For three variables (top and bottom soil moisture, $W_t$ and $W_b$; dry soil height at the top, $h_t^d$), we describe a linear relaxation of the values $X_j$ within each soil column towards their grid-cell average $X_j$, with a timescale $\tau_h$ of one day:
\begin{equation}\label{eq:hdiff}
\Delta X_j = - \frac{\Delta t}{\tau_h}(X_j-\overline{X}).
\end{equation}
