With a decrease in Volume for our simple bathtub model, change in water depth is easily found as:

Volume = Surface Area x Depth

In our simple bathtub model, the surface area stays the same and increasing or decreasing the water volume simply raises or lowers the water depth.

Change in Volume = Surface Area x Change in Depth

Change in Depth = Change in Volume/Surface Area

Change in Depth = -194,700 acre-ft/230,000 acres = -0.85 ft

The water level will drop 0.85 ft each year.