ABSTRACT
Soil evaporation not only determines partitioning of available energy between sensible and latent heat fl ux for bare soil surfaces but can also signifi cantly infl uence energy fl ux partitioning of partially vegetated surfaces. This latter effect occurs via the impact of soil evaporation on the resulting surface soil moisture and temperature. These, in turn, strongly infl uence the microclimate in partially vegetated canopies, indirectly affecting plant transpiration.[1] Over a growing season, soil evaporation can be a signifi - cant fraction of total water loss for agricultural crops.[2] On a seasonal basis in semiarid and arid regions, soil evaporation can signifi cantly alter the relative fraction of runoff to rainfall, which in turn has a major impact on the available water for plants.[3] In deserts, in spite of its small magnitude, soil evaporation can introduce signifi cant errors in meteorological forecasting if neglected.[4]
The measurement of soil evaporation at fi eld scale is typically obtained using standard micrometeorological techniques, namely Bowen ratio and eddy covariance methods. Traditionally, due to fetch and measurement requirements, under partial canopy cover conditions, these techniques are not able to partition the total evapotranspiration into its soil evaporation and plant transpiration components. Recently, a novel procedure for partitioning evapotranspiration through utilizing the measured high-frequency time series of carbon dioxide and water vapor concentrations has been developed and tested.[5] This approach relies upon the simple
assumption that contributions to the time series of carbon dioxide and water vapor concentrations derived from stomatal processes (i.e., photosynthesis and transpiration), and nonstomatal processes (i.e., respiration and direct evaporation) separately conform to fl ux-variance similarity. Vegetation water-use effi ciency is the only parameter needed to perform the partitioning. Further work is needed to evaluate the utility of this technique with eddy covariance data collected over a variety of land cover and climate conditions.
