g = Graph[{
      Property[DirectedEdge[0, 1], {"WaterFlow" -> x, "PipeWidth" -> 10.0}],
      Property[DirectedEdge[1, 2], {"WaterFlow" -> x + y, "PipeWidth" -> 8.5}],
      Property[DirectedEdge[2, 1], {"WaterFlow" -> y, "PipeWidth" -> 8.5}],
      Property[DirectedEdge[2, 3], {"WaterFlow" -> x, "PipeWidth" -> 10.0}]
    }]