During $2012,$ the following hydrologic data were collected for a $85-$mi watershed total precipitation, $27$ in$.$; total combined loss due to evaporation and evapotranspiration, $10$ in: estimated groundwater outflow $($including groundwater depletion$),7$ in; and mean surface runoff, $9$ in$.$ What is the change in volume of water remaining in storage in the watershed during $2012?($Volume in ac$-$ft$)$