The Courant Number reflects the portion of a cell that a solute will traverse by advection in one time step:



where:

C_r = Courant number

delta l = dimension of the grid cell at each location

v =average linear velocity at that location

delta t = maximum time step size

When Advection dominates dispersion, designing a model with a small (<1) Courant number will decrease oscillations, improve accuracy & decrease numerical dispersion.