Heat and water transport in soils and across the soil-atmosphere interface: 1. Theory and different model concepts

Evaporation is an important component of the soil water balance. It is composed of water flow and transport processes in a porous medium that are coupled with heat fluxes and free air flow. This work provides a comprehensive review of model concepts used in different research fields to describe evaporation. Concepts range from nonisothermal two-phase flow, two-component transport in the porous medium that is coupled with one-phase flow, two-component transport in the free air flow to isothermal liquid water flow in the porous medium with upper boundary conditions defined by a potential evaporation flux when available energy and transfer to the free airflow are limiting or by a critical threshold water pressure when soil water availability is limiting. The latter approach corresponds with the classical Richards equation with mixed boundary conditions. We compare the different approaches on a theoretical level by identifying the underlying simplifications that are made for the different compartments of the system: porous medium, free flow and their interface, and by discussing how processes not explicitly considered are parameterized. Simplifications can be grouped into three sets depending on whether lateral variations in vertical fluxes are considered, whether flow and transport in the air phase in the porous medium are considered, and depending on how the interaction at the interface between the free flow and the porous medium is represented. The consequences of the simplifications are illustrated by numerical simulations in an accompanying paper.