Predicting bare soil temperature. I. Theory and models for the multi‐day mean diurnal variation

Summary The energy balance equation for the soil‐air interface is expressed, by suitable approximations, in a form linear in the surface temperature T 0 . This forms the basis of two predictive models for the diurnal variation of T 0 , averaged over a many‐day interval, for potentially evaporating bare soil. Input data include total solar radiation, air temperature and vapour pressure, windspeed u a , cloud cover c , and soil radiative, aerodynamic and thermal properties (the latter assumed uniform). In the simpler model, based on harmonic analysis, u a and c are assumed constant. For N t observations per day, an algebraic solution is found as a set of equations for the parameters of up to the first N t /2 harmonics. In the second model, dynamic wind‐speed is introduced, and a more complicated linear algebraic method of solution is required. The models have agroclimatic value, since by averaging they predict the thermal ‘climate’ of soil, in contrast to instantaneous models which describe the time‐specific thermal ‘weather’.