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Port CLM cryosphere and hydrology fixes into ELM #8638

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Port CLM cryosphere and hydrology fixes into ELM #8638

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c304fff
Port CLM fractional-snow energy and melt-compaction fixes
daliwang Aug 6, 2026
3f7fd0f
Port CLM bedrock heat capacity fix in SoilTemperatureMod
daliwang Aug 6, 2026
5bdf5f2
Port CLM active-element masking for ELM accumulators
daliwang Aug 6, 2026
4b7a01a
Bug fixes for surface water runoff calculation
Aug 7, 2026
fbfcc93
Fix snow balance accounting issue
Aug 7, 2026
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21 changes: 21 additions & 0 deletions components/elm/src/biogeophys/BalanceCheckMod.F90
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Expand Up @@ -469,6 +469,27 @@ subroutine ColWaterBalanceCheck( bounds, num_do_smb_c, filter_do_smb_c, &
snow_sinks(c) = qflx_sub_snow(c) + qflx_evap_grnd(c) + qflx_snow_melt(c) &
+ qflx_snwcp_ice(c) + qflx_snwcp_liq(c) + qflx_sl_top_soil(c)

if (lun_pp%itype(l) == istice .or. lun_pp%itype(l) == istice_mec) then
! With fractional frac_sno_eff on glacier columns, only the
! snow-covered fraction receives dew/rain and loses sub/evap.
if (do_capsnow(c) .and. .not. use_firn_percolation_and_compaction) then
snow_sources(c) = qflx_prec_grnd(c) &
+ frac_sno_eff(c) * (qflx_dew_snow(c) + qflx_dew_grnd(c))

snow_sinks(c) = frac_sno_eff(c) * (qflx_sub_snow(c) + qflx_evap_grnd(c)) &
+ qflx_snwcp_ice(c) + qflx_snwcp_liq(c) &
+ qflx_snow_melt(c) + qflx_sl_top_soil(c)
else
snow_sources(c) = qflx_snow_grnd_col(c) &
+ frac_sno_eff(c) * (qflx_rain_grnd_col(c) &
+ qflx_dew_snow(c) + qflx_dew_grnd(c))

snow_sinks(c) = frac_sno_eff(c) * (qflx_sub_snow(c) + qflx_evap_grnd(c)) &
+ qflx_snwcp_ice(c) + qflx_snwcp_liq(c) &
+ qflx_snow_melt(c) + qflx_sl_top_soil(c)
endif
endif

if (col_pp%is_lake(c)) then
if (.not. use_firn_percolation_and_compaction) then
if ( do_capsnow(c)) then
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15 changes: 8 additions & 7 deletions components/elm/src/biogeophys/CanopyHydrologyMod.F90
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Expand Up @@ -642,14 +642,15 @@ subroutine CanopyHydrology(bounds, &
end if !end of do_capsnow construct

! set frac_sno_eff variable
if (col_pp%is_soil(c) .or. col_pp%is_crop(c)) then
if (subgridflag ==1) then
frac_sno_eff(c) = frac_sno(c)
else
frac_sno_eff(c) = 1._r8
endif
else
! Port of CLM fractional-snow energy fix: when subgrid fluxes are on,
! allow fractional frac_sno_eff for all non-urban, non-lake columns
! (including glacier and wetland), matching soil/crop treatment.
if (subgridflag == 1 .and. .not. urbpoi(l) .and. ltype(l) /= istdlak) then

@bishtgautam bishtgautam Aug 17, 2026

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Is the default value of subgridflag 1 or 0?

frac_sno_eff(c) = frac_sno(c)
else if (frac_sno(c) > 0._r8) then
frac_sno_eff(c) = 1._r8
else
frac_sno_eff(c) = 0._r8
endif

if (ltype(l) == istwet .and. t_grnd(c)>tfrz) then
Expand Down
9 changes: 8 additions & 1 deletion components/elm/src/biogeophys/SnowHydrologyMod.F90
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Expand Up @@ -568,6 +568,7 @@ subroutine SnowCompaction(bounds, num_snowc, filter_snowc, &
!
! !LOCAL VARIABLES:
integer :: j, l, c, fc, t ! indices
! (urbpoi / lakpoi used for CLM-consistent fractional melt compaction gate)
! parameters
real(r8), parameter :: c2 = 23.e-3_r8 ! [m3/kg]
real(r8), parameter :: c3 = 2.777e-6_r8 ! [1/s]
Expand Down Expand Up @@ -605,6 +606,8 @@ subroutine SnowCompaction(bounds, num_snowc, filter_snowc, &
snl => col_pp%snl , & ! Input: [integer (:) ] number of snow layers
n_melt => col_pp%n_melt , & ! Input: [real(r8) (:) ] SCA shape parameter
ltype => lun_pp%itype , & ! Input: [integer (:) ] landunit type
urbpoi => lun_pp%urbpoi , & ! Input: [logical (:) ] true => landunit is an urban point
lakpoi => lun_pp%lakpoi , & ! Input: [logical (:) ] true => landunit is a lake point

forc_wind => top_as_inst%windbot , & ! Input: [real(r8) (:) ] atmospheric wind speed (m/s)
t_soisno => col_es%t_soisno , & ! Input: [real(r8) (:,:) ] soil temperature (Kelvin)
Expand Down Expand Up @@ -703,9 +706,13 @@ subroutine SnowCompaction(bounds, num_snowc, filter_snowc, &
endif

