Replenishment and use

What was there, and what changed?

Stock is the amount stored at one time. Collection adds water; use removes water. Our records cover one week and use litres throughout. Assume there are no leaks, overflow or other changes

1. 1Remaining stock Add the collected water to the starting amount, then subtract the water used
2. 2Change Collection greater than use raises the stock. Equal amounts leave it unchanged. Use greater than collection lowers it

Store A begins with more water than Store B. Must A also gain more water during the week?

Account for all the water

A store starts with 40 L, collects 15 L and uses 10 L during one week

1. 1Add collection 40 L + 15 L = 55 L before the water is used
2. 2Subtract use 55 L − 10 L = 45 L remaining
3. 3Remaining is not the gain The store gained 15 L − 10 L = 5 L. Adding that gain to the original 40 L also gives 45 L

At the start (L)

\(\displaystyle 45\)

Collected this week (L)

\(\displaystyle 25\)

Used this week (L)

\(\displaystyle 15\)

How much water remains after this week? Assume no other gains or losses

Use litres, not another unit. Add collection to the starting amount, then subtract use

Each modelled week starts with 60 L. Try two different equal pairs of collected and used water. Then hold collection fixed and increase use. Each control change starts a new one-week model; it does not add another week. Assume no leaks or overflow

When collection equals use, how does the ending stock compare with the start?

Compare what was gained

For each store, subtract use from collection. The starting amount is not part of that calculation

1. 1Store A A starts with 90 L, collects 20 L and uses 15 L. Its gain is 20 L − 15 L = 5 L
2. 2Store B B starts with 45 L, collects 30 L and uses 10 L. Its gain is 30 L − 10 L = 20 L
3. 3Which gained more? B gained more: 20 L is greater than 5 L. A still holds more water at the end, but that answers a different question

Both stores gained water during the same week. Assume no other gains or losses. Which gained more?

Which store gained more during this week? Assume no other gains or losses

Which store gained more during this week? Assume no other gains or losses

A store ends a week with the same amount as at the start. What can this result alone show?

Stock and change answer different questions

Remaining stock includes what was already there. Gain compares what entered and left during the period

1. 1Remaining Starting stock + collection − use
2. 2Gain Collection − use. Compare this result for each store
3. 3Limits Use the same period and unit. A one-week balance does not establish future supply or all environmental impacts