Method to Calculate Soil Heat Flux That Accounts for Sunlit and Shaded Soil Beneath Row Crops.

Soil heat flux (G) is a component of the soil-plant-atmosphere energy balance, and can have significant impact on evapotranspiration (ET), especially for incomplete canopies. Most ET models calculate G as a fraction of net radiation (Rn), which is usually suitable for full canopy cover and spatial scales on the order of tens of meters or greater. However, sensor-driven ET models often consider much smaller spatial scales, where G has been shown to have large variation across interrows of row crops with partial canopy cover. This is related to differences in soil illumination and shading, which varies with row orientation, and may be of importance where rows are circular such as in center pivot fields. A method is proposed to calculate G that accounts for sunlit and shaded soil but does not require additional information beyond that typically required to calculate Rn. The method is tested by comparing calculated surface G to that derived from calorimetric correction of soil temperature and heat flux measured below the surface. Measurements were obtained at five locations across a crop interrow for two row orientations. The method resulted in closer agreement between calculated and calorimetrically-derived G values compared with calculations by models that ignored the effects of sunlit and shaded soil. This new method should result in more accurate modeling of ET at within-field scales.