Evapotranspiration, Transpiration, and Evaporation Processes and Modeling in the Soil–Residue–Canopy System

Accurate quantification of evapotranspiration (ET) in irrigated and rainfed agriculture under different soil, water, and crop management practices and climatic conditions has been, and will always be, paramount for effective management of water resources. This is especially important for enhancing crop and water productivity for the world's rapidly increasing population. While changing global climatic conditions further exacerbate this challenge, scientific investigations and discoveries will be necessary to address these large-scale challenges and develop practical solutions at the scale of management. ET, which is a combination of evaporative losses from the plant canopy through transpiration and evaporation from the plant–soil interface and other surfaces in agroecosystems, is one of the most complex processes in terrestrial system water vapor and energy exchange and hydrologic processes. Its accurate quantification is important for various applications, including irrigation and water management, crop growth modeling, climate change studies, assessing the impact of climate change on agricultural productivity and water resources, assessing the impact of land use change on agroecosystem hydrology and water balance, plant physiology and agronomy studies, groundwater resources replenishment, agricultural water rights transfers, and others. This chapter focuses on recent developments and improvements in modeling of ET, transpiration (T), and soil evaporation (E) processes occurring from the soil–residue–canopy system in the light of changing climate conditions. While various models have been introduced over the decades, ET models must be capable of responding to the changes to which the agricultural ecosystems have been subjected to in the past and will be in the future. We describe the formulations and applications (across agricultural, horticultural, and natural landscapes) of the most-used single-source and multisource combination ET models, namely the Shuttleworth–Wallace (SW) and Penman–Monteith models and their recent refinements and applications. Major complements and critiques of these models are also discussed, especially when compared with our current theoretical level of understanding. In general, ET models are limited in their application to comprehensively reflect the physiological and biophysical changes in plants in response to elevated CO 2 concentrations, the implications of which are discussed. Alternative approaches that compensate for or bypass these limitations are discussed as well, with recommendations for future lines of research and applications.