Two Alternatives to the Two‐Source Energy Balance Evapotranspiration Model

Abstract

The storage and flux of terrestrial water between reservoirs significantly affect nearly all land-atmosphere coupling processes. These reservoirs and fluxes are also vital elements of the Earth's surface energy balance. They are essential for scientific research and practical applications, including managing the availability and supply of freshwater for societies, determining ecological water requirements, and simulating global climate change. Evapotranspiration (ET) is the second-largest hydrologic flux in the terrestrial water cycle. It globally returns approximately 60%-65% of terrestrial precipitation directly to the atmosphere (Brutsaert, 2005) and also affects the weather and climate through vegetation-atmosphere interaction. Accurate estimates of the ET are necessary to form a theoretical understanding of vegetation-atmosphere interaction and also to simulate its influence on weather, climate, and water resource utilization and management.An environmental model is a simplified representation of a natural system, and several important assumptions are required to allow the actual system to be simulated as realistically as possible. However, these assumptions can yield biased model predictions and compromise system modeling. As the scientific community's understanding of the interplay between ecological, geological, and hydrometeorological systems improves, new physically realistic and accurate assumptions are constantly proposed, which can, in turn, be used to further improve environmental models. For the ET model, Penman (1948) originally developed the Penman equation for estimating the potential evaporation from a wet surface. Later, Monteith (1965) adopted a surface or stomatal resistance term to reflect the effect of partially dry surfaces on evaporation, which led to the formalization of the Penman-Monteith (PM) model (Monteith, 1965;Penman, 1948). The initial development of the Penman equation and PM model used an implicit surface energy equation for evaporation. In order to analytically solve the energy balance equation (EBE) in the Penman equation and PM model, Penman (1948) and Monteith (1965) both used a major assumption by linearizing the Clausius-Clapeyron relation (CCR) around air temperature, which produced an approximate solution. However, linearization of the CCR introduces both empirical and conceptual errors into ET estimates (McColl, 2020; Paw U & Gao, 1988). Vallis et al. (2019) proposed a new approach to approximate