Modelling the soil

The soil is important since it represents the main land storage of heat and water available for release into the atmosphere later.  The multi-layer soil model has a fairly realistic representation of the vertical density and temperature profiles of the soil which allows a good representation of its thermal properties.  

Structure of the soil

The structure of the soil is not usually uniform throughout the top layers of the earth but for a given location is normally similar within the top ~1.3m of soil.  Variations of density of the soil and fluxes of heat and moisture are more related to the texture and the water or ice content, and vegetation roots have an impact on the retention of water.

There is an exchange of heat, moisture and momentum between the atmosphere and underlying surface according to the vegetation type.   

The multi-layer soil model

The IFS multi-layer soil model uses four layers to represent the top ~1.3m of soil and the complex heat fluxes and interactions between them.   These are sufficient to represent correctly all timescales from one day to one year.   The soil model represents the vertical structure of the soil and the evolution of soil temperature and liquid water content in each layer.  Heat and moisture energy flux:

• at the top of the surface model soil layer (Layer 1):
  • thermal coupling across the soil-atmosphere interface is the balance of upward and downward energy fluxes at the soil surface.  These are computed as a weighted average over the 'tiles' representing vegetation type, and includes the appropriate albedo and the effect of any water evaporation.   Upward heat fluxes cross the snow-soil interface of any overlying snow - this allows a representation of changes in permafrost.

  • an interception layer collects water from precipitation and dew fall.  Infiltration and run-off are represented depending on soil texture and standard deviation of subgrid orography.   A fraction of the water flux (rain or snow melt) is considered runoff according to the soil texture, soil water content and the standard deviation of orography (runoff can be up to 30% of rainfall in complex orography or mountainous regions).

• between all layers:
  • heat transfers upwards or downwards.  The effects of frozen water, or freezing and melting of water has an effect upon the transfer, release and absorption of heat in each layer.
  • water percolates downwards but soil water transfer dependent on a soil water potential.  Water is removed by the roots at all levels according to the root depth and transpiration of surface vegetation.  Transpiration is suppressed in frozen ground.
• at the base of the lowest model soil layer (Layer 4):
  • there is no flux of heat.
  • free drainage is assumed with no modelling of bedrock (geographical distribution of bedrock depth not used).

The fluxes are illustrated and explained in Figs2.  

The characteristics of each grid box are updated through the forecast period (e.g. model snowfall might increase the area or depth of snow cover; model rainfall might increase soil moisture rather than be removed by run-off).  The areal extent of each land surface tile type (listed above) can vary in a rapid, interactive way during the model run, as rain falls then evaporates or snow accumulates then melts, etc.  The slope and aspect of orography within each grid box (e.g. south-facing, steepness) is not taken into account and HTESSEL may consequently under- or over-estimate solar heating and runoff.

The soil type for each land grid box is defined by an offline dataset and this soil type is used for all the layers.   Each soil type has its own physical characteristics:

• soil texture (defines water retention and hydraulic conductivity in the soil):
  • Coarse.
  • Medium.
  • Medium-Fine.
  • Fine.
  • Very Fine.
  • Extra-tropical Organic.
  • Tropical Organic.
• hydraulic properties (define amount of water in the soil and availability for vegetation):
  • saturation.
  • field capacity.
  • permanent wilting point.
  • residual moisture.
  • plant available soil moisture.
• infiltration capacity:
  • ability of water to percolate downwards from one layer to another (rain or dew at the surface).
  • surface evaporative fluxes consider separately the contributions from snow cover, wet and dry vegetation and bare soil.
  • base of lowest layer is considered as free draining.
• surface run-off:

  • when the water flux at the surface exceeds the maximum infiltration rate the excess water is considered surface runoff.

• extraction of water by plant root:
  • root depth varies according to plant species as defined by the "tile".

Considerations

• Soil characteristics can be very variable within a grid box.  Users and forecasters should take into account the peculiarities of a location when interpreting model output.
• The assigned average soil type for a grid box is not necessarily representative of an individual location.
• Runoff can be up to 30% of rainfall in complex orography or mountainous regions. 

Additional sources of information

(Note: In older material there may be references to issues that have subsequently been addressed)

Fig2.1.16: Schematic of four-level soil model with land surface tiles.  Surface heat and moisture fluxes using maximum selection of six different surfaces ('tiles').  The four layers of soil have differing moisture contents which vary:

• as water transfers directly (mostly downwards through gravity).
• by evapotranspiration (upwards) via the roots which penetrate to different soil depths.
• by freezing or melting.
• by runoff on the surface (assumed to be "lost" via river flow).
• by free drainage at the base of the model soil.

Table1: List of symbols for parameters shown in Figs2.

Fig2.1.xx: Example soil moisture chart VT 00UTC 14 Feb 2023 showing moisture in soil level 1, the surface layer.  The legend shows:

• Sandy shades: Soil moisture SM < Permanent wilting point PWP.  Living vegetation cannot be sustained. Values show soil moisture as a percentage of the permanent wilting point  value.
• Yellow/Green shades: Permanent wilting point PWP < soil moisture SM < field capacity CAP.  Evapotranspiration efficiency in percent increases as soil moisture increases.
• Blue shades: Capacity CAP < Saturation SAT.  Soil moisture super-saturation. Dark blue suggests flooding (in the model).

See the current soil moisture chart.  Select "Layer 1 2 3" from the drop down menu for the average moisture in the top metre of the earth.