fix(mfmodel): fix budgetkey for transport models (#2176)

* add budgetkey param to MF6Output and pass it down to MF6ListBudget in MF6Output.__list()
* update MFModel.output to set budgetkey to "MASS..." instead of volume for transport models
* todo: revisit and generalize for additional model types

flopy/mf6/mfmodel.py

@@ -682,10 +682,13 @@ def laycbd(self):
 
     @property
     def output(self):
+        budgetkey = None
+        if self.model_type == "gwt6":
+            budgetkey = "MASS BUDGET FOR ENTIRE MODEL"
         try:
-            return self.oc.output
+            return MF6Output(self.oc, budgetkey=budgetkey)
         except AttributeError:
-            return MF6Output(self)
+            return MF6Output(self, budgetkey=budgetkey)
 
     def export(self, f, **kwargs):
         """Method to export a model to a shapefile or netcdf file

flopy/mf6/utils/output_util.py

@@ -23,7 +23,7 @@ class MF6Output:
 
     """
 
-    def __init__(self, obj):
+    def __init__(self, obj, budgetkey=None):
         from ..modflow import ModflowGwfoc, ModflowGwtoc, ModflowUtlobs
 
         # set initial observation definitions
@@ -39,6 +39,11 @@ def __init__(self, obj):
         self._obj = obj
         self._methods = []
         self._sim_ws = obj.simulation_data.mfpath.get_sim_path()
+        self._budgetkey = (
+            "VOLUME BUDGET FOR ENTIRE MODEL"
+            if budgetkey is None
+            else budgetkey
+        )
         self.__budgetcsv = False
 
         if not isinstance(obj, (PackageInterface, ModelInterface)):
@@ -382,7 +387,7 @@ def __list(self):
         if self._lst is not None:
             try:
                 list_file = os.path.join(self._sim_ws, self._lst)
-                return Mf6ListBudget(list_file)
+                return Mf6ListBudget(list_file, budgetkey=self._budgetkey)
             except (AssertionError, OSError):
                 return None