diff --git a/rvic/core/history.py b/rvic/core/history.py
index d559159..d0c428d 100644
--- a/rvic/core/history.py
+++ b/rvic/core/history.py
@@ -120,7 +120,8 @@ def __init__(self, time_ord, caseid, rvar, tape_num=0,
             self._units_mult = grid_area[self._outlet_y_ind,
                                          self._outlet_x_ind]
             self._units_mult *= SECSPERDAY
-            self._units_mult /= WATERDENSITY
+            # divide by water density (1000 kg/m3) and multiply by
+            # 1000 mm/m gives 1 mm * m2 / kg
             self._units_mult /= self._outlet_upstream_area
         elif units in ['gal/day', 'gpd', 'gal d-1']:
             self._units_mult = grid_area[self._outlet_y_ind,