# Atmospheric Boundary Conditions

Often flow models need to include the atmosphere as the top boundary. The atmospheric boundary conditions can be specified using Dirichlet boundary elements with very large volumes. For EOS9, no atmospheric boundary element is needed since EOS9 uses Richards’ equation to describe variably saturated flow of a single aqueous phase and treats the gas phase as a passive bystander at constant pressure. A single atmospheric element can be connected to all elements at the ground surface, and users may use AddBound.exe for this purpose (free software available from the TOUGH website).

&#x20;Atmospheric pressure and temperature are used as initial condition in atmospheric elements. A relative humidity of 100% is conveniently specified by initializing the atmospheric block as a two-phase point with a liquid saturation smaller than the residual liquid saturation (so relative permeability is zero, preventing liquid flow into soil). For relative humidities less than 100%, a single-phase gas point must be specified with an appropriate air mass fraction (1.0 for dry air; the minimum value depends on vapor pressure, which is a function of temperature; intermediate values determine relative humidity). The material properties, such as relative permeability and capillary pressure functions, for the atmospheric boundary element should be appropriately selected so that (1) liquid relative permeability is zero, (2) gas relative permeability is one, and (3) capillary pressure is zero. In addition, mobilities need to be upstream weighted.&#x20;

Infiltration can be simulated by specifying the infiltration rate in a row of elements below the atmospheric boundary element using the [GENER ](https://lbl-2.gitbook.io/tough4-user-manual/preparation-of-model-input/keywords-and-input-data/gener)block. Evaporation can be simulated (a) as a binary diffusion process (atmosphere at < 100% relative humidity), (b) by specifying the evaporation rate in a row of elements below the atmospheric boundary element using the GENER block, or (c) by assigning a capillary pressure according to Kelvin’s equation in the atmospheric element (Ghezzehei et al., 2004).
