Index: wflow-py/wflow/wflow_sbm.py
===================================================================
diff -u -r34c2b30d41c043c3a36fb6b265acea8484cd201e -r0fc7439329d0ea7ec14ea9360d97a18ebbcf82c0
--- wflow-py/wflow/wflow_sbm.py	(.../wflow_sbm.py)	(revision 34c2b30d41c043c3a36fb6b265acea8484cd201e)
+++ wflow-py/wflow/wflow_sbm.py	(.../wflow_sbm.py)	(revision 0fc7439329d0ea7ec14ea9360d97a18ebbcf82c0)
@@ -1231,7 +1231,7 @@
         self.SurfaceRunoffMM = self.SurfaceRunoff * self.QMMConv  # SurfaceRunoffMM (mm) from SurfaceRunoff (m3/s)
         self.updateRunOff()
         self.InflowKinWaveCell = upstream(self.TopoLdd, self.SurfaceRunoff)
-        self.MassBalKinWave = (self.KinWaveVolume - self.OldKinWaveVolume) / self.timestepsecs + self.InflowKinWaveCell + self.Inwater - self.SurfaceRunoff
+        self.MassBalKinWave = (-self.KinWaveVolume + self.OldKinWaveVolume) / self.timestepsecs + self.InflowKinWaveCell + self.Inwater - self.SurfaceRunoff
 
         Runoff = self.SurfaceRunoff