Surface energy balance partitioning in tilled and non-tilled bare soils

Knowledge of the partitioning of the surface energy balance (SEB) components is essential in understanding heat and water budgets at the soil-plant-atmosphere interface. Agriculturally, changes in soil structure due to soil tillage in the fall and spring affects the magnitude of these components. SEB partitioning determined by modeled and measured studies usually assumes a constant near surface bulk density values for extended periods of time. The objectives of this study is to determine the effect of soil bulk density changes on net radiation, soil heat flux (G), latent heat flux (LE) and sensible heat flux (H) of a tilled and non-tilled bare soil with time. Micro-Bowen ratio (MBR) system were set up for 91 days on a tilled (Till) soil and a non-tilled (NT) soil at the Iowa State University Agronomy and Agricultural Engineering Research farm near Ames. MBR systems measured water vapor and air temperature at 0.01 and 0.06 m heights above the soil surface, to determine atmospheric gradients for LE and H calculations. Net radiation was obtained by a net radiometer positioned at 1.25 m above the surface, while soil heat flux measurements were obtained from soil heat flux plates at a 0.06 m depth and soil heat storage calculations (0- 0.06 m). Evaporation rates were also determined by micro-lysimeters. Two time periods, were selected early and late in the season (relative to tillage), to determine the effect of changing bulk density with time. Bulk density showed little to no change following tillage in Period 1 but increased by 0.11 g cm-3 following substantial rainfall events at the Till plot during Period 2. In Period 1, Rn and G fluxes did not differ between plots as bulk density and water contents were similar. The Till soil LE flux was 12% higher than in the NT according to the MBR measurements and 15% higher according to the ML measurements. In Period 2 (DOY 262-266), Rn and G fluxes showed relatively larger daytime difference between Till and NT. As bulk density and water content increased during this period, G fluxes represented 7% more Rn at the Till soil than in the NT soil. A subsequent 7% lower available energy was observed at the Till plot leading to 10% lower LE values for Period 2. The results of this research highlight the value of considering dynamic bulk density measurements with time when determining the distribution of energy at the soil surface