Soil Humidity Assessment over large Regions using C- and Ku-band Scatterometers

Soil moisture is an important component of the global energy and water cycles. It is a limiting factor in agricultural production, puts an important control on ecosystems, influences runoff, affects weather patterns, and is a crucial element in climate change studies. It can be measured accurately on the ground but only few measurements programs have accumulated substantial soil moisture data because of the high costs. Microwave Remote sensing may resolve this problem because soil moisture maps of large areas could be produced at relatively low costs, but methods to separate the contributions from other influencing parameters on the remotely sensed signal need to be found. Although initially designed for measuring wind speed and wind directions over oceans, spaceborne scatterometry holds considerable potential for monitoring soil moisture conditions over large areas. Spaceborne scatterometers are radars with a short revisit time and with a spatial resolution in the order of tens of kilometres. The ERS Scatterometer is operated in C-band (5.3 GHz) and has been flown on the European Remote Sensing Satellites ERS-1 and ERS-2. It has provided an uninterrupted stream of high-quality backscatter measurements over land and oceans since the launch of ERS-1 in July 1991. NASA is planning to launch SeaWinds, a Ku-band scatterometer (13.4 Ghz), on-board of QuikSCAT on May 19, 1999 and on-board of ADEOS II in 2000. The registered backscattering coefficient is dependent on vegetation, and over regions with low vegetation cover (grassland, agricultural areas, bare soil) on soil moisture and surface roughness. To estimate soil moisture the other geophysical parameters need to be accounted for. For the ERS Scatterometer, a soil moisture retrieval algorithm that accounts for heterogeneous land cover and vegetation phenology is already available. It has been tested over a number of regions, but has not yet been tried on a continental scale. Due to the limited lifetime of past Ku-band missions the backscattering behaviour of land surfaces in Ku-band is not yet well understood. More fundamental research is therefore needed before soil moisture retrieval methods can be proposed. The objective of the project is to advance our knowledge of the backscattering behaviour over land surfaces in C- and Ku-band and to apply this knowledge for the purpose of large-scale soil moisture mapping based on scatterometer data. The aim is also to produce high-quality soil moisture data sets that are of value in applications such as agronomy, meteorology, hydrology, and climate change studies.

The aim of SHARCKS was the retrieval of high quality soil moisture information from Scatterometer data. The study lead to the first global soil moisture data set which will be available to researchers of various disciplines. The aim of SHARCKS was the retrieval of high quality soil moisture information from remotely sensed Scatterometer data. Scatterometers are active microwave sensors with a low spatial (a few kilometers) but a high temporal (daily coverage of the earth surface) resolution. These properties make them an ideal instrument for monitoring global, highly dynamic processes like soil moisture. Soil moisture exerts prominent control on the interactions between the hydrosphere, biosphere and atmosphere. Being basic to all surface bio-geophysical processes an accurate assessment of the spatial and temporal variation of soil moisture is important for numerous applications and for answering diverse research questions. However, except a few large-scale measurement networks providing widespread information on soil moisture, continuous soil moisture information is all but nonexistent as area representative precise in-situ measurements are in general expensive and tedious to collect. Remote sensing of soil moisture from the vantage point of space might bypass the limitations of traditional methods by naturally providing a spatial measure at relative low cost covering large areas. Especially microwave technology has high potential to measure soil moisture under a variety of environmental conditions so that it could be extended to a routine measurements system But still soil moisture remote sensing is fraught with challenges and currently no methods are available to retrieve high quality soil moisture information from the various datasets. The lack of missing information on soil moisture variability is felt as a pressing deficiency in various applications (yield forecast, weather forecast, drought and flood early warning ) with far reaching significance to human society. Therefore both the American as well as the European Space Agency have implemented special satellite programs dedicated to the earth surface hydrology, the Aqua (NASA) and the Soil Moisture and Ocean Salinity SMOS (ESA) program. Both programs rest on passive microwave systems. Within SHARCKS the potential of scatterometers and a soil moisture retrieval algorithm especially developed for this type of sensors has been investigated. Global application of the algorithm lead to the retrieval of the first global, high quality soil moisture dataset. This dataset will be available to researchers in climatic, agronomic, meteorological and hydrologic disciplines. Furthermore experience gathered will directly impact the success of multinational large scale projects like AQUA, SMOS and upcoming satellite programs like the Advanced Scatterometer which will be launched in 2005