diff --git a/src/python/geoclaw/topotools.py b/src/python/geoclaw/topotools.py
index c54baddc9..28e645eda 100644
--- a/src/python/geoclaw/topotools.py
+++ b/src/python/geoclaw/topotools.py
@@ -788,8 +788,8 @@ def read_header(self):
             
         return num_cells
 
-    def write(self, path, no_data_value=None, topo_type=None, masked=True, 
-                header_style='geoclaw'):
+    def write(self, path, topo_type=None, no_data_value=None, masked=True, 
+                header_style='geoclaw', Z_format="%9.3f"):
         r"""Write out a topography file to path of type *topo_type*.
 
         Writes out a topography file of topo type specified with *topo_type* or
@@ -799,13 +799,17 @@ def write(self, path, no_data_value=None, topo_type=None, masked=True,
 
         :Input:
          - *path* (str)  - file to write
-         - *no_data_value* - values used to indicate missing data
          - *topo_type* (int) - GeoClaw format topo_type 
+         - *no_data_value* - values used to indicate missing data
          - *masked* (bool) - unused??
          - *header_style* (str) - indicates format of header lines
              'geoclaw' or 'default'  ==> write value then label 
              'arcgis' or 'asc' ==> write label then value  
                         (needed for .asc files in ArcGIS)
+         - *Z_format* (str) - string format to use for Z values
+           The default format "%9.3f" gives mm precision and gives a
+           smaller files than the previous default of "%22.15e" used in
+           GeoClaw version 5.3.1 and earlier.
 
         """
 
@@ -883,23 +887,25 @@ def write(self, path, no_data_value=None, topo_type=None, masked=True,
 
                 # Write out topography data
                 if topo_type == 2:
+                    Z_format = Z_format + "\n"
                     if masked_Z:
                         Z_filled = numpy.flipud(self.Z.filled())
                     else:
                         Z_filled = numpy.flipud(self.Z)
                     for i in xrange(self.Z.shape[0]):
                         for j in xrange(self.Z.shape[1]):
-                            outfile.write("%22.15e\n" % Z_filled[i,j])
+                            outfile.write(Z_format % Z_filled[i,j])
                     if masked_Z:
                         del Z_filled
                 elif topo_type == 3:
+                    Z_format = Z_format + " "
                     if masked_Z:
                         Z_flipped = numpy.flipud(self.Z.filled())
                     else:
                         Z_flipped = numpy.flipud(self.Z)
                     for i in xrange(self.Z.shape[0]):
                         for j in xrange(self.Z.shape[1]):
-                            outfile.write("%22.15e   " % (Z_flipped[i,j]))
+                            outfile.write(Z_format % (Z_flipped[i,j]))
                         outfile.write("\n")
                     if masked_Z:
                         del Z_flipped
diff --git a/tests/test_topotools.py b/tests/test_topotools.py
index 0f2824b55..e1d30074b 100644
--- a/tests/test_topotools.py
+++ b/tests/test_topotools.py
@@ -63,7 +63,7 @@ def test_read_write_topo_bowl():
     try:
         for topo_type in xrange(1, 4):
             path = os.path.join(temp_path, 'bowl.tt%s' % topo_type)
-            topo.write(path, topo_type=topo_type)
+            topo.write(path, topo_type=topo_type,Z_format="%22.15e")
 
             topo_in = topotools.Topography(path)
             assert numpy.allclose(topo.Z, topo_in.Z), \
@@ -172,7 +172,7 @@ def test_read_write_topo_bowl_hill():
 
         for topo_type in xrange(1,4):
             file_path = os.path.join(temp_path, 'bowl_hill.tt%s' % topo_type)
-            topo.write(file_path, topo_type=topo_type)
+            topo.write(file_path, topo_type=topo_type,Z_format="%22.15e")
             topo_in = topotools.Topography(path=file_path, topo_type=topo_type)
             assert numpy.allclose(topo.Z, topo_in.Z), \
                    "Written file of topo_type=%s does not equal read in" + \
@@ -207,7 +207,7 @@ def test_netcdf():
         # Write out NetCDF version of file
         ascii_topo = topotools.Topography(path=local_path)
         ascii_topo.read()
-        ascii_topo.write(nc_path, topo_type=4)
+        ascii_topo.write(nc_path, topo_type=4,Z_format="%22.15e")
 
         # Read back in NetCDF file
         nc_topo = topotools.Topography(path=nc_path)
@@ -313,7 +313,7 @@ def func(x, y):
     # Load (and save) test data and make the comparison
     test_data_path = os.path.join(testdir, "data", "unstructured_test_data.tt3")
     if save:
-        topo.write(test_data_path)
+        topo.write(test_data_path,Z_format="%22.15e")
 
     compare_data = topotools.Topography(path=test_data_path)