Default waterThickness/Min. effectConducEdge limit

Establishes a default waterThickness to be imposed as an initial
condition if no waterThickness variable exists. Forces
effectiveConducEdge to zero if below 1e-30 to avoid too small of
diffusivity/waterVelocity values.

Author: alexolinhager

components/mpas-albany-landice/src/Registry_subglacial_hydro.xml

@@ -152,7 +152,7 @@
                 <!-- state vars -->
 		<var name="iceThicknessHydro" type="real" dimensions="nCells Time" units="m"
 			description="ice thickness used by the hydrology model. Same as 'thickness' but with potential differences along domain boundaries that inhibit the formation of local hydropotential minima on boundaries." /> 
-		<var name="waterThickness" type="real" dimensions="nCells Time" units="m"
+                <var name="waterThickness" type="real" dimensions="nCells Time" units="m" default_value="0.01"
                      description="water layer thickness in subglacial hydrology system" />
                 <var name="waterThicknessOld" type="real" dimensions="nCells Time" units="m"
                      description="water layer thickness in subglacial hydrology system from previous time step" />

components/mpas-albany-landice/src/mode_forward/mpas_li_subglacial_hydro.F

@@ -845,6 +845,7 @@ subroutine calc_edge_quantities(block, err)
       integer :: i, j, iVertex, iCell
       real (kind=RKIND) :: velSign
       integer :: numGroundedCells
+      real(kind=RKIND), parameter :: SMALL_CONDUC = 1.0e-30_RKIND
       integer :: err_tmp
       
       err = 0
@@ -1079,6 +1080,10 @@ subroutine calc_edge_quantities(block, err)
          enddo
       endif
 
+      where (effectiveConducEdge < SMALL_CONDUC)
+              effectiveConducEdge = 0.0_RKIND
+      end where
+
       ! calculate diffusivity on edges
       diffusivity(:) = rho_water * gravity * effectiveConducEdge(:) * waterThicknessEdge(:)