diff --git a/doc/api.rst b/doc/api.rst
index 444fe5a..22be503 100644
--- a/doc/api.rst
+++ b/doc/api.rst
@@ -50,12 +50,18 @@ Name             Description
 ===============  =========================================
 GO_BC_NONE       No boundary conditions are applied.
 GO_BC_EXTERNAL   Some external forcing is applied. This must be implemented by a kernel.
+                 The domain can be specified using a ``tmask``, but if no ``tmask`` is
+                 specified, a dummy ``tmask`` is created that will define an all ocean
+                 domain.
 GO_BC_PERIODIC   Periodic boundary conditions are applied.
 ===============  =========================================
 
 The infrastructure requires this information in order to determine the
 extent of the model grid.
 
+Note that at this stage ``GO_BC_PERIODIC`` is not supported when
+using distributed memory. This is tracked in issue #54.
+
 The index offset is required because a model (kernel) developer has
 choice in how they actually implement the staggering of variables on a
 grid. This comes down to a choice of which grid points in the vicinity
@@ -98,8 +104,12 @@ object. This is done via a call to the ``grid_init`` subroutine::
     !! wet (1), dry (0) or external (-1).
     integer, dimension(m,n), intent(in), optional :: tmask
 
-It should be noted that currently only grids with constant
-resolution in *x* and *y* are supported by this routine.
+
+If no T-mask is supplied then this routine configures the grid
+appropriately for an all-wet domain by allocating a default
+T-mask. It should also be noted that currently only grids with
+constant resolution in *x* and *y* are supported by this routine.
+
 
 .. _gocean1.0-fields:
 
diff --git a/finite_difference/src/grid_mod.f90 b/finite_difference/src/grid_mod.f90
index 4a05d86..5cbdcc2 100644
--- a/finite_difference/src/grid_mod.f90
+++ b/finite_difference/src/grid_mod.f90
@@ -96,9 +96,9 @@ module grid_mod
      !!                        -1 == wet outside simulated region
      !! This is the key quantity that determines the region that
      !! is actually simulated. However, we also support the
-     !! specification of a model consisting entirely of wet points
-     !! Since this does not require a T-mask, we do not allocate
-     !! this array for that case.
+     !! specification of a model consisting entirely of wet points.
+     !! In this case a dummy tmask will be allocated, set to indicate
+     !! an all wet domain.
      integer, allocatable :: tmask(:,:)
      !> Pointer to tmask on remote device (if any)
      type(c_ptr) :: tmask_device
@@ -324,7 +324,9 @@ end function grid_constructor
   !! @param[in] dxarg Grid spacing in x dimension
   !! @param[in] dyarg Grid spacing in y dimension
   !! @param[in] tmask Array holding the T-point mask which defines
-  !!                  the contents of the local domain.
+  !!                  the contents of the local domain. Need not be
+  !!                  supplied if domain is all wet, in which case a
+  !!                  dummy all-wet tmask will be created internally.
   subroutine grid_init(grid, dxarg, dyarg, tmask)
     use decomposition_mod, only: subdomain_type, decomposition_type
     use parallel_mod, only: map_comms, get_rank, get_num_ranks, on_master