What a roof can deliver

SLOP4761 · Week 7 · Water sourcing, and the specialist handover

Where we are — read this order carefully

  1. Today, Monday: the water method, and a preview of what a measurement year produced
  2. Friday, 5 pm: Assignment 1 is due
  3. After that: the full specialist handover. Release B, identical for everyone on your home

Nothing in today’s preview changes what your register should say. Nothing you compute before Friday is an observation of this house.

This week’s question

You know what the garden wants (week 2) and what the household wants (week 1).

How much water does the site actually produce, and in which months?

Not the annual total. The months. Annual totals are how people end up with a tank that runs dry in September.

By the end you can

  1. Apply the enHealth runoff formula per month, and validate it against its own worked example
  2. Deduct a first flush by a stated rule
  3. Lay a supply curve against week 2’s demand curve in the same units

enHealth Appendix B

runoff (L) = A × (rainfall − B) × roof area

  • A — collection efficiency, 0.80–0.85 in the source, citing Martin (1980). This course uses 0.80
  • B — initial loss, 2 mm per month, or 24 mm a year

The source’s own worked example:

0.8 × (750 − 24) × 200 = 116,160 L

If your spreadsheet does not reproduce that line exactly, fix it before going further.

Apply it per month. First flush: about 10 L per 50 m² of roof per event. And the identity behind all of it: 1 mm of rain on 1 m² of roof is 1 L.

Worked example: Alice Springs, the hardest site

350 m² of roof across house and machinery shed. 284 mm of rain a year.

0.80 × (284 − 24) mm × 350 m² = 72,800 L = 73 kL/year

Domestic demand is 219 kL. The roof covers a third of the household, and none of the garden.

Alice Springs is the only site with a second source: a bore capped at 100 kL a year. That gives 173 kL against a domestic demand of 219 kL and a garden asking 660 kL — total demand approaching 880 kL.

What a capped allocation does

It converts a resource question into an allocation question.

You are no longer asking how much water exists. You are choosing who gets it.

That is why the excellent Alice Springs reference design reaches 97 % water closure with 16 % garden water satisfaction — it closes the loop by watering almost nothing.

Both numbers are published together. Always.

Your tutorial task

  1. Validate: 750 mm, 200 m², expect 116,160 L
  2. Twelve rows, never the annual total. A month below the 2 mm initial loss contributes zero, not negative
  3. Deduct first flush and name the rule
  4. Overlay supply on demand, same axis, kilolitres
  5. Last pass on Assignment 1 — both cross-checks, then the $1,500 total

Output: a twelve-row supply curve, and a register ready to submit.

Before you leave

  • Does your sheet return 116,160 L for the published example?
  • Did you use connected or guttered roof area, not roof plan area?
  • Is any monthly contribution negative? Floor it
  • Is Assignment 1 finished and costed for Friday?

Full sources in the week 7 notes.