UZF unit specific factors seem to assume a feet-meter conversion

[Huite]

This is all really quite pedantic, but I was going through UfzCellGroup.f90, and I noticed a few things:

  !> @brief Calculate unit specific tolerances
  !<
  subroutine factors(feps1, feps2)
    ! -- dummy
    real(DP), intent(out) :: feps1
    real(DP), intent(out) :: feps2
    real(DP) :: factor1
    real(DP) :: factor2
    !
    ! calculate constants for uzflow
    factor1 = DONE
    factor2 = DONE
    feps1 = DEM9
    feps2 = DEM9
    if (ITMUNI == 1) then
      factor1 = DONE / 86400.D0
    else if (ITMUNI == 2) then
      factor1 = DONE / 1440.D0
    else if (ITMUNI == 3) then
      factor1 = DONE / 24.0D0
    else if (ITMUNI == 5) then
      factor1 = 365.0D0
    end if
    factor2 = DONE / 0.3048
    feps1 = feps1 * factor1 * factor2
    feps2 = feps2 * factor1 * factor2
    !
    ! -- Return
    return
  end subroutine factors

...
    ! -- check for falling or rising water table
    if ((thick - thickold) > feps1) then

...
    ffcheck = (this%surflux(icell) - this%uzflst(this%nwavst(icell), icell))
    !
    ! -- increase new waves in infiltration changes
    if (ffcheck > feps2 .OR. ffcheck < -feps2) then

The following things are a bit remarkable:

• factor2 is defined twice, first as DONE, then again as DONE / 0.3048 (which is the number of feet per meter).
• feps1 and feps2 are identical, as both come down to DEM9 * factor1 * factor2
• factor1 obviously depends on the ITMUNI value (0=undefined, 1=seconds, 2=minutes, 3=hours, 4=days, 5=years)

The result is then used as tolerances for falling/rising water tables (feps1) or infiltration changes (feps2).

At some places in MODFLOW 6, lenuni (1=feet, 2=meters, 3=centimeters, 4=unknown) is used, but not for the UZF package as far as I can tell. I checked the IO docs: there's no mention of time or length units anywhere in the UZF section, unlike the length_conversion and time_conversion entries for the lake package.

It seems to me like the code here wants to define a tolerance of 1e-9 in meter/day, and it assumes the thickness is given in feet (which would then be the general MODFLOW 6 length unit, I guess).
There are a lot more hardcoded tolerances in this module which are compared to time step durations, lengths, speeds, etc., which were probably written with a specific set of units in mind.

It probably doesn't matter much for actual outcomes, since most of these:

  use ConstantsModule, only: DZERO, DEM30, DEM20, DEM15, DEM14, DEM12, DEM10, &
                             DEM9, DEM7, DEM6, DEM5, DEM4, DEM3, DHALF, DONE, &
                             DTWO, DTHREE, DEP20

reflect "a small number".

But the feet/meter thing got me a little confused...