Time Information

Time Units

Time units [T] to be used throughout the application (years, days, hours, min, sec). When units are changed during or after data entry, then all input variables are converted automatically into the new units.

Initial Time        

Starting time [T] of the calculation.

Final Time

Final time [T] of the calculation.

Initial Time Step

Initial time increment, Δt [T]. The recommended value for the initial time step depends on the type of simulation and boundary conditions used. When simulating a process that starts with a large initial pressure head or concentration gradient at the boundary (e.g., ponded infiltration or a sudden change of boundary concentration), use a small value of the initial time step (e.g., 1 s). When simulating a long term process with variable boundary conditions (e.g., seasonal or multiyear simulation), start with a larger time step (e.g., 15 min). This is because this initial time step is used whenever time variable boundary conditions significantly change (e.g., the water flux changes by 25% or more). If needed (if there is no convergence for Δtinit), the program will still use a smaller time step than Δtinit, but starting with larger Δtinit leads to more efficient calculations. In general smaller initial time steps must be used for soil with more nonlinear soil hydraulic properties (e.g., course textured soils) and larger initial time steps can be used for soil with less nonlinear soil hydraulic properties (e.g., loam).

Minimum Time Step

Minimum permitted value of the time increment, Δtmin [T]. The minimum time step must be smaller than a) the initial time step, b) interval between print times, and c) interval between time-variable boundary condition records. Always specify a small minimum allowed time step, on the order of 1 s. This value may never be used, but it provides the code with flexibility when it may be needed, e.g., when there is a sudden change in boundary fluxes and HYDRUS may not converge with larger time steps.

Maximum Time Step

Maximum permitted value of the time increment, Δtmax [T]. This is relatively unimportant parameter and a large value may be specified. Since HYDRUS automatically selects its optimal time step, there is usually no need to constraint that. The only time when there is a need to constrain the time step is likely for cases when HYDRUS is asked to generate internally intra-daily variations in temperature, or in evaporation and transpiration fluxes. Then there is a need to have time step smaller (e.g., 1 h) so that these daily variations can be properly modeled.