How much depth would a lake lose per year if it uniformly covered an area of 50 square kilometers and supplied a local town with a population of 40,000 people?

Question

A typical family of 4 uses roughly 1200 liters of water per day.

Answer 1

1 person uses 1200/4=300 litres per day. That is, 109,500 litres per year.

40000 people use 40000*109500=4.38*10^9 litres.

1 litre=1000 cubic centimetres=1000/1000000=0.001 cubic metres and 4.38*10^9 litres=4.38*10^6 cu m.

1km=1000m and 1 sq km=10^6 sq m. 50 sq km=5*10^7 sq m. So depth changes by 4.38*10^6/5*10^7=0.0876m=8.76cm.