For the calculation of soil temperature, SWAT utilizes a mechanistic approach, that starts from the computation of a maximum damping depth, with the following equation:

 

where ddmax is the maximum damping depth (mm), and ρb is the soil bulk density (t m-3).

 

Then, starting from this equation, the impact of soil water content on the damping depth is considered via a scaling factor, calculated with the equation:

 

     

 

where φ is the scaling factor for water content (unitless), SW is the amount of water in the soil profile expressed as depth of water in the profile (mm), ρb is the soil bulk density (t m-3) and ztot is the depth from the soil surface to the bottom of soil profile (mm).

 

After this, the actual value of the damping depth is calculated with this equation:

 

 

where dd is the actual damping depth (mm), ddmax is the maximum damping depth (mm) and φ is the scaling factor for water content (unitless).

 

Then, a depth factor is calculated via the equation:

 

 

where df is the depth factor that quantifies the influence of depth below surface on soil temperature (mm) and zd is the ratio of the depth at the center of the soil layer to the dumping depth, calculated via this equation:

 

 

where z is the depth at the center of the soil layer (mm) and dd is the actual damping depth (mm).

 

Finally, the average daily temperature in the middle of each soil layer can be calculated with the following equation:

 

where Tsoil(z,dn) is the soil temperature (°C) at depth z (mm) and day of the year dn, l is the lag coefficient (ranging from 0.0 to 1.0) that controls the influence of the previous day's temperature on the current day's temperature, Tsoil(z,dn-1) is the soil temperature (°C) in the layer from the previous day, df is the depth factor that quantifies the influence of depth below surface on soil temperature (mm),

TAAair is the average annual air temperature (°C), and Tssurf is the soil surface temperature on the day. SWAT sets the lag coefficient, l , to 0.80.

 

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