This chapter for the most part has been confined to providing an overview of the retrieval of H and LE heat fluxes as well as soil water content using information derived from ther- mal and optical remote sensing sensors, with an emphasis placed on the so-called triangle method of Gillies et al� (1997) and Carlson (2007a)� As was clearly evidenced from the review of the Ts/VI methods, which was also undertaken here, the state-of-the-art in the retrieval of surface heat fluxes from optical and thermal remote sensing yields the retrieval of LE and H with a 10−30% accuracy, a range probably approaching the limit of accuracy currently pos- sible using satellite measurements, as was also indicated by Jiang, Islam, and Carlson (2004)�
This has been generally considered to be reasonable, given that the accuracy in the measure- ment of these fluxes using ground instrumentation is generally around 10−15% (as referred to in Jiang, Islam, and Carlson 2004)� However, in terms of the surface soil water content esti- mation, unlike radar-derived estimates, it appears that optical and thermal remote sensing are not able to achieve the accuracy of approximately ±4% vol/vol in the retrieval of surface soil water content recommended for a large range of applications, as reported, for instance, by Engman (1992), Calvet and Noilhan (2000), and Walker and Houser (2004)�
From all the methods employed today in the retrieval of the LE and H fluxes as well as M0 using optical and thermal remote sensing data, those based on the triangular (or trap- ezoid) space that emerges from a satellite-derived surface temperature (Ts) and VI appear to possess certain advantages� The ability of these methods to relate the patterns encapsu- lated by the Ts/VI pixel envelope to key biophysical properties explains the large number of studies concerned with their implementation for retrieving spatially explicit maps of H, LE fluxes, and M0� Of course, care must be taken in interpreting the surface moisture availability, as the precise meaning of any soil water content derived from satellite is fun- damentally uncertain, as pointed out in reference to Figure 19�1�
The most efficient and possibly the most consistently accurate one of these Ts/VI methods for estimating surface variables is the so-called triangle method of Gillies and Carlson (1995) and Gillies et al� (1997), the workings of which are presented in this chapter, along with some examples of validation exercises� Results from various validation exercises that have been conducted have shown that this method is able to provide results of at least similar, or in some occasions, better accuracy compared to other methods available� However, this method has several advantages in its use compared to other methods discussed in this chap- ter, which make it ideal for use by in the operational estimation of soil water content from the Visible/Infrared Imager/Radiometer Suite (VIIRS) and the Microwave Imager Sounder (MIS) under NPOESS starting in the year 2016 (Chauchan et al� 2003)�
It is understandable that arguments for a further comprehensive validation of this method are now of a high priority and scientific interest� It also appears that to best promote the
use of Ts/VI approaches, a number of technological/theoretical/practical hurdles must be overcome� The spatial and temporal resolution of satellite instruments with appropri- ate specifications for Ts/VI methods should be improved to allow the study of the surface heat fluxes and soil surface moisture at the spatiotemporal frequencies required� Also, the development of techniques for the implementation of these methods over cloudy conditions and conditions representative of a non-full-range of surface conditions, which is at pres- ent holding these methods from operational applications, should be further investigated�
Further work toward the development of methods for the operational retrieval of surface heat fluxes and surface soil water content utilizing remotely sensed data becomes more indispensable considering the forthcoming launch of new satellites� The VIIRS instrument planned to be placed in orbit by the NPOESS/NASA Preparatory Project in 2011, as well as the Sentinel-3 mission of the European Space Agency (ESA), which is planned to be launched in 2012, providing thermal infrared observations from space at 750 and 1 km, respectively, are expected to be highly valuable in estimating land atmosphere energy fluxes and surface soil water content from remote sensing in the coming years� The impor- tance of the use of data from the above sensors is even strengthened by the fact that no future mission has been yet formalized regarding the continuation of spaceborne thermal infrared data acquisitions at very high spatial resolutions and at a global scale, as a succes- sion of the Landsat and ASTER missions�
Acknowledgments
The authors wish to thank the Greek Scholarships Foundation (IKY) of the Ministry of Education of Greece for providing PhD scholarships that assisted in completing the inclu- sive verification exercises of some of the results presented here� In addition, the authors are grateful to the site managers of the CarboEurope IP sites for providing the validated ground truth measurements and to the ASTER Remote Sensing Data Analysis Center (ERSDAC) team of Japan for the free provision of the ASTER images over the CarboEurope IP sites used in this verification study� The authors also wish to express their gratitude to Dr� Marc Kennedy from DEFRA, United Kingdom, for providing the BACCO GEM SA software, which allowed the SA study implementation to the SimSphere model� Closing, Dr� Petropoulos wishes to thank Professor Martin Wooster and Dr� Nick Drake from King’s College, London, for their contributions to the content of this manuscript�
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