thetaS: $0\le \theta_S \le 1$

Implicitness parameter for vertical scalar advection and diffusion. Note that this value of thetaS does not necessarily share the same stability properties as the free-surface solver since stability for scalar transport is governed by the horizontal Courant number even when $\theta_S>0.5$.

• $\theta_S=0$: Fully explicit
• $\theta_S=0.5$: Crank-Nicolson (Ensures no vertical Courant limitation for advection or diffusion)
• $\theta_S=1.0$: Fully implicit (same as above but first-order accurate)
• $\theta_S=\theta$: To guarantee consistency with continuity