Replaced some magic numbers with local and global constants, following @bishtgautam's suggestions.

Author: mpaiao

components/elm/src/biogeophys/CanopyFluxesMod.F90

@@ -17,6 +17,8 @@ module CanopyFluxesMod
   use elm_varctl            , only : use_hydrstress
   use elm_varpar            , only : nlevgrnd, nlevsno
   use elm_varcon            , only : namep
+  use elm_varcon            , only : mm_epsilon
+  use elm_varcon            , only : pa_to_kpa
   use pftvarcon             , only : crop, nfixer
   use decompMod             , only : bounds_type
   use PhotosynthesisMod     , only : Photosynthesis, PhotosynthesisTotal, Fractionation, PhotoSynthesisHydraulicStress
@@ -318,6 +320,9 @@ subroutine CanopyFluxes(bounds,  num_nolakeurbanp, filter_nolakeurbanp, &
     ! Indices for raw and rah
     integer, parameter :: above_canopy = 1         ! Above canopy
     integer, parameter :: below_canopy = 2         ! Below canopy
+
+    ! Lower bound for VPD (based on CLM)
+    real(r8), parameter :: vpd_min = 50._r8
     !------------------------------------------------------------------------------
 
     associate(                                                               &
@@ -863,15 +868,15 @@ subroutine CanopyFluxes(bounds,  num_nolakeurbanp, filter_nolakeurbanp, &
             ! Done each iteration to account for differences in eah, tv.
 
             svpts(p) = el(p)                         ! Pa
-            eah(p) = forc_pbot(t) * qaf(p) / 0.622_r8   ! Pa
+            eah(p) = forc_pbot(t) * qaf(p) / mm_epsilon   ! Pa
             rhaf(p) = eah(p)/svpts(p)
 
             ! variables for history fields
             rah_above(p)  = rah(p,above_canopy)
             raw_above(p)  = raw(p,above_canopy)
             rah_below(p)  = rah(p,below_canopy)
             raw_below(p)  = raw(p,below_canopy)
-            vpd(p)        = max((svpts(p) - eah(p)), 50._r8) * 0.001_r8 ! kPa
+            vpd(p)        = max((svpts(p) - eah(p)), vpd_min) * pa_to_kpa ! kPa
          end do
 
          ! Modification for shrubs proposed by X.D.Z

components/elm/src/main/elm_varcon.F90

@@ -72,6 +72,10 @@ module elm_varcon
   real(r8) :: tlsai_crit = 2.0_r8                           ! critical value of elai+esai for which aerodynamic parameters are maximum
   real(r8) :: watmin = 0.01_r8                              ! minimum soil moisture (mm)
 
+  real(r8), parameter :: mm_epsilon = 0.622_r8              ! Molar mass ratio (water:dry air)
+                                                            !   This is set to 0.622 for bit-for-bit compatibility, but
+                                                            !   this should be defined as SHR_CONST_MWWV/SHR_CONST_MWDAIR
+
   real(r8) :: re = SHR_CONST_REARTH*0.001_r8                ! radius of earth (km)
 
   real(r8), public, parameter :: degpsec = 15._r8/3600.0_r8 ! Degree's earth rotates per second
@@ -81,6 +85,13 @@ module elm_varcon
   integer , public, parameter :: ispval = -9999             ! special value for int data 
                                                             ! (keep this negative to avoid conflicts with possible valid values)
 
+
+  !------------------------------------------------------------------
+  ! Unit conversion constants
+  !------------------------------------------------------------------
+
+  real(r8), parameter :: pa_to_kpa = 0.001_r8               ! Conversion factor (Pa to kPa) [kPa/Pa]
+
   ! These are tunable constants from clm2_3
 
   real(r8) :: zlnd = 0.01_r8        ! Roughness length for soil [m]