A tank with a tap and a drain
- The tap20 litres a dayThe tankThe drain20 litres a day
Picture a water tank on a farm. A tap fills it: the rain off the roof, about 20 litres a day. A drain empties it: the animals drink, about 20 litres a day. The level in the tank tells you one thing. It tells you whether the drain is running faster than the tap.
- In, a day20 litresOut, a day20 litres
Twenty in and twenty out: the level holds. Twenty in and thirty out: the level drops, by ten litres, every day, until the tank is dry. People call the first one sustainable. The word just means it can keep going, because what goes out comes back.
- In, a day20 litresOut, a day25 litres
The tap stays at 20 litres a day. The animals now drink 25. What happens to the level?
It rises, because the tap is still running.
Not yet. The tap was running before too. What changed is the drain, and the drain is now the bigger number.It drops a little every day.
Right. Five litres more go out than come in, every day. A small gap is still a gap, and a gap that runs every day empties the tank in the end.It holds, because 25 is close to 20.
Not yet. Close is not equal. The tank does not round. Five litres a day is 150 litres a month, and the tank only had so many. - 0 litres336 litres672 litresDay 0Day 30
Tap 20, drain 30: a tank that started full The tank hides the speed. A full tank with a fast drain looks fine for weeks. The level answers one question: how much is left. The tap and the drain answer a different one: how fast it is going. Both matter, and a full tank only answers the first.
Picture the tank at 600 litres with the tap fixed at 20 a day. Move the drain and watch what is left after thirty days. The numbers are arithmetic on that rule.
Start600 litresAfter 30 days900 litresDrain, litres a day: 10
Drain 10, tap 20. More comes in than goes out. The tank is fuller after a month than it started.
Start600 litresAfter 30 days600 litresDrain, litres a day: 20
Drain 20, tap 20. In equals out. The level has not moved in thirty days.
Start600 litresAfter 30 days300 litresDrain, litres a day: 30
Drain 30, tap 20. Ten litres more go out than come in. After thirty days, 300 litres are left.
Start600 litresAfter 30 days0 litresDrain, litres a day: 40
Drain 40, tap 20. Twenty more out than in, every day. On day thirty the tank is empty.
At which drain settings is the tank sustainable?
40 as well. It still had water for a whole month.
Not yet. A month of water is the level, not the rate. At 40 the tank was always going to be empty on day thirty, and it was.All of them. Rain always comes back.
Not yet. The rain is the tap, and the tap is 20 a day whatever the drain does. A bigger drain does not make it rain harder.20 or less. What goes out comes back.
Right. Sustainable is about the two rates, not the level. At 10 and 20 the drain is no bigger than the tap, so the tank can run like this for ever.Move the control.
- A fisherybreed vs catchA forestgrow vs cutGroundwatersoak in vs pump
The farm tank is every real tank. A fishery: the fish breed, that is the tap; the boats catch, that is the drain. A forest: trees grow, trees are cut. Groundwater: rain soaks in, pumps draw out. Same two numbers every time.
- 200 grow a year300 cut a year? fewer after ten years
Picture a forest of 10,000 trees. Every year 200 grow to full size. Every year a company cuts 300. After ten years, how many fewer trees are standing than at the start?
treesFind the gap for one year first, then count the years.
Right. A thousand. The gap is 100 a year, and ten years of it is a thousand trees. The forest still looks like a forest. It is a tenth smaller, and the gap has not closed. - Bigger boatsthe drain growsThe cod stockthe tap does not2 July 1992about 30,000 jobs lost
Off Newfoundland the cod were the drain nobody watched. For nearly 500 years the catch came back every spring. The boats grew bigger, the catch grew, and the stock did not. On 2 July 1992 Canada shut the fishery. About 30,000 people in Newfoundland and Labrador lost their jobs.
Slide the marker to how big the breeding cod stock was in 1992, as a share of the biggest it had ever been.
Slide the marker to where you think the answer is, then press Lock. Nothing is scored.
your guess · 0%the answer · 1%- 1Gone0%
- 2Half50%
- 3All100%
Your guess: —
About one in a hundred of its estimated peak, by the Canadian Encyclopedia's count. Guesses land near the middle, because a fishery that still had boats on it feels half full. The level was the last thing to show. A small fishery was allowed again in 2024. The stock is still a fraction of what it was.- Refills100 a dayFifty houses150 a dayThe wellten years deep
Picture a village well. Rain refills it by 100 litres a day. Fifty houses draw 3 litres each a day. The well holds ten years of water at that rate. Is the well sustainable?
Yes, if it rains more next year.
Not yet. That is hoping the tap grows to match the drain. The tap is the rain's size, not the village's wish.Yes. Ten years is a long time.
Not yet. Ten years is how much is left, not how fast it goes. A tank with a gap is not sustainable; it is a tank with a date.No. 150 goes out and 100 comes in. The depth only says when it ends.
Right. The two rates decide it, and the drain is the bigger one. Ten years is the level, and it is counting down.
Lesson complete
Sustainable means used no faster than it comes back. The level says how much is left, not how fast.