23 August 2007

ESA's Earth Observation Missions,
Part 2: Groundwater

Primary Sources:
30 October 2006: NASA
Gravity Recovery and Climate Experiment (GRACE) mission
January 2007: National Academies Press
Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond
19 July 2007: European Space Agency
Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission
27 July 2007: BBC
'Space arrow' to map Earth's tug
22 August 2007: EurekAlert!
Climate change goes underground
23 August 2007: Vadose Zone Journal (open-access section)
Special Section: Groundwater Resources Assessment under the Pressures of Humanity and Climate Change
Additional sources include Wikipedia.

Although my first post in this series was intended to focus on the European Space Agency's push for Earth Observation missions, I recognize that I placed almost equal focus on US (NASA) efforts toward similar measurements. I want to provide context as well as commentary, and NASA has led the way in EO missions thus far. ESA, however, is the up-and-coming competitor in this area of science and remote sensing, and their efforts deserve recognition for innovation as well as the extensive partnership within the European Union that brings these missions about. That work is truly collaborative, and for the benefit of all. In the meantime, the bulk of NASA funding remains mired in the so-called "Vision for Space Exploration" proposed some time ago by the Bush administration, which is only now coming around to recognize that heavy investment in science and technical programs at both educational and professional levels will ensure American competitiveness in the coming years. The administration's approach to climate change is another story, but one that I think will show significant changes soon, and for very specific reasons that I will discuss in that series of posts in the coming weeks.

On the topic of groundwater remote sensing, the GRACE mission is a partnership between NASA and the German Space Agency and has been in orbit for more than five years now. The mission is actually past it's nominal operational lifetime, and has repeatedly shown its value in the measurement of total-column water storage changes, which translate primarily to changes in groundwater storage when other factors (atmospheric and surface water) are removed from the calculation. The twin satellites flying in formation that make up the GRACE mission were, in fact, designed primarily to measure spatial differences in total gravity of the Earth, of which water constitutes only a miniscule portion. Given all the core and mantle material that make up nearly 6360 km of the Earth's radius to the underside of the crust, and that the other approximately 11 km of Earth radius through the Earth's crust is nearly solid bedrock, albeit plastic in many regions but with proportionately small water content almost everywhere, the geologists' noise becomes the hydrologists' signal. GRACE is capable of capturing changes in total water storage at spatial resolutions near 800 km, and has been used for studies of water balance in large drainage basins such as the Mississippi, Amazon, Ganges, and others. A great deal of information on GRACE data, science and publications is available at the NASA Jet Propulsion Laboratory's GRACE Tellus site.

As mentioned in Part 1 of this brief series of posts on ESA EO missions, the US National Research Council released a "Decadal Survey" of NASA and community Earth Science missions earlier this year. That report called for a GRACE follow-on, dubbed "GRACE-II," as a medium-cost mission (approx. $450M) for launch in the 2016-2020 time period. For data continuity, the community and NASA should hope for a follow-on mission sooner. In fact, given the nominal lifetime for the original GRACE mission, launch of GRACE-II should have been planned for sometime in the next three years, not 9+ years out! However, the report also suggested upgrades to the mission concept, including an even better laser ranging system for the twin satellites that will allow for more accurate measurements and significant improvements in spatial resolution. Since it takes time and money to develop that kind of technology, following one ground-breaking mission with another deserves a little slack. But only a little! Remember, data continuity is key to change detection.

In the meantime, ESA has begun construction and testing of the first Earth Explorer Core mission of its Living Planet Programme. The mission is a single-satellite design with highly sensitive accelerometers and 12-channel GPS receivers, and aims to measure the Earth's geoid at spatial resolutions of 100 km or better. Such an anticipated improvement over GRACE is welcome in the science community, and will allow for more detailed studies of both large- and medium-sized river basins for groundwater, surface water, and glacial cover, as well as oceanographic factors such as sea-level changes and deep-layer currents for which both GRACE and GOCE are also designed.

The GOCE mission is scheduled for launch in Spring 2008 and may actually provide the data continuity that the community wants, provided the measurements are sufficiently compatible. Then, I suppose, it's up to GRACE-II to advance the science and technology even further. The first GRACE was selected for funding under NASA's Earth System Science Pathfinder (ESSP) program as its first mission (ESSP-1) in 1997. GRACE was launched in 2002, the Science Team was subjected to recompetition for funding in 2007, and the mission has already been extended through 2009. There is no funding for development of the follow-on mission in the NASA FY2008 budget request. Neither is there funding requested to support the next program Announcement of Opportunity (AO), for ESSP-4. The AOs were supposed to come biennially, but it takes even longer than that just to collect the proposals and decide on the finding selections, and the latest has been held in anticipation since ESSP-3 selections were announced in 2002, when the ESSP-4 AO would have been scheduled for release.

In related news, scientists in two collaborative groups based primarily in Europe have begun looking at the potential impacts of climate change on groundwater resources. Based on climate results fed to detailed land surface models (LSMs) that account for subsurface storage and aquifer recharge, as well as runoff and vegetation, both groups have concluded that the response of groundwater to climate forcing is essentially a positive feedback: greater storage and recharge in wetter regions, reduced storage and recharge in dry areas. Many dry areas of the world were once wetter, though, and fossil aquifers exist in many regions (US Midwest, Sub-Saharan Africa, Saudi Arabia) where the geology is favorable for aquifer formation. These sources are seeing growing rates of withdrawal and exploitation. A study by Danish and Greenland scientists has found:
"The magnitude of the water response to the simulated climate change was highly dependent on the geological setting. In the study area characterized by sandy top soils and large, interconnected aquifers, the groundwater levels rose significantly. For the other study, with low-permeable top soils and thick clay layers, the groundwater levels only showed minor changes. The primary effect in this area was the change in river discharge with up to 50% increase in winter and 50% decrease in summer."
Several studies by these and other groups were presented in April 2006 at a UNESCO conference on Groundwater Resources Assessment under the Pressures of Humanity and Climate Change (GRAPHIC) and appear in a special open-access section of Vadose Zone Journal beginning today.

Though the subsurface reacts much more slowly to climate changes in the atmosphere and at the land surface, groundwater is a vital and less-understood link in the global hydrologic cycle, and provides countless people with their only water source in many areas of the world, including those struggling just to survive, let alone grow food and make a living. Other than individual well measurements and lots of mathematical guesswork, remote observations from orbital platforms provide an ideal way to get at the present state of groundwater resources, improve our understanding of the dynamics involved, and get a better grasp on the future of these resources under the influence of inevitable climate change.

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