In a dam lake $($reservoir$)$ having a surface area of $50$ km$2$

$,$ the precipitation is $20$ mm during $10$

days. Qin and Qout are $11$ and $10$ m$3$

$/$s$,$ respectively, and the infiltration depth is $10$ mm within this period. In case

of the decreasing of the water level of $8$ mm in the lake, calculate the evaporation depth within this period.