HR: 0800h
AN: H21D-0744    [Abstracts]
TI: Soil-Water Evaporation Dynamics Determined From Soil Sensible Heat Transfer Measurements
AU: * Heitman, J L
EM: jlheitman@ncsu.edu
AF: Soil Science Dept., North Carolina State University Campus Box 7619, Raleigh, NC 27695, United States
AU: Horton, R
EM: rhorton@iastate.edu
AF: Agronomy Dept., Iowa State University Agronomy Hall, Ames, IA 50011, United States
AU: Sauer, T J
EM: tom.sauer@ars.usda.gov
AF: USDA-ARS, National Soil Tilth Laboratory 2150 Pammel Drive, Ames, IA 50011, United States
AU: DeSutter, T M
EM: thomas.desutter@ndsu.edu
AF: Soil Science Dept., North Dakota State Univ. Walster Hall, Fargo, NC 58105, United States
AB: Soil-water evaporation is important in both the hydrologic cycle and the surface energy balance and for processes ranging from microbial ecology to global climate change. Yet, routine measurements are unable to capture rapidly shifting near-surface soil heat and water transfer processes involved in soil-water evaporation. Recent advancements in fine-scale measurement of soil thermal properties provide a new opportunity to observe heat transfer associated with soil-water evaporation in the upper centimeters of the vadose zone. The objective of this study was to determine the depth and location of the evaporation zone within soil using observations of sensible heat transfer. Three-needle heat-pulse sensors were used to monitor soil heat capacity, thermal conductivity, water content, and temperature below a bare soil surface in Central Iowa during natural wetting/drying cycles. Soil heat flux and changes in heat storage were calculated from these data to obtain a balance of sensible heat components. The residual from this balance (i.e., the net heat flux minus the change in heat storage) was attributed to latent heat from water evaporation, and thus, provided estimates of in situ water evaporation. As the soil dried following rainfall, results showed divergence in the soil sensible heat flux with depth. Divergence in the heat flux indicated the location of a heat sink associated with soil-water evaporation. Evaporation estimates from the sensible heat balance provided depth and time patterns consistent with observed soil-water depletion patterns. Evaporation occurred near the soil surface immediately after rainfall and the evaporation zone proceeded below 3 mm in the profile within 2-3 d of rainfall events. As the soil dried, the evaporation zone continued to penetrate deeper into the soil extending below 13 mm within 6 d after rainfall. Peak evaporation rates as high as 0.42 mm/h were observed at the 3-mm depth near midday, with evaporation declining by late afternoon. Evaporation occurred simultaneously at multiple soil depth increments, but with time lag between peak evaporation rates for the deeper depths. Daily heat-balance evaporation estimates compared well with microlysimeter evaporation estimates taken 3 or more d after rainfall providing root mean square error of 0.11 mm/d and r2 = 0.90. Implementation of fine-scale measurement techniques for the soil sensible heat balance provides a new opportunity to improve understanding of soil-water evaporation.
DE: 1818 Evapotranspiration
DE: 1843 Land/atmosphere interactions (1218, 1631, 3322)
DE: 1866 Soil moisture
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: 2007 Fall Meeting