According to the SWAT approach, the soil surface temperature is a function of the previous day's temperature, the amount of ground cover and the temperature of the surface when no cover is present.

For the calculation of surface soil temperature, SWAT firstly considers the emissivity term, calculated with the following equation:

where εsr is the emissivity term (unitless), Hday is the solar radiation reaching the ground on the current day (MJ m-2 d-1),and α is the albedo for the day.

Then, this term is used for the computation of the temperature of a bare soil surface:

where Tbare is the temperature of the soil surface with no cover (°C), Tav is the average temperature on the day (°C), Tmx is the daily maximum temperature (°C), Tmn is the daily minimum temperature (°C), and εsr is the emissivity term (unitless).

Then, the influence of plant canopy or snow cover on soil temperature is incorporated with a weighting factor, bcv (unitless), calculated as:

where CV is the total aboveground biomass and residue present on the current day (kg ha-1) and SNO is the water content of the snow cover on the current day (mm H2O). SWAT considers as 0.0 the weighting factor for a bare soil and approaches 1.0 as cover increases.

Finally, the soil surface temperature can be calculated as:

where Tssurf is the soil surface temperature for the current day (°C), bcv is the weighting factor for soil cover impacts (unitless), Tsoil(1,dn-1) is the soil temperature of the first soil layer on the previous day (°C), and Tbare is the temperature of the bare soil surface (°C). The influence of ground cover is to place more emphasis on the previous day.s temperature near the surface.

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