improve hydro stability

[xuchongang]

This pull request improves the numerical stability in the situation of extreme dry conditions

Description:

Hydro uses the transpiration flux from last time step. For numerical stability, this pull request make sure that transpiration flux is less than 80% of total water availability. If it is larger than that, it will adjust the transpiration flux and record the excessive demand to the plant water storage pool, which will be dumped later in HLM.This should not affect the science of hydrodynamics as this basically delays the water limitation by a few steps with a benefit of numerical stability.

Collaborators:

@bchristo @rgknox

Expectation of Answer Changes:

This will not affect most of simulation and should only affect simulations with very dry soil.

Checklist:

• My change requires a change to the documentation.
• I have updated the in-code documentation .AND. (the technical note .OR. the wiki) accordingly.
• I have read the CONTRIBUTING document.
• FATES PASS/FAIL regression tests were run
• If answers were expected to change, evaluation was performed and provided

Test Results:

CTSM (or) E3SM (specify which) test hash-tag:

CTSM (or) E3SM (specify which) baseline hash-tag:

FATES baseline hash-tag:

Test Output:

[rgknox]

@xuchongang , I think we might need to subtract the trans_err term to the plant stored water output boundary condition, here:

fates/biogeophys/FatesPlantHydraulicsMod.F90

Lines 1810 to 1812 in aa290b3

	bc_out%plant_stored_h2o_si = csite_hydr%h2oveg + csite_hydr%h2oveg_dead - &
	csite_hydr%h2oveg_growturn_err - &
	csite_hydr%h2oveg_hydro_err

That way when E3SM and CTSM do their calculations on mass conservation, it will include this water we needed to create.

Thoughts?

[xuchongang]

@xuchongang , I think we might need to subtract the trans_err term to the plant stored water output boundary condition, here:

fates/biogeophys/FatesPlantHydraulicsMod.F90

Lines 1810 to 1812 in aa290b3

	bc_out%plant_stored_h2o_si = csite_hydr%h2oveg + csite_hydr%h2oveg_dead - &
	csite_hydr%h2oveg_growturn_err - &
	csite_hydr%h2oveg_hydro_err

That way when E3SM and CTSM do their calculations on mass conservation, it will include this water we needed to create.

Thoughts?

@rgknox , you are correct. I forget to copy that over as I am testing on the ELM, which is not the most recent compared to master. I just added the line of the code.