Solution
= Solution
{c}
Return to dimensional variables. The river at base level imposes the drained <boundary condition>, while the right-hand end is impermeable:
$$
\boxed{h(0,t)=0,\qquad q(L,t)=-Kh(L,t)h_x(L,t)=0.}
$$
As long as the right-hand end remains wet, the latter is $h_x(L,t)=0$. The <initial condition> for the drought is the height profile inherited at $t=t^*$; stopping rainfall changes the forcing but does not instantaneously remove stored water.