# HydroGeoSphere/Transport Conditions
## Transport time weighting

* twc Time-weighting factor for the transport solution.
Assigns a new value for the time-weighting factor for the transport solution, which defaults to 0.5. Values can range from 0.0 (explicit) to 0.5 (central or Crank-Nicolson) or 1.0 (fully implicit). Fully implicit time-weighting is less prone to exhibit oscillations but more prone to numerical smearing than central time-weighting.

## Peclet number

* pectol Peclet number.
Assigns a new value for the Peclet number, which defaults to 1 × 10$^{20}$. The Peclet number does not influence the solution in any way, but is merely used to generate warning messages. The Peclet number can be computed as:
    $P_{e}=v{\frac {{\Delta }x}{D}}$
where $v$ is a flow velocity, ${\Delta }x$ is an element length, in this case in the x-direction, and $D$ is a dispersion coefficient. It is a measure of the adequacy of mesh fineness, with large numbers indicating poor spatial discretization.
The large default value suppresses the generation of warning messages.

## Output peclet number

This instruction causes HydroGeoSphere to write the Peclet number for each element to the file prefixo.peclet_number, at the first timestep only.

## Courant number

* courtol Courant number.
Assigns a new value for the Courant number, which defaults to 1 × 10$^{20}$. The Courant number does not influence the solution in any way, but is merely used to generate warning messages. The Courant number can be computed as:
    $C_{e}=v{\frac {{\Delta }t}{{\Delta }x}}$
where $v$ is a flow velocity, ${\Delta }x$ is an element length, in this case in the x-direction, and ${\Delta }t$ is the timestep length. It is a measure of the adequacy of timestep size, with large numbers indicating poor temporal discretization.

## Transport solver convergence criteria

* restolc Transport solver convergence criteria.
Assigns a new value for the transport solver convergence criteria, which defaults to 1 × 10$^{−10}$. Convergence is obtained when this criteria is less than the absolute maximum value of the residual (error) of the latest transport solution.

## Transport solver detail

* isolv_infoc Transport solver detail level.
Assigns a new value for the transport solver detail level, which defaults to 0. This value controls the level of detail of solver performance information printed to the screen and listing file, and can have values of 0 (no information) or 1 (full information).

## Transport solver maximum iterations

* maxtit Maximum number of transport solver iterations.
Assigns a new value for the maximum number of transport solver iterations, which defaults to 2000.

## Upstream weighting of velocities

* almax,btmax,gammax Upstream weighting factors for the x-, y-, and z-directions respectively.
Causes upstream-weighting of velocities to be used, as opposed to the default, central weighting of velocities. Values can range from 0.0 (no upstream-weighting) to 1.0 (full upstream weighting). Note that these variables do not apply for the control volume case, where full upstream weighting is always applied when the switch upstrvel is .TRUE.

## Flux limiter for transport

This instruction causes HydroGeoSphere to use the van Leer flux limiter as described in Section 3.9.1.

## Iteration parameters flux limiter

* maxiter_flim, resmax_flim, delmax_flim Maximum number of iterations, residual and absolute convergence criteria respectively for the non-linear flux limiter iteration procedure.
Assigns new values to the parameters that affect the behaviour of the van Leer flux limiter, and which default to 15, 1 × 10$^{−5}$ and 1 × 10$^{−8}$ respectively.

## Overland advective solute exchange only

This instruction only affects transport in coupled surface/subsurface systems, and causes HydroGeoSphere to neglect dispersive/diffusive exchange between the two domains. This is intended to be used to gauge the relative importance of advective vs. dispersive exchange or when comparing HydroGeoSphere to codes that neglect dispersive exchange.

## Fracture advective solute exchange only

This instruction only affects transport in coupled fracture/subsurface systems, and causes HydroGeoSphere to neglect dispersive/diffusive exhange between the two domains. This is intended to be used to gauge the relative importance of advective vs. dispersive exchange or when comparing HydroGeoSphere to codes that neglect dispersive exchange.
The following three instructions can be used to define a threshold concentration for flagging output and optionally stopping a run.

## Detection threshold concentration

* detection_threshold_conc Detection threshold concentration [M L$^{−3}$]. This instruction sets the value which will be used to control the behaviour of the next two instructions.

## Flag observation nodes if exceed detection threshold concentration

Causes HydroGeoSphere to tag observation well output with the string > detection threshold if the concentration at the node exceeds detection_threshold_conc, defined above.

## Stop run if flux output nodes exceed detection threshold concentration

Causes HydroGeoSphere to halt execution if the concentration at any observation well node exceeds detection_threshold_conc, defined above.