1 Spatial Modelling of the Terrestrial Environment - Georeferencial

60 Spatial Modelling of the Terrestrial Environment
land surface, such as remotely sensed estimates of soil moisture, will find value. Coupling
a description of microwave emission to the land surface models used in an LDAS should
allow direct assimilation of measured microwave brightness temperatures and improve estimates
of the soil moisture fields calculated by the LDAS (Houser et al., 1998; Reichle
et al., 2001; Galantowicz et al., 1999; Burke et al., 2003).
Remotely sensed observations of global surface soil moisture using an L-band (1.4 GHz)
passive microwave radiometer are likely to become available within the next decade. Hence,
there is considerable interest in developing methods to use this information effectively.
However, such measurements have limitations, specifically:
1. The relationship between the measured microwave brightness temperature and soil moisture
is strongly influenced by the presence of vegetation. A simple one-parameter (optical
depth) model is usually used to specify the effect of the vegetation, with the optical depth
taken to be proportional to the vegetation water content. However, the constant of proportionality
(the opacity coefficient) is an uncertain function of vegetation type and
radiometer characteristics.
2. L-band microwave brightness temperatures are only directly related to soil moisture in
the top few centimetres of the soil and information about the soil moisture deeper in the
soil profile has to be inferred indirectly through the use of land surface models.
3. In the near future, any potential L-band mission will likely have a resolution ∼30–60
km for technical reasons. At this resolution, the vegetation cover, soil type, and soil
water content within each pixel will necessarily be heterogeneous for all pixels and this
could introduce errors. For some regional and local-scale applications, such as modelling
runoff from a catchment, for example, or for initializing a mesoscale model to predict
convective precipitation, higher resolution fields of soil moisture are important.
This chapter describes progress towards potential solutions to these issues through the use
of coupled land surface and microwave emission models, where possible in the context of
potential upcoming L-band satellite missions (SMOS – Kerr et al., 2001 and HYDROS –
HYDROS homepage).
4.2 Models and Methods
4.2.1 Coupled Land Surface and Microwave Emission Models
Coupled land-surface and microwave emission models can be used as a tool to explore
the potential of L-band microwave radiometry and to evaluate the relationship between
microwave brightness temperatures and soil moisture. The land surface model is forced by
meteorological data (incoming solar radiation, incoming longwave radiation, air temperature,
relative humidity, precipitation and wind speed). It uses these data to provide calculations
of the evolving (profiles of) soil water content and soil temperature and vegetation
temperature, which are input to the microwave emission model. The microwave emission
model then calculates the microwave brightness temperature of the soil–vegetation–
atmosphere interface. Such forward modelling of microwave brightness temperature incorporates
model physics neglected in simple retrievals of near surface soil moisture, including
representation of the effect of non-uniform near-surface profiles of soil moisture, and