A residual means little until you know how much surprise is normal. At 10:15 a.m., a pressure forecasting system reports a discrepancy. At one monitoring point in a water distribution network, the model expects 34.0 mca. The sensor reports 36.5 mca, a residual of +2.5 m. Fifteen minutes later, it is still there. Why would 2.5 mca matter? In a distribution network, pressure is a managed quantity. Too little affects service, too much stresses pipes and typically increases leaka