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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Soil, Water & Air Resources Research » Research » Publications at this Location » Publication #224054

Title: Sensible Heat Measurements Indicating Depth and Magnitude of Subsurface Soil Water Evaporation

item Sauer, Thomas - Tom

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2008
Publication Date: 10/18/2008
Citation: Heitman, J.L., Xiao, X., Horton, R., Sauer, T.J. 2008. Sub-Surface Evaporation Determined from Sensible Heat Balance. Water Resources Research. 44. W00D05. [doi:10.1029/2008WR006961]

Interpretive Summary: Accurate measurement of evaporation of water from soil is important because the amount of water in the soil affects plant growth and many other processes. Methods were developed to measure soil water evaporation based on the amount of energy it takes to evaporate water or measurement of changes in soil water content. Attempts to combine these two methods have never been successful under field conditions. The objective of this study was to use new, small-scale sensors near the soil surface to measure both soil thermal properties and soil water content at the same time and calculate evaporation from individual soil layers. The sensors were placed in the soil very close to the surface and then monitored over time as the soil wetted and dried with rain and sunshine. Soil water evaporation was measured separatedly either by weighing cores of soil or by standard meteorological techniques. The new method succeeded in accurately measuring soil water evaporation and in identifying which layers in the soil the water was evaporating from. This research is important for scientists interested in measuring soil water evaporation directly from small areas and especially from specific layers in the soil.

Technical Abstract: Soil water evaporation is typically determined by techniques that assume the latent heat flux originates from the soil surface. Here, we describe a new technique for determining in situ soil water evaporation dynamics from fine-scale measurements of soil temperature and thermal properties with heat pulse-sensors. A sensible heat balance is computed using soil heat flux density at two depths and the change in sensible heat storage between. The residual to this balance is attributed to latent heat from water evaporation. Comparisons between near-surface heat flux and Bowen ratio energy balance measurements suggest that evaporation originates below the surface several days after rainfall. The sensible heat balance is able to account for this evaporation dynamic in mm-scale depth increments within the soil. Comparisons of heat balance daily evaporation estimates to Bowen ratio and mass balance estimates indicate strong agreement and point to potential of this new method.