Hydrographs and pattern loads are two distinct ways to describe how flow varies over time. Ultimately, you can attain the same results using either type but there are some behavioral and semantic differences that should be noted.

Pattern loads consists of a single average base load and a series of dimensionless multipliers used to delineate how the load varies over time. A hydrograph, simply, is a time-discharge series.

Hydrographs are usually applied as wet weather loads, and are generated using hydrologic methods, while patterns are more typically applied to sanitary loads. Pattern multipliers are usually developed based on flow monitoring data for the system under consideration. The multipliers are used to account for time-of-day variations in sewer loads. Usually a handful of patterns are developed (e.g. residential area, commercial area, large industry) and these patterns are assigned to the appropriate nodes. These statements represent typical usage of both loading types; they do not represent hard and fast rules.

During an extended period simulation if the duration of the simulation exceeds the duration of a pattern then the pattern will repeat itself. If the duration of the simulation exceeds the duration of a hydrograph the last point of the hydrograph will remain constant for the extent of the remaining time.

The following figure shows the difference between a hydrograph and pattern load.

Difference between a Hydrograph and Pattern Load