- 9 July 2002: NASA
- Pathfinder Missions to Enhance Our Understanding of Earth
- 16 December 2005: NASA
- Hydrosphere State Mission
- January 2007: National Academies Press
- Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond
- 16 July 2007: European Space Agency
- Extreme weather monitoring boosted by space sensor
- 30 July 2007: European Space Agency
- ESA mission highlighted at remote sensing conference
It was in December 2005 (see above) that research scientists at NASA received news that the Hydrosphere State Mission ("Hydros") would not be funded forward into its construction and launch phases. The goal of Hydros was the observation of soil moisture content and freeze/thaw states using active and passive microwave instruments (scatterometer/radar and radiometer, respectively). To be fair, Hydros was an alternate selection for funding in its program, number three of Earth System Science Pathfinder mission selections on a list released in 2002 for which only the top two had been promised funding. With the National Academies' recent "Decadal Survey" of NASA and community Earth Science missions, and its recommendation for funding a mission equivalent to Hydros, it is likely that Hydros will be reincarnated as the "Soil Moisture Active/Passive" (SMAP) mission when funding for such becomes available. Hydros was a well-developed mission concept with extensive community support, and one hopes it will not go away quietly.
In the meantime, the European Space Agency (ESA) has released results of recent observations of soil moisture over sub-Saharan Africa at high spatial resolution (1 km) from ENVISAT, using synthetic aperture radar methods. This is a banner result of the ESA-supported SHARE project (Soil Moisture for Hydrometeorological Applications in the Southern African Development Community Region--how you spell SHARE with that, I don't know). SHARE is now moving out of its pre-operational phase and looking for continuation funding from the African community that may benefit from such detailed datasets, and so the results and impacts discussed in that news release are oriented directly on such a financial campaign. I certainly wish the ESA project, and especially its African partners, the best of fortune and luck.
SAR-based soil moisture retrievals have been around for almost two decades in the US and Canada, based on numerous airborne and satellite-based instruments and methods that have been published in trade journals for almost as long. The problem is, SAR is prone to error due to surface roughness, including vegetation, and it can see only the uppermost layer of the soil column when nothing else is in the way. One can get the general idea of spatial patterns in soil moisture from results like those shown at ESA's website (see sources above) but realistic, ground-truthed values are troublesome. The high resolution that is touted by ESA may be this method's only redeeming quality, and its full calibration and research-to-applications value may only be realized when a mission such as NASA's Hydros/SMAP (with approx. 3 km active resolution and 40 km passive resolution) and its ESA counterpart, the Soil Moisture and Ocean Salinity (SMOS) mission, are finally in orbit. While SMOS is scheduled for launch sometime in 2008, Hydros/SMAP has no definitive timeline and still awaits the go-ahead for further mission development.
These active/passive missions will employ different microwave bands from those on SAR missions, and will be able to see somewhat deeper into the soil column (and with less trouble over rough and vegetated surfaces) in order to give more useful soil moisture estimates. With their attendant surface monitoring sites to provide ground-truth calibration, these missions might be used in conjunction with the ENVISAT-type observations to eliminate differences and errors in the higher-resolution products, making them significantly more useful in application to drought and flood forecasting and even irrigation management. In modeling of the land surface, the depth (or layer thickness) of observation makes all the difference when it comes to assimilation and/or calibration, specifically of hydraulic parameters related to the accurate modeling of soil moisture transport, retention and vegetation uptake. The uppermost "skin" layer responds quickly to changes in air temperature, humidity, and precipitation at the surface, while layers farther down in the soil column respond more slowly. Yes, even a couple of centimeters in depth can make a big difference in the accuracy of assimilation and calibration, and thus in the feedback to the atmosphere for coupled modeling efforts.
Note the quote in ESA's news release about early use of the ENVISAT products by Chinese meteorological scientists. China has been investing heavily in resources throughout Africa, especially oil and metals, in order to feed their growing domestic demands. Some in the economic and security community see this (rightly, in my opinion) as Chinese support for development in Africa, which produces positive strategic feedback to China in support for increasing population and industrial demands. Others see China's work in Africa as economic and military competition for the US and the rest of the developed world, another Great Powers struggle that will bring us back to the Cold War. You know what? That's exactly what it is, except now it's dynamic economic investment in Africa instead of an ideological stalemate. It's the best kind of Great Powers competition: who can pour more money and manpower into African science, technology and development the fastest? Will it be the US, the EU, or China? In any case, all parties will benefit, none more so than the African community, and at least there's science in it--hydrology, no less!