Do You Have Difficulty Calibrating Dry Days in Your Model?

This paper addresses a problem experienced by both modelers and practitioners of Infiltration Inflow studies; the imbalance of metered flow between upstream and downstream sites. In most cases where this is discovered, the data analyst will make some adjustment to the data or calculation to obtain a ‘positive net’ flow between the meters. But to a modeler, merely seeing a positive net flow is not adequate; the net flow has to equal the known or estimated flow contribution from the sewer shed.Getting the model to match metered dry day flow usually involves some measure of population in the sewer shed multiplied by an estimate of per capita water use. Population estimates can be based on census values, Equivalent Dwelling Units (EDU), number of known lateral connections or some other measure that can be associated with population. In addition, modelers have the ability to adjust Base Infiltration to help match measured flow. So the modeler has essentially three ‘knobs’ that can be adjusted to achieve a dry-day match; 1) population, 2) per capita water usage and 3) Base Infiltration.It is not uncommon for modelers to twist these ‘knobs’ into unlikely or even unbelievable ranges to match measured dry day flow. But what if the flow data are incorrect? How can the modeler diagnose incorrect flow data and avoid improper fiddling with the knobs? This paper will show the modeler a novel technique to determine if flow data can be the cause of calibration problems; not the assumptions about the population.