Wednesday, November 15, 2006

Estimation of Bare-Soil Evaporation Using a Calorimetric Approach with Heat Flux Measured at Multiple Depths.

Joshua Heitman, Iowa State Univ, 1718 Crawford Street, Boone, IA 50036, Thomas Sauer, National Soil Tilth Lab USDA-ARS, 2150 Pammel Dr., Ames, IA 50011-4420, United States of America, Robert Horton, Iowa State Univ, 2543 Agronomy Hall, Ames, IA 50011, and Thomas DeSutter, USDA-ARS, 1524 Duff Ave., Ames, IA 50010.

An assumption in calorimetric methods for soil heat flux is that sensible heat terms can be balanced (i.e., if the heat flux is known at one depth, the heat flux at another depth may be determined by monitoring the change in heat storage). Latent heat from water evaporation is assigned to the energy balance at the surface. Despite this convention, evaporation occurs within the soil and represents a large potential heat sink. A soil energy balance would include a latent heat term in addition to the calorimetric terms. This provides an additional unknown term that could be calculated by monitoring soil heat flux at two depths. Experiments were performed in a bare field environment to evaluate this calculation of latent heat.  We used three-needle heat-pulse sensors to monitor soil heat capacity, thermal conductivity, and temperature from 0.6 mm to 7 cm below the soil surface. From these data we calculated soil heat flux and changes in heat storage. This information was used in the calorimetric approach, but with the soil heat flux known at both the upper and lower depths. The residual from this calculation (i.e., the net heat flux minus the change in heat storage) was treated as latent heat associated with water evaporation. These residuals provide an estimate of the temporal pattern of evaporation for a given depth increment. Daily summation of the residuals was compared to independent measurements of evaporation from mini-lysimeters. Our approach showed strong agreement with estimates from the mini-lysimeters (within 0.1 mm per d) between rainfall events, but poor agreement for periods immediately after rainfall events. Possible reasons for this disparity will be discussed. The calorimetric approach used here shows promise for providing additional information in the surface energy balance and showing temporal patterns of evaporation.