Write a difference equation for the amount of water in a reservoir behind a dam on a river.

The reservoir holds 1 billion gallons of water when full

Annual water loss through evaporation equals 10% of the total in the reservoir.

Rainfall of 50 million gallons falls on the reservoir.

River water input   varying between 300- 500 million gallons passes through the reservoir each year. The water can be captured behind the dam or released to the river below.

200 million gallons is released annually for use in agriculture and water supply for cities.

To maintain the health of the river, we need to have an annual flow of at least 100 million gallons of water after the agricultural and city uses have been taken.

Write a difference equation to calculate the amount of water in the reservoir