Soil Hydrology Model
Objectives
Regional-scale atmospheric models require the initialization of soil
water (SW) content over the model domain. Routine SW measurements over
an extended area are unfeasible. As a result, climatological estimates
of soil moisture have often been used to provide initial conditions for
the models. Unfortunately, this approach may result in large errors
during periods of excessive rainfall or drought. In the absence of
observations, an indirect method is needed to determine SW fields from
routine meteorological observations. This need is being met for SRBEX
by the development of the Penn State Soil Hydrology Model (SHM).
Current Activities
The SHM, developed by Bill Capehart and Toby Carlson (Capehart and Carlson,
1994), uses a one-dimensional soil-water diffusion and gravitation
scheme to determine the temporal evolution of the SW as a function of
depth. The model includes three modules: precipitation,
evapotranspiration, and sub-surface diffusion-gravitation. The
precipitation processes include leaf interception, infiltration,
ponding, and surface runoff. Evapotranspiration is modeled following a
Penman-type equation. The subsurface portion of the SHM follows the
Darcy's law description of SW movement. The SHM is driven by routine
meteorological observations at the surface (i.e., the atmospheric
forcing: air temperature, moisture, and wind speed, as well as rainfall
and cloud cover), and conventional topographic information,
surface-cover parameters (vegetation type, vegetation fraction), and
soils information (soil texture). The soil column depth, vertical grid
spacing and the nature of the lower boundary (impermeable, permeable or
saturated barrier) in the model are specified by the user.
The SHM is initialized with an arbitrary SW profile, usually chosen to
be 50% of saturation. The model, driven by the atmospheric forcing,
converges toward a common solution regardless of the chosen initial SW
value. This convergence takes place over a balancing period that may
range from several weeks to a few months, depending mostly upon the
precipitation amounts.
To provide the three-dimensional SW fields needed for initialization of
the mesoscale atmospheric model (MM5) used by
SRBEX, the one-dimensional SHM must be run at each grid point of the MM5
domain. Some of the MM5 pre-processing modules (those which grid land
surface properties data and objectively analyze meteorologoical
observations) have been adapted to generate the atmospheric forcing and
surface properties data for each grid point, and then pass them to a
driver program which runs the SHM at each point(Smith et al., 1994). This
publication also describes the linkage between the SHM system and the
MM5.
We are currently developing a real-time version of the SHM which will
provide updated SW fields to the real-time MM5 (Lakhtakia et al., 1994).
Testing of this linkage will include comparison of MM5-simulated
afternoon temperature and specific humidity close to the ground with
actual observations.
To facilitate the study of rapid drying of the surface soil layer, to
which remote sensing instruments appear to be
most sensitive, the SHM is also being modified to accept vertical grid
resolutions of less than 1 cm.
Results
The recent publication of high spatial resolution soils and landcover
databases for the 48 conterminous United States has permitted running
the SHM system with significantly more realistic surface characteristics
data for the parts of the MM5 domain within the 48 states. The
resulting simulated SW fields appear to fit actual measurements more
closely than the simulated values obtained using older, less accurate
soils and landcover data (Lakhtakia and Miller,
1995).
A 5-minute video has been prepared which shows the time evolution of the
SHM-generated SW fields over the SRBEX domain for the specific
application described by Smith et al. (1994). A copy is available for
loan from M. N. Lakhtakia, lakht@essc.psu.edu.
Last change: 10 May 1995,
R. A. White / raw@essc.psu.edu