## 1. Introduction

[2] Evapotranspiration (ET) is arguably the most challenging hydrological process to predict. Even though the basic physics of ET is well understood [e.g., *Shuttleworth*, 1993; *Brutsaert*, 1982], we are still facing some difficulties in modeling ET, a crucial component of the land surface water and energy balance [*Desborough et al.*, 1996; *Henderson-Sellers et al.*, 2003]. Efforts to improve ET simulation models, too numerous to be summarized here, have primarily focused on improving the parametrization of physical processes, in particular the turbulence in the atmospheric boundary layer [e.g., *Tillman*, 1972; *Katul et al.*, 1996], and on incorporating more field and remote sensing observations [e.g., *Kalma et al.*, 2008]. Toward the same goal, in this study we propose a different kind of ET model taking advantage of the emerging theory of maximum entropy production (MEP) [*Dewar*, 2005] as a derivative of the maximum entropy (MaxEnt) theory [*Jaynes and Bretthorst*, 2003].

[3] The MaxEnt theory was developed as a general inference tool for any systems that need to be described probabilistically. The MEP is derived from applying the MaxEnt to nonequilibrium thermodynamic systems. An excellent overview of the MEP theory and its applications to a range of subjects is given by *Kleidon and Lorenz* [2005]. More insightful views about the potential applications of the MEP theory in land surface hydrology are reported by *Kleidon and Schymanski* [2008]. The MEP method differs conceptually from traditional “physically based” approaches [e.g., *Sellers et al.*, 1997]. Basically, the MEP theory addresses the question “what is the best prediction based on the available information?,” while the classical physical theories deal with the question “what are the fundamental laws governing the physical world?.” A proof-of-concept MEP model of surface heat fluxes over a dry soil [*Wang and Bras*, 2009] has demonstrated the usefulness and potential of the MEP theory in modeling the land surface energy balance. The MEP theory offers a possibility of a new approach to predicting ET (and heat fluxes). We attempt to realize that possibility by formulating an MEP model of ET guided by the case study of heat fluxes over a dry soil.

[4] A description of the MEP formalism is given by *Wang and Bras* [2009], and hence not repeated here. Section 2 focuses on model formulation starting from bare soil evaporation followed by transpiration from a canopy. The MEP solution of evaporation and transpiration together with sensible and ground heat fluxes are expressed as implicit or explicit analytical functions of surface temperature, humidity, and net radiation. In particular, no gradient variables are used as model input. Section 3 presents model validation using observations from several field experiments. Section 4 discusses some important properties of the MEP model. Section 5 gives a brief summary and our view on the potential applications of the MEP model.