Evapotranspiration estimation performance of root zone water quality model: evaluation and improvement

Root zone water quality model (RZWQM) has proven to be useful in evaluating the agricultural systems. However, it has previously been recognized that its evapotranspiration estimates somewhat depart from the measured values indicating the potential for some improvement in this module. Estimation procedure is based on the dual surface approach of Shuttleworth and Wallace, that although having a solid theoretical basis, it is greatly empirical in its application due to the lack of accurate quantitative knowledge of the resistance terms that control the heat fluxes in the canopy. Analysis of the formulation used in RZWQM allowed to detect some weakness in the calculation of bulk surface resistance (r_s^c), which is based on stomatal resistance averaged by effective leaf area index (LAI_eff). In this work, an alteration to the definition of LAI_eff is proposed, which leads to an improvement of the estimates. It is also discussed that a greater improvement could be obtained by introducing a model of stomatal resistance response to environmental conditions, which would increase the complexity of the model and be difficult to apply given the lack of adequate knowledge of the quantitative behavior of stomata. Alternatively, the simple crop coefficient approach could be incorporated to define an upper limit to evapotranspiration fluxes and avoid some otherwise unrealistically high estimates of the current version of the model.