An average family of four uses roughly 1200 liters of water per day (1L = 1000 Com). A lake that covers 50 km with a uniform depth and supplies water to a local town with a population of 40,000 people would lose
depth every year due to water consumption form
the locals. Ignore evaporation, rain, creeks and rivers

A family of 4 uses 1200 liters of water per day

1200 liters/day * 365 days/year = 438000 liters per year

If the town has 40,000 people, this equals 10,000 families

Explanation:

So the total town usage in one year = 10000*438000 = 4.38E9 liters = 4.38E12 cm^3 (scientific notation)

The area of the lake in cm^2 is 50 km^2 * (100 cm / m)^2 * (1000 m / km)^2 = 5E11 cm^2

The loss of volume of the lake is 5E11 cm^2 * h, where h is the change in depth

The total volume of water used by the town = the volume lost by the lake.

Therefore 4.38E12/5E11 = h = 8.76 cm

moisture is lackin, and organic acids are scarce.