|Norman, John - UNIV OF WISCONSIN|
Submitted to: Agriculture Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 10, 1998
Publication Date: N/A
Interpretive Summary: Monitoring evapotranspiration (ET) of agricultural crops is important for assessing water stress and productivity. A soil-vegetation model has been developed that can use remotely-sensed surface temperatures for computing soil evaporation and canopy transpiration over partial vegetated covered surfaces. Such remotely-sensed observations are currently available from several satellite systems and therefore could be used operationally with this simplified model. It is shown that two key model parameters require modification for application in arid climates. Another version of the model that uses canopy and soil temperatures is shown to be useful in estimating these model parameters. Therefore, this model has potential in monitoring ET operationally over the continental U.S. The information will help to assess crop and rangeland conditions and provide an operational means for monitoring water use and impacts of climate change on agricultural lands.
Technical Abstract: A two-source model developed to use radiometric temperature observations for predicting component surface energy fluxes from soil and vegetation was evaluated with data from a row crop (cotton)in an arid climate. The original formulation for canopy transpiration uses the Priestley-Taylor assumption with the universal constant = 1.26. The total or combined heat fluxes from the soil and vegetation agreed to within 20% of the observed values, on average. However, an analysis of the component heat fluxes from the soil and vegetation predicted by the model indicated that soil evaporation was higher than canopy transpiration, on average, even though nearly three weeks had elapsed since the last irrigation. In order to obtain more physically realistic soil and vegetation component heat fluxes and better agreement between the predicted and observed soil and canopy temperatures, two model parameterizations required modification. One adjustment was to the value of the Priestley-Taylor coefficient. The othe modification was to the soil resistance to sensible heat flux transfer equation based on the recent experimental results. The model was also run using the observed soil and canopy temperatures, thus avoiding the need for the Priestley-Taylor formulation. The component heat fluxes using the observed temperatures support these modifications. Studies indicate that component temperatures can be estimated using radiometric temperatures at significantly different view angles, and it is proposed that with meaurements appropriate values of model parameters could be derived for different ecosystems.