Energy and Mass Exchange Over Incomplete Vegetation Cover

Competition for fresh water between agriculture and domestic and industrial uses is increasing worldwide. This is forcing subsistence and commercial agriculture to produce more with less water. Consequently, it is crucial to properly and efficiently manage water resources. This requires accurate determination of crop water loss into the atmosphere, which is greatly influenced by the exchange of energy and mass between the surface and the atmosphere. Measurement of these exchange processes can best be accomplished by micrometeorological methods. However, most micrometeorological methods are very expensive, difficult to set up, require extensive post-data collection corrections and/or involve a high degree of empiricism. This review discusses estimation of evapotranspiration using relatively inexpensive micrometeorological methods in temperature-variance (TV), surface renewal (SR) and mathematical models. The TV and SR methods use high frequency air temperature measurements above a surface to estimate sensible heat flux (H). The latent heat flux (λE), and hence evapotranspiration, is calculated as a residual of the shortened surface energy balance using measured or estimated net radiation and soil heat flux, assuming surface energy balance closure is met. For crops with incomplete cover, the disadvantage of these methods is that they do not allow separation of evapotranspiration into soil evaporation and plant transpiration. The mathematical models (single- and dual-source) involve a combination of radiation and resistance equations to determine evapotranspiration from inputs of automatic weather station observations. Single-source models (Penman-Monteith type equations) are used to determine evapotranspiration over homogeneous surfaces. The dual-source models, basically an extension of single-source models, determine soil evaporation and plant transpiration separately over heterogeneous or sparse vegetation. These mathematical models have also been modified to accommodate inputs of remotely-sensed radiometric surface temperatures that enable estimation of evapotranspiration on a regional and global scale.