Diurnal Covariation in Soil Heat Flux and Net Radiation

Abstract

Diurnal variation in soil heat flux is a key constraint on the amount of energy available for sensible and latent heating of the lower troposphere. Many studies have demonstrated that soil heat flux G is strongly correlated with net radiation Rn. However, methods to parameterize G based on this relationship typically do not account for the dependency of G on soil properties and ignore asymmetry in the diurnal variation of G relative to Rn. In this paper, the diurnal behavior of G as a function of Rn is examined for sparse cover and bare soil conditions, focusing on patterns of diurnal variation as well as on the effects of soil moisture and soil type. To this end, information from field data is combined with simulations from a multilayer, diffusion-based soil model over a range of soil conditions and vegetation densities. The results show that a relatively simple function can be used to capture the first-order diurnal covariation between G and Rn. Within this framework, soil moisture exerts an important control on this relationship. When soils make the transition from stage-1 (atmosphere limited) to stage-2 (soil limited) evaporation, the ratio of G to Rn tends to increase. Further, soils in stage-2 evaporation exhibit positive G later in the day relative to moist soils. Data from several field experiments show that the amplitude of diurnal surface temperature can be used to predict the magnitude and behavior of G/Rn by integrating the effects of soil type and moisture. Based on these results, a method to estimate G/Rn is proposed that provides a robust representation of G/Rn on hourly timescales for varying soil conditions. This method provides improvement over previous semiempirical treatments for G for which diurnal energy balance closure is required.