! Compaction occurring during melt
! Port of CLM fractional-snow melt compaction fix: use the
! fractional ddz3 form for all non-lake, non-urban columns when
! subgridflag is on (includes glacier and wetland).

if (imelt(c,j) == 1) then
if(subgridflag==1 .and. (col_pp%is_soil(c) .or. col_pp%is_crop(c))) then
l = col_pp%landunit(c)
if (subgridflag == 1 .and. (.not. lakpoi(l) .and. .not. urbpoi(l))) then
! first term is delta mass over mass
ddz3 = max(0._r8,min(1._r8,(swe_old(c,j) - wx)/wx))

Expand Down
22 changes: 11 additions & 11 deletions components/elm/src/biogeophys/SoilHydrologyMod.F90
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Expand Up @@ -554,23 +554,23 @@ subroutine Infiltration(bounds, num_hydrologyc, filter_hydrologyc, num_urbanc, f
!5. surface runoff from h2osfc
if (h2osfcflag==1) then
! calculate runoff from h2osfc -------------------------------------
if (use_modified_infil) then
!if (use_modified_infil) then
if (frac_h2osfc_act(c) <= pc .and. frac_h2osfc(c) <= pc) then
frac_infclust=0.0_r8
else
if (frac_h2osfc(c) <= pc) then
if (frac_h2osfc_act(c) <= pc) then
frac_infclust=(frac_h2osfc_act(c)-pc)**mu
else
frac_infclust=(frac_h2osfc(c)-pc)**mu
frac_infclust=(frac_h2osfc_act(c)-pc)**mu
endif
Comment on lines 555 to 565

@bishtgautam bishtgautam Aug 17, 2026

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I agree with Copilot's review: Why is the second frac_infclust=(frac_h2osfc(c)-pc)**mu being changed to frac_infclust=(frac_h2osfc_act(c)-pc)**mu?

endif
else
if (frac_h2osfc(c) <= pc) then
frac_infclust=0.0_r8
else
frac_infclust=(frac_h2osfc(c)-pc)**mu
endif
endif
!else
! if (frac_h2osfc(c) <= pc) then
! frac_infclust=0.0_r8
! else
! frac_infclust=(frac_h2osfc(c)-pc)**mu
! endif
!endif
Comment on lines +557 to +573

@bishtgautam bishtgautam Aug 17, 2026

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It seems to me that we are making the use_modified_infil scheme as the default. So, why are we not doing it through namelist instead of commented out the old code and making the use_modified_infil as the default?

Or, if use_modified_infil needs to be made default, let's remove it from the namelist and delete the code corresponding to .not. use_modified_infil.

@donghuix donghuix Aug 21, 2026

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use_modified_infil is still used to control how the infiltration capacity is computed for different sub fractions. This modification just remove the control of use_modified_infil on the surface water runoff generation.

endif

if (lun_pp%ispolygon(col_pp%landunit(c))) then
Expand All @@ -596,7 +596,7 @@ subroutine Infiltration(bounds, num_hydrologyc, filter_hydrologyc, num_urbanc, f
! limit runoff to value of storage above S(pc)
if(h2osfc(c) >= h2osfc_thresh(c) .and. h2osfcflag/=0) then
! spatially variable k_wet
k_wet=1.0_r8 * sin((rpi/180._r8) * col_pp%topo_slope(c))
k_wet=1.0e-4_r8 * sin((rpi/180._r8) * max(col_pp%topo_slope(c), 1.0e-3_r8))

@bishtgautam bishtgautam Aug 17, 2026

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What is the logic of reducing k_wet by four-orders of magnitude? Was this a bug identified by CLM that we are fixing here?

@daliwang daliwang Aug 20, 2026

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A bug from the misuse of unit

qflx_h2osfc_surf(c) = k_wet * frac_infclust * (h2osfc(c) - h2osfc_thresh(c))

qflx_h2osfc_surf(c)=min(qflx_h2osfc_surf(c),(h2osfc(c) - h2osfc_thresh(c))/dtime)
Expand Down
4 changes: 4 additions & 0 deletions components/elm/src/biogeophys/SoilTemperatureMod.F90
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Expand Up @@ -1044,6 +1044,10 @@ subroutine SoilThermProp (bounds, num_nolakec, filter_nolakec, &
.AND. col_pp%itype(c) /= icol_sunwall .AND. col_pp%itype(c) /= icol_shadewall .AND. &
col_pp%itype(c) /= icol_roof) then
cv(c,j) = csol(c,j)*(1._r8-watsat(c,j))*dz(c,j) + (h2osoi_ice(c,j)*cpice + h2osoi_liq(c,j)*cpliq)
! Port of CLM bedrock heat-capacity fix: do not reduce solid-rock
! heat capacity by soil porosity below the soil-bedrock boundary.
! csol is already set to bedrock mineral heat capacity for these layers.
if (j > nlevbed) cv(c,j) = csol(c,j)*dz(c,j)
else if (lun_pp%itype(l) == istwet) then
cv(c,j) = (h2osoi_ice(c,j)*cpice + h2osoi_liq(c,j)*cpliq)
if (j > nlevbed) cv(c,j) = csol(c,j)*dz(c,j)
Expand Down
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