Week 2: Water is the yield limit, not sunlight

Week 2 — evapotranspiration, effective rain and the monthly deficit that decides how much of your growing area is actually productive

1 March 2027

Open slides — week 2

The question

You have between 130 and 500 m² of growable ground depending on your site. The naive sizing — area × days × yield — says Alice Springs, with 400 m² and a long warm season, should out-produce Canberra. It does not, and the reason is that the naive sizing never asked where the water comes from.

So: month by month, how much of the garden’s water demand does rain meet, and what does the shortfall do to yield?

The method: FAO-56, three equations

Reference evapotranspiration, Hargreaves, FAO-56 Equation 52:

ETo = 0.0023 × (Tmean + 17.8) × √(Tmax − Tmin) × Ra

Ra is extraterrestrial radiation from latitude and day of year (Equations 21–25). FAO warns this over-predicts in humid conditions, which is why Brisbane’s figure carries a caveat and the others do not.

Crop demand: ETc = ETo × Kc. This course uses Kc = 0.95 for a mixed garden, a blend inside FAO-56 Table 12’s range and a course assumption.

Effective rain, FAO Irrigation Training Manual 3, Annex 1:

Pe = 0.8P − 25   for P > 75 mm/month
Pe = 0.6P − 10   for P ≤ 75 mm/month

The deficit is ETc − Pe, floored at zero, and only in months warm enough to grow.

Worked example: Canberra

Canberra’s annual ETo is 1,244 mm against 616 mm of rain: P/ETo = 0.50. Run the monthly deficit across the growing season and it sums to 932 mm of net irrigation per square metre per year.

Over the full 450 m² growable area:

932 mm × 450 m² = 419,400 L = 419 kL/year

The roof delivers 142 kL, and the household has first call on it. Canberra’s garden is water-limited long before it is land-limited, and the excellent reference design still only meets 20 % of the garden’s deficit. That is not a failure of the design. It is the site.

Where Canberra wins, and the model cannot see it

Two advantages sit outside every number this course scores.

Rainfall evenness. 28 % of Canberra’s rain falls in its driest four months — the highest share of the five sites, against Darwin’s 1.4 %. Even rain means storage does less work, which is why Canberra’s excellent design needs 36 kL of tank where Darwin’s needs 76 kL.

Chill. Chill hours cannot be computed from monthly means: the sine-curve method gives Canberra 1,979 hours against the ~720 that regional guides report, and gives Brisbane zero against ~150. So this course uses regional chill categories, not a formula. Canberra sits in the high-chill band, which opens apples, pears, cherries and walnuts. Brisbane is low-chill and Darwin is effectively nil, so their orchards are a different list of species entirely.

Five sites: net irrigation demand per m² of garden

SiteNet irrigationP/EToChill categoryWhat it means for the planting plan
Canberra932 mm/yr0.50high (~720 h)Temperate fruit and nuts available; frost defines the annual season, not water
Alice Springs1,650 mm/yr0.16lowNearly twice Brisbane’s demand per m². Irrigated area, not total area, is the real design variable
Brisbane651 mm/yr0.85low (~150 h)Lowest demand per m², but only 130 m² exists to plant, including the shared strata garden
Adelaide952 mm/yr0.43medium (~500 h)Rain falls in winter and demand peaks in summer; the two barely overlap
Darwin832 mm/yr1.07nilWet season needs no irrigation at all; the dry season needs almost all of it

What this week hands you

A planting plan with a monthly irrigation demand curve underneath it: area committed by month, deficit by month, and an honest statement of how much of your growable area you are choosing to leave unplanted because you cannot water it.

Weeks 7 to 9 size the supply that curve is asking for. Do not adjust the curve to suit a tank you have not yet sized.

Everything here is a scoping estimate from long-term climate records, not a measurement of this garden. What the garden actually draws is one of the things Assignment 1 has to buy.

Teaching team

Back to week 2 overview