Learning About Growth and Depletion
A young professional couple living in a McMansion in Phoenix, AZ read about Peak Oil and they are worried. They have some cash, a lot of equity in their house, but are not self sufficient. They discover an old ranch out in the desert with a verv good well that produces 10,000 gallons of water a month. They get an idea. "We can sell our house, buy this ranch and start a small fruit orchard using irrigation water from the well- grow our own garden too." They do it.
Year 1- The well produces the 10,000 gallons as advertised.
They figure they need
2000 gal of water for the house and garden
At 100 gal of water per tree they plant 50 trees = 5000 gal
They figure they can start a road side fruit stand and sell enough fruit to pay taxes and get by with just 25 trees, but plant 50 to have extra cash.
That leaves 3000 gal of water for a swimming pool. The fun life begins.
What they don't know is the exponential law.
As the trees grow, each year they require 5% more water. They also did not know that as they pumped water from this well at 10,000 gallons a month, the local water table drops and they get 10% less water each year. ( There are small rounding errors in the following calculations)
Year 2-The well produces only 9000 gal. Trees need 105 gal each.
The trees now take 5250 gal and they notice they can't wash the car as often as there is only a 1750 gal surplus
Year-3.The well produces only 8100 gal. Trees need 110 gal each.
The trees now take 5513 gal and the surplus is down to 588 gal. They tear out the swimming pool. Shorter showers too.
Year-4 The well produces only 7290 gal. Trees need 116 gal each
The trees now take 5788 gal, and there is a shortage of water for the garden. They cut the house water down to 1502 gal. They divert grey waste water to the garden.
Year-5 The well produces only 6561 gal. Trees need 122 gal each
The trees now take 6078 gal. That is only 456 gal for the house. They garden will have to be small, but their fruit stand is making money. They can buy some extra food.
Year-6 The well produces only 5905 gal. Trees need 128 gal each
The trees will need 6381 gal with NONE for the house. They cut down 7 trees leaving 43 trees( needing only 5488 gal)and 417 gal for the house.
Year-7 The well produces only 5314 gal. Trees need 134 gal each
They learn about the exponental law. If you divide the percent into 70 you get the doubling time, or in this case the time to drop by about half. Divide 10% into 70 and it is 7 years.
They cut down more trees leaving 37( 4858 gal) leaving 456 gal for the house.
Year- 8. The well produces only 4783 gal. Trees need 141 gal each
They cut down more trees leaving 31 (4362 gal) leaving 421 gal for the house
Year-9. The well produces only 4309 gal. Trees need 148 gal each
They cut down more trees leaving only 27 (3989 gal) leaving only 320 gal for the house
Year-10. They get out their old PC and do the math using an Excel spreadsheet. They realize it is hopeless. This year they will not have enough trees to grow enough fruit to sell in order to pay the taxes.
They sell their Ranch. They lost their McMansion, their equity, and their dream. They came out of it wiser and knowing about the effects of exponential growth and exponential depletion.
They learned about Peak Oil the hard way. Increasing percent demand colliding with reduced percent supply except this time it is Peak Water.
In this simplified example I just used simple arithmetic to approximate the answers. ( I realize there are more precise formulas in Excel for this calculation, but I tried to keep this idea simple)
Well Water = previous year *0.9
Water for trees = no of trees * (gal/tree previousyear * 1.05)
Total water used = total water for trees + house water which must be less than well produces.
Let some economist tell that well that the couple's "demand" for water was enough for 50 trees- It should deliver enough to meet demand. I had a well like this one once. It did not give a damn about our water needs! Local water was limited by fractures in hard rock

An expert is someone who has made every mistake possible in their field and learned how to prevent them.