Week 1: What crosses the fence line

Week 1 — the household's annual demand, the definition of closure the course scores, and the site sheet every later week writes into

22 February 2027

Open slides — week 1

The question#

Stand at the fence line of a suburban block for a year and count what crosses it. Water in. Food in. Fertiliser in. Rubbish out, sewage out, stormwater out. Which of those flows could be turned around to feed another system on the same block, and how much of each would that actually absorb?

That is the whole course. Everything after this week is one flow, sized properly.

The household, fixed for everyone#

Every student designs for the same four people — two adults and two children aged 9 to 13 — so that five different climates are the only thing that differs.

Read that table twice. It is the denominator of every score you will get.

The method: a closure ratio that cannot be gamed#

A self-sufficiency ratio that divides production by demand has two well-known faults. It can exceed 100 % — so a design is rewarded for growing food nobody eats — and it ignores imports entirely, so a system running on bought-in fish feed scores the same as one running on its own.

Closure is defined to remove both:

Closure = what the site supplied and the household actually used ÷ everything the household used, including imports. Capped at 100 %.

Three consequences worth stating now, because they will each surprise someone in about week 5:

1. Imports sit in the denominator. Carting water in lowers water closure; it does not simply vanish from the sum.
2. Animal products count only in proportion to feed grown on site. Fish fed on bought pellets are bought fish.
3. A bigger garden cannot lower your water score. An earlier version of this model put the student’s chosen garden in the denominator, and a more ambitious design scored worse. That was a bug in the metric, not a finding about gardens.

Worked example: the roof, at every site#

The one calculation everybody can do in week 1. Roof runoff, from enHealth’s Appendix B, citing Martin (1980):

runoff (L) = A × (rainfall − B) × roof area, with A = 0.80 and B = 2 mm per month

Canberra, 300 m² of roof, 616 mm of rain a year:

0.80 × (616 − 24) mm × 300 m² = 142,080 L = 142 kL

Domestic demand is 219 kL. So the largest roof in the set, in a climate nobody calls dry, delivers 65 % of the household’s water before a single plant is watered. Alice Springs delivers 73 kL, a third of demand. Darwin delivers 411 kL, nearly twice demand — and still runs out in September, for reasons week 9 is entirely about.

What is out of scope#

Energy and shelter. Not because they do not matter, but because crediting them here would let a design claim self-sufficiency it has not demonstrated. Biogas is a case in point: your digester will make it, you will record it, and it earns you nothing in this course. Only the digestate’s nutrients count. Those belong to SLOP4762 — Designing the Closed-Loop Household II: Energy and Shelter, a proposed follow-on course that adds energy physics and introductory thermodynamics to the prerequisites.

What this week hands you#

The site sheet: your preset’s plot, growing area, roof, existing tanks and bore allocation, with twelve rows of Bureau of Meteorology monthly climate behind it and a source for every value. Every later week writes into it. Bring it to every tutorial.

Back to week 1 overview