Water Fluxes Determined from Sensible Heat and Mass Balance.

The sensible heat balance (SHB) method has been developed to measure in situ subsurface soil water evaporation at a fine (mm) scale using heat-pulse (HP) sensors. Numerical analysis suggests that, in combination with mass balance, SHB methodology can be extended for measuring liquid soil water fluxes associated with evaporation. The objective of this study was to measure soil liquid water flux during evaporation. Laboratory experiments were conducted with sand columns under constant upper boundary conditions but with varying lower boundary conditions via adjustments to water table position. Soil temperature was measured at 2.5 mm depth increments from the surface with thermocouples, and HP sensors were used to measure thermal properties and water content with depth and time over soil layers extending from the surface to 37.5 mm depth. Scales were used to continuously monitor system mass balance. Preliminary results indicated evaporation from mass balance agreed well with that from SHB when the water table was positioned > 30 cm, but showed some discrepancy between mass and heat balance when the water table depth was shifted subtly within the upper 30 cm of the soil. We hypothesized that discrepancies were associated with uncertainty in thermal property measurement resolution in the near surface soil derived from the impact of the soil-atmosphere interface on HP sensor measurements. Subsequent experiments examined the effects of the surface interface on thermal property measurements with the goal of quantifying uncertainty in heat and mass balance. Results from these on-going experiments will be presented and discussed.