama, 2005; Wu et al., 2006). Moreover, the inversion of geodetic observations for surface mass changes is hampered by model inconsistencies, which limit the full exploitation of the geodetic observations (Plag et al., 2007).
Exploring the linkage between the signals in gravity, shape and rotation of the Earth, GRACE can be used to validate new methods using GPS data on Earth's shape to produce the first ever estimates of decadal scale variation in continental scale water storage. Unlike GRACE, high quality GPS data now spans >10 years, and the global GPS network with ever increasing spatial resolution and accuracy provides longer term stability toward studies of global climate change and its effect on terrestrial water storage. Moreover, by combining the geodetic observations with hydrological models, the geodetic techniques are the basis for a mass transport monitoring system. In order to fully exploit the geodetic potential, a number of science issues need to be addressed. These issues are discussed below.
A continuation of GRACE-like missions is a fundamental prerequisite for a geodetic monitoring of the global water cycle. A major effort needs to be made to ensure subsequent missions when the current GRACE mission stops operation potentially as early as 2010. While on longer time scales, new and improved gravity missions are considered, the immediate need for continuation may be best met by a second GRACE-type mission. Such a mission could be deployed in close cooperation with and potentially under the lead of one of the emerging space agencies in Africa or Asia. Moreover, in the frame of a virtual constellation for water cycle monitoring, additional gravity missions could be considered, thereby increasing spatial resolution considerably. An important goal of the proposed project will be to explore the various options and to discuss with CEOS members an appropriate path to ensure continuous gravity missions.