For lakes, when reading restart file: set frac_sno=1 if snow_depth>0

Due to ESCOMP#783, old restart files can have frac_sno == 0 for
lake points despite having a snow pack. This can cause other problems,
so fix that here. (This mainly impacts restart files produced by
versions prior to the initial fix of ESCOMP#783 - i.e., prior to
ctsm1.0.dev057. However, in principle, restart files produced by
versions after that tag could still have this issue, since this
backwards compatibility code wasn't in place at that point, so frac_sno
could have persisted at 0 if there were no new snow since the last
restart file was written. So, to be safe, this backwards compatibility
code should be left in place until we can rely on all restart files
being from versions later than the tag where this backwards
compatibility was first implemented.)

The criteria snow_depth > 0 seems as good as any for determining if
there was actually snow present in a given column on the restart file.

src/biogeophys/WaterDiagnosticBulkType.F90

@@ -608,9 +608,9 @@ subroutine RestartBulk(this, bounds, ncid, flag)
     ! !USES:
     use spmdMod          , only : masterproc
     use clm_varcon       , only : pondmx, watmin, spval, nameg
-    use landunit_varcon  , only : istcrop, istdlak, istsoil  
+    use landunit_varcon  , only : istdlak
     use column_varcon    , only : icol_roof, icol_sunwall, icol_shadewall
-    use clm_time_manager , only : is_first_step
+    use clm_time_manager , only : is_first_step, is_restart
     use clm_varctl       , only : bound_h2osoi
     use ncdio_pio        , only : file_desc_t, ncd_io, ncd_double
     use restUtilMod
@@ -623,6 +623,7 @@ subroutine RestartBulk(this, bounds, ncid, flag)
     !
     ! !LOCAL VARIABLES:
     logical  :: readvar
+    integer  :: c
     !------------------------------------------------------------------------
 
 
@@ -672,6 +673,28 @@ subroutine RestartBulk(this, bounds, ncid, flag)
          long_name=this%info%lname('fraction of ground covered by snow (0 to 1)'),&
          units='unitless',&
          interpinic_flag='interp', readvar=readvar, data=this%frac_sno_col)
+    ! BACKWARDS_COMPATIBILITY(wjs, 2019-10-15) Due to ESCOMP/ctsm#783, old restart files
+    ! can have frac_sno == 0 for lake points despite having a snow pack. This can cause
+    ! other problems, so fix that here. (This mainly impacts restart files produced by
+    ! versions prior to the initial fix of ESCOMP/ctsm#783 - i.e., prior to
+    ! ctsm1.0.dev057. However, in principle, restart files produced by versions after that
+    ! tag could still have this issue, since this backwards compatibility code wasn't in
+    ! place at that point, so frac_sno could have persisted at 0 if there were no new snow
+    ! since the last restart file was written. So, to be safe, this backwards
+    ! compatibility code should be left in place until we can rely on all restart files
+    ! being from versions later than the tag where this backwards compatibility was first
+    ! implemented.)
+    if (flag == 'read' .and. .not. is_restart()) then
+       do c = bounds%begc, bounds%endc
+          if (col%lun_itype(c) == istdlak .and. &
+               this%frac_sno_col(c) == 0._r8 .and. this%snow_depth_col(c) > 0._r8) then
+             ! Often the value should be between 0 and 1 rather than being 1, but 1 is at
+             ! least better than 0 in this case, and it would be tricky or impossible to
+             ! figure out the "correct" value.
+             this%frac_sno_col(c) = 1._r8
+          end if
+       end do
+    end if
 
     call restartvar(ncid=ncid, flag=flag, &
          varname=this%info%fname('FWET'), &