Water is the yield limit

SLOP4761 · Week 2 · Soil, plants and irrigation demand

Where we are

Week 1 counted what crosses the fence line. Week 2 asks what the garden wants.

Still scoping. Every number today comes from long-term climate records, not from your client’s garden. What that garden actually draws is a measurement, and it goes on your register in week 6.

This week’s question

You have between 130 and 500 m² of growable ground.

The naive sizing — area × days × yield — says the warm site with the long season wins.

It does not. Where does the water come from?

By the end you can

  1. Compute ETo for a month by hand from Tmax, Tmin and latitude
  2. Turn twelve months of climate into an irrigation deficit in millimetres
  3. Choose a planted area you can defend on water rather than on ambition

Three equations, and one habit

Reference evapotranspiration, FAO-56 Equation 52 (Hargreaves):

ETo = 0.0023 × (Tmean + 17.8) × √(Tmax − Tmin) × Ra [mm/month]

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

Effective rain, FAO Irrigation Training Manual 3:

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

Deficit = ETc − Pe, floored at zero.

Worked example: Canberra

Annual ETo 1,244 mm against 616 mm of rain. P/ETo = 0.50.

Sum the monthly deficits across the growing season:

932 mm of net irrigation per m² per year 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.

What the five sites show

Net irrigation demand per m² is a climate property, not a design choice. It is highest where ETo is high and rain is low — and the sites do not rank the way intuition suggests.

Two Canberra advantages no indicator scores: rainfall evenness (28 % of rain in the driest four months, against Darwin’s 1.4 %) and chill, which the course handles as a regional category rather than a formula, because the sine-curve method gives Canberra 1,979 hours against a reported ~720.

Your tutorial task

  1. One month by hand, first. January. Ra, then ETo, then ETc, then Pe, then deficit
  2. Then the twelve rows, from your workbook’s sheet 2
  3. Then choose a planted area, and check it against your client’s retained area

Output: a twelve-row irrigation demand curve in kilolitres, on sheet 2.

Before you leave

  • Is every deficit computed from effective rain, not total rain?
  • Is any deficit negative? Floor it at zero
  • Is the last column in kL, with the unit written on it?
  • Does your hand-computed January match the spreadsheet?

Sources: FAO-56 Equations 21–25 and 52, Table 12; FAO Irrigation Training Manual 3 Annex 1. Both cited in full in the week 2 notes.