If only one soil layer is present, then  $h_t^d = h_b^d$, and we define: 
\begin{equation}\label{eq:us1}
U_s=  \exp (-c_j \, h_t^d )
\end{equation}
If there are two soil layers, we compute $U_s$ as the average of the values defined by the two dry soil depths $h_t^d$ and  $h_b^d$. The weights depend on $a_j^\mathrm{sat}$, which defines the "transpiring" fraction of the top soil layer:
\begin{equation}\label{eq:us2}
U_s= a_j^\mathrm{sat} \exp (-c_j \, h_t^d ) + (\mathrm{1}-a_j^\mathrm{sat})  \exp (-c_j \, h_b^d )
\end{equation}

The "transpiring" fraction is defined by redistributing the top layer's moisture $W_t$ in a virtual layer of depth $h_t^\mathrm{sub}$ (set to 1 mm in hydrolc), what defines a totally dry and a totally saturated fraction ($a_j^\mathrm{sat}$):
\begin{itemize}
\item if  $W_t > h_t^\mathrm{sub} \, \theta^\mathrm{max}$, then $a_j^\mathrm{sat}$=1, and $U_s$ depends on $h_t^d$ only (Eq. \ref{eq:us1}), % $U_s=\exp (-c_j \, h_t^d )$,
\item if $W_t$ is very small, $a_j^\mathrm{sat}$ decreases from 1 to 0 as the top layer's moisture vanishes, and it serves to describe a smooth transition to the case where the top soil layer has vanished and $U_s$ depends on  $h_t^d = h_b^d$.
\end{itemize}

