# Understanding the Water Quality Time Step

 Product(s): WaterGEMS, WaterCAD Version(s): 08.11.XX.XX, 10.XX.XX.XX Area: Calculations

# Problem

How does the Water Quality Time Step work, between choosing "True" vs. "False" for "Set Water Quality Time Step?" in the calculation options?

When a user defined water quality time step is defined in the calculation options, why are the results different when compared to the default of "False" for "Set water quality time step"?

# Solution

By default, the calculation option “Set water quality time step?” is set to "False", which tells the program to use a water quality timestep equal to one tenth of the “hydraulic time step”. So E.g. if the hydraulic time step is 0.25 hr then a water quality time step of 0.025 hr will be used for the water quality calculations.

When the "Set water quality time step?" option is set to "True", a user defined water quality time step can be entered.

For example, consider a case where the hydraulic time step is set to 0.25 hr and you are comparing results with the "Set water quality time step" set to "False" to the results when it is set to "True", with a user defined water quality timestep of 0.25 hr. One might think that the results should be the same, but they will actually be using two different water quality time steps: 0.25 hr and 0.025 hr. Hence, the water quality results (age, trace or constituent) may be different.

Generally speaking, the smaller the water quality timestep, the more accurate the results, and a smaller water quality timestep is typically best in challenging situations or when some unexpected water quality results are seen.

In the below graph it can be seen that the results are more accurate with a water quality timestep of 0.01 hr (set water quality timestep = true), when compared to 0.25 hr (set water quality timestep = true) or 0.025 hr (set water quality timestep = false).