Bulk density effects on soil hydrologic and thermal characteristics: A numerical investigation

Soil bulk density ((b)) is commonly treated as static in studies of land surface dynamics. Magnitudes of errors associated with this assumption are largely unknown. Our objectives were to (a) quantify (b) effects on soil hydrologic and thermal properties and (b) evaluate effects of (b) on surface energy balance and heat and water transfer. We evaluated 6 soil properties, volumetric heat capacity, thermal conductivity, soil thermal diffusivity, water retention characteristics, hydraulic conductivity, and vapour diffusivity, over a range of (b), using a combination of 6 models. Thermal conductivity, water retention, hydraulic conductivity, and vapour diffusivity were most sensitive to (b), each changing by fractions greater than the associated fractional changes in (b). A 10% change in (b) led to 10-11% change in thermal conductivity, 6-11% change in saturated and residual water content, 49-54% change in saturated hydraulic conductivity, and 80% change in vapour diffusivity. Subsequently, 3 field seasons were simulated with a numerical model (HYDRUS-1D) for a range of (b) values. When (b) increased 25% (from 1.2 to 1.5 Mg m(-3)), soil temperature variation decreased by 2.1 degrees C in shallow layers and increased by 1 degrees C in subsurface layers. Surface water content differed by 0.02m(3)m(-3) for various (b) values during drying events but differences mostly disappeared in the subsurface. Matric potential varied by >100m of water. Surface energy balance showed clear trends with (b). Latent heat flux decreased 6%, sensible heat flux increased 9%, and magnitude of ground heat flux varied by 18% (with a 25% (b) increase). Transient (b) impacted surface conditions and fluxes, and clearly, it warrants consideration in field and modelling investigations.