Atmospheric Boundary

When atmospheric boundary conditions are implemented, time-dependent input data for the precipitation rate, Prec, and the evaporation rate, rSoil, must be specified in the input file ATMOSPH.IN (given in the major module). The potential fluid flux across the soil surface is determined by rAtm= rSoil - Prec. The actual surface flux is calculated internally by the program. Two limiting values of the surface pressure head must be provided: hCritS which specifies the maximum allowed pressure head at the soil surface (usually 0.0), and hCritA which specifies the minimum allowed surface pressure head (defined from equilibrium conditions between soil water and atmospheric vapor). The program automatically switches between Dirichlet and Neumann boundary conditions if one of these two limiting points is reached. The following table summarizes the use of the variables rAtm, hCritS and hCritA during program execution. Width(n) in this table denotes the length of the boundary segment associated with node n.

Definition of the variables Kode(n), Q(n) and h(n) when an atmospheric boundary condition is applied.

See also the "How to Edit Boundary Conditions" topic.

See available Water Flow Boundary Conditions.

How does one determine infiltration and actual evaporation rates from HYDRUS outputs? Since HYDRUS uses both precipitation and potential evaporation as the limiting fluxes, how does one know from the actual surface flux that the maximum values have been reached?

When both precipitation (irrigation) and evaporation are specified at the same boundary (atmospheric BC) at the same time, HYDRUS calculates the potential flux across the boundary as “Boundary Potential Flux = Precipitation - Potential Evaporation” and then applies this resulting flux. One can see based on the sign whether the flux is into (infiltration) or out of (evaporation) the transport domain. HYDRUS prints into the output files this potential flux (as well as precipitation and evaporation fluxes), as well as the actual flux. When one compares the potential and actual fluxes, one can immediately see whether the potential flux was reduced due to the soil conditions.