Deriving the surface soil heat flux from observed soil temperature and soil heat flux profiles using a variational data-assimilation approach.

A novel approach to infer surface soil heat fluxes from measured profiles of soil temperature, soil heat flux, and observations of the vegetation canopy temperature and the incoming short wave radiation, is evaluated for the Cabauw measurement facility in the Netherlands. The approach is a variational data-assimilation approach which uses the applied measurements to optimize on a daily basis parameter values of a model that describes the heat transport between the vegetation canopy and the surface and within the soil column. Inserting error characteristics that are either inferred from the field data itself or derived from literature, leads to valid estimates of the cost function for about 100 days in the year 2003. It appears that the approach gives values of the model parameters that compare well to values derived from literature, though at the end of 2003 values for the soil conductivity and the volumetric heat capacity of the soil start to differ from the literature values, possibly to specific soil characteristics and the extreme dryness of the summer of 2003. It appears that the model gives estimates of the surface soil heat flux that compare well with estimates using the currently operational Lambda approach, provided that the latter is adapted to account for the disturbance of the soil heat flux at the locations of the heat flux plates. Only when the surface soil heat flux is very small or very large, the new approach gives estimates of the surface soil heat flux that differ from the Lambda-approach.