Models & Qualitative Behavior · Hard problem

Water drains from a cylindrical tank through a hole in its base, and by Torricelli's law the depth $h$ (in feet) satisfies $h' = -k\sqrt{h}$ for a constant $k > 0$. The depth is $16$ ft at $t = 0$ and $9$ ft at $t = 1$ hour. At what time $t$ (in hours) is the tank empty?