diff --git a/opm/models/discretization/common/fvbaseproblem.hh b/opm/models/discretization/common/fvbaseproblem.hh
index 76701c007..98ab65703 100644
--- a/opm/models/discretization/common/fvbaseproblem.hh
+++ b/opm/models/discretization/common/fvbaseproblem.hh
@@ -37,6 +37,7 @@
 
 #include <dune/common/fvector.hh>
 
+#include <functional>
 #include <iostream>
 #include <limits>
 #include <string>
@@ -268,7 +269,9 @@ public:
      *         the next regular parameter. If this is less than 1, it indicated that the
      *         positional parameter was invalid.
      */
-    static int handlePositionalParameter(std::set<std::string>&,
+    static int handlePositionalParameter(std::function<void(const std::string&,
+                                                            const std::string&)>,
+                                         std::set<std::string>&,
                                          std::string& errorMsg,
                                          int,
                                          const char** argv,
diff --git a/opm/models/utils/parametersystem.hh b/opm/models/utils/parametersystem.hh
index b5366884a..8457eb8cd 100644
--- a/opm/models/utils/parametersystem.hh
+++ b/opm/models/utils/parametersystem.hh
@@ -41,6 +41,7 @@
 
 #include <charconv>
 #include <cstdlib>
+#include <functional>
 #include <fstream>
 #include <iostream>
 #include <list>
@@ -449,7 +450,8 @@ inline void printUsage(const std::string& helpPreamble,
 }
 
 /// \cond 0
-inline int noPositionalParameters_(std::set<std::string>&,
+inline int noPositionalParameters_(std::function<void(const std::string&, const std::string&)>,
+                                   std::set<std::string>&,
                                    std::string& errorMsg,
                                    int,
                                    const char** argv,
@@ -613,8 +615,11 @@ std::string parseCommandLineOptions(int argc,
             || argv[i][1] != '-')
         {
             std::string errorMsg;
-            int numHandled = posArgCallback(seenKeys, errorMsg, argc, argv,
-                                            i, numPositionalParams);
+            int numHandled = posArgCallback([](const std::string& k, const std::string& v)
+                                            {
+                                                MetaData::tree()[k] = v;
+                                            }, seenKeys, errorMsg,
+                                            argc, argv, i, numPositionalParams);
 
             if (numHandled < 1) {
                 std::ostringstream oss;
diff --git a/opm/simulators/flow/FlowGenericProblem.hpp b/opm/simulators/flow/FlowGenericProblem.hpp
index 2531cd6b2..5d3be7bbd 100644
--- a/opm/simulators/flow/FlowGenericProblem.hpp
+++ b/opm/simulators/flow/FlowGenericProblem.hpp
@@ -52,7 +52,8 @@ class EclipseState;
 class Schedule;
 template<typename Grid, typename GridView> class LookUpData;
 
-int eclPositionalParameter(Dune::ParameterTree& tree,
+int eclPositionalParameter(std::function<void(const std::string&,
+                                              const std::string&)> addKey,
                            std::set<std::string>& seenParams,
                            std::string& errorMsg,
                            const char** argv,
diff --git a/opm/simulators/flow/FlowGenericProblem_impl.hpp b/opm/simulators/flow/FlowGenericProblem_impl.hpp
index 7c0fa2ed1..6c5f6b9b6 100644
--- a/opm/simulators/flow/FlowGenericProblem_impl.hpp
+++ b/opm/simulators/flow/FlowGenericProblem_impl.hpp
@@ -45,7 +45,7 @@
 
 namespace Opm {
 
-int eclPositionalParameter(Dune::ParameterTree& tree,
+int eclPositionalParameter(std::function<void(const std::string&, const std::string&)> addKey,
                            std::set<std::string>& seenParams,
                            std::string& errorMsg,
                            const char** argv,
@@ -73,7 +73,7 @@ int eclPositionalParameter(Dune::ParameterTree& tree,
         return 0;
     }
 
-    tree["EclDeckFileName"] = argv[paramIdx];
+    addKey("EclDeckFileName", argv[paramIdx]);
     seenParams.insert("EclDeckFileName");
     return 1;
 }
diff --git a/opm/simulators/flow/FlowProblem.hpp b/opm/simulators/flow/FlowProblem.hpp
index e64120946..a7125da59 100644
--- a/opm/simulators/flow/FlowProblem.hpp
+++ b/opm/simulators/flow/FlowProblem.hpp
@@ -228,14 +228,16 @@ public:
     /*!
      * \copydoc FvBaseProblem::handlePositionalParameter
      */
-    static int handlePositionalParameter(std::set<std::string>& seenParams,
+    static int handlePositionalParameter(std::function<void(const std::string&,
+                                                            const std::string&)> addKey,
+                                         std::set<std::string>& seenParams,
                                          std::string& errorMsg,
                                          int,
                                          const char** argv,
                                          int paramIdx,
                                          int)
     {
-        return eclPositionalParameter(Parameters::MetaData::tree(),
+        return eclPositionalParameter(addKey,
                                       seenParams,
                                       errorMsg,
                                       argv,