Meltwater production over an advancing footprint ($\dot{\mathcal{V}}=s_m\mathcal{L}$)

For constant water-production rate per wetted area $s_m$, dry zero-flux noses and footprint length $\mathcal{L}(t)$, the moving-boundary conservation of mass gives $\dot{\mathcal{V}}=s_m\mathcal{L}$ per out-of-plane span. Consequently $\ddot{\mathcal{V}}=s_m\dot{\mathcal{L}}$. Equivalently the melt volume is $s_m{\int }_0^t\mathcal{L}(t\prime )dt\prime$; a point begins producing melt only after the front arrives.