A materially conserved quantity in adiabatic, frictionless flow that accounts for changes of vorticity due to vortex line stretching and Coriolis effects. In the shallow water approximation, the potential vorticity is q=(f+zeta)/(H+eta), in which f is the Coriolis parameter, zeta is the relative vorticity, H is the mean fluid depth, and eta is the surface elevation above the mean.