Bugfix: Introducing nested if-conditions in ch4Mod (#69)

* Fix subroutine ch4_tran by introducing two nested if-conditions

Related to issue #68  #68 (comment)

See also the error messages mentioned in issue comments
- #68 (comment)
- #68 (comment)

---------

Co-authored-by: AGonzalezNicolas <157027467+AGonzalezNicolas@users.noreply.github.com>

Author: jjokella

src/clm5/biogeochem/ch4Mod.F90

@@ -2473,12 +2473,14 @@ subroutine ch4_prod (bounds, num_methc, filter_methc, num_methp, &
             end if
 
             ! If switched on, use pH factor for production based on spatial pH data defined in surface data.
-            if (.not. lake .and. usephfact .and. pH(c) >  pHmin .and.pH(c) <  pHmax) then
+            if (.not. lake .and. usephfact) then
+             if (pH(c) >  pHmin .and.pH(c) <  pHmax) then
                pH_fact_ch4 = 10._r8**(-0.2235_r8*pH(c)*pH(c) + 2.7727_r8*pH(c) - 8.6_r8)
                ! fitted function using data from Dunfield et al. 1993  
                ! Strictly less than one, with optimum at 6.5
                ! From Lei Meng
                f_ch4_adj = f_ch4_adj * pH_fact_ch4
+             end if
             else
                ! if no data, then no pH effects
             end if
@@ -3552,10 +3554,11 @@ subroutine ch4_tran (bounds, &
                      pondz = h2osfc(c) / 1000._r8 / frac_h2osfc(c) ! Assume all h2osfc corresponds to sat area
                      ! mm      /  mm/m
                      pondres = pondres + pondz / ponddiff
-                  else if (.not. lake .and. sat == 1 .and. frac_h2osfc(c) > 0._r8 .and. &
-                       h2osfc(c)/frac_h2osfc(c) > capthick) then ! Assuming short-circuit logic will avoid FPE here.
+                  else if (.not. lake .and. sat == 1 .and. frac_h2osfc(c) > 0._r8) then
+                     if (h2osfc(c)/frac_h2osfc(c) > capthick) then ! Assuming short-circuit logic will avoid FPE here.
                      ! assume surface ice is impermeable
                      pondres = 1/smallnumber
+                     end if
                   end if
 
                   spec_grnd_cond(c,s) = 1._r8/(1._r8/grnd_ch4_cond(c) + snowres(c) + pondres)