Retrieval of soil moisture at global scale from satellite data acquired by passive and active microwave sensors

Soil moisture plays an important role in land surface processes, such as partitioning of incoming radiation into latent and sensible heat fluxes and partitioning of precipitation into surface runoff and infiltration. It affects vegetation processes such as the transpiration and plant growth. Then, the evapotranspiration-precipitation feedback influences the local weather. Therefore, long term and continuous soil moisture information at small, as well as large, scale is a key variable to run, calibrate and validate hydrological and agronomical models, weather forecast and climate models. The traditional measurement methods, such as gravimetric sampling and automatic probes, are suitable for soil moisture monitoring at small scale (i.e. local and field scale), but the use of this methods to estimate the typical high spatial variability of soil moisture over large areas (i.e. from regional to global scale) is out of question.

Passive and active microwave sensors on board satellite platforms have shown the capability to retrieve the soil moisture of the surface layer over large areas. Although, both surface emission and surface scattering are sensitive to other factors than the soil moisture, such as soil roughness, vegetation physical and structural properties and surface temperature. The space agencies are supporting new satellite missions, whose goal is the retrieval of soil moisture at global scale, such as ESA’s Soil Moisture and Ocean Salinity mission (SMOS) launched on 2nd November 2009 and NASA’s Soil Moisture Active and Passive mission (SMAP) expected in 2015.

The main objective of this research is to develop a new method for the retrieval of soil moisture at global scale, which is based on the synergistic use of active and passive microwave data. The possibility to integrate active and passive microwave data is theoretically based on the relationship between the fundamental processes that determine the surface emission and scattering.

As satellite data with a course resolution (more suitable for large scale soil moisture monitoring) will be employed, the validation could be a critical task due to the large scale gap between in-situ soil moisture measurements and satellite data spatial resolution (25-50km) and due to the typically high spatial variability of soil moisture. For this reason, the soil moisture retrieval algorithm will be validated over two large soil moisture networks: Maqu site in China and Twente site in The Netherlands.

Maqu site is located on the northern Tibetan plateau, in Gansu province in China, at the first major meander of the Yellow river, with hilly topography and uniform short grassland at an altitude of approximately 3439m.a.s.l (Figure 1). 20 soil moisture and soil temperature stations were installed in summer 2008 in an area of approximately 40km*80km, in collaboration with the CARRERI institute of the Chinese Academy of Sciences. Twente site is located in The Netherlands. The Twente region is relatively flat between 9 and 80m.a.s.l (Figure 3). The land use is mainly agriculture (71%) on sandy soils with a small forest patch (11%). 20 soil moisture and soil temperature stations have been installed between October 2008 and June 2009 in an area of approximately 45km*60km. Soil moisture data collected in both sites at 5 cm depth are shown in Figure 3 and 4.

The two test sites are located in different climatic regions and are characterised by different topography, vegetation cover and soil hydrological properties. This gives the possibility to have a more accurate validation of the proposed retrieval algorithm for different conditions of the surface and covering different soil moisture ranges.

Figure 1: Left: Maqu area, location of the 20 soil moisture and soil temperature stations and of the micrometeorological tower of CAREERI-CAS; Right: Typical landscape of Maqu area: hills, grassland and bare areas, river valleys, Yellow River, wetlands.

Figure 2: Calibrated volumetric soil moisture measured at 5 cm depth in all Maqu network stations. The highest values are measured in wetland areas and the lowest in sandy soils.

Figure 3: Left: Twente area and location of the 20 soil moisture and soil temperature stations; Right: Typical landscape of Twente area: pasture and agricultural fields, forest patches and urban areas.