Week 2 Tutorial: The irrigation demand curve

Week 2 tutorial — turn twelve rows of climate normals into a monthly irrigation demand curve, and decide on the numbers how much ground you will actually plant

4 March 2027

Time

90 minutes in the room, plus about 30 to tidy the output

Graded

Checkpoint 1 of 10, 2 % — a good-faith attempt at the output below

Due

5 pm, 5 March 2027

What this tutorial is for#

The naive way to size a garden — area times days times yield — never asks where the water comes from. Today you replace it with a monthly balance, and the curve that falls out decides how much of your growable ground is worth planting.

Story stage. Scoping. The deficit you compute comes from long-term records, not from your client’s garden. What that garden actually draws is one of the things Assignment 1 will propose to measure.

Open these first#

• Your client evidence workbook, sheet 2 (climate normals) — twelve rows of rain_mm, tmax_c, tmin_c, eto_mm and effective_rain_mm.
• Week 2’s lecture, for the three FAO-56 equations. You need its method section open beside you.
• Your home’s dossier at clients, for the growing envelope and any retained area the client will not let you plant.

If you missed week 1#

Build the workbook now from the week 1 tutorial — it takes about twenty minutes. If you would rather catch up afterwards, use the labelled practice climate instead and redo it on your own home later:

PRACTICE — Wattle Street, invented round numbers, not a real climate. Rain 40, 40, 50, 50, 60, 60, 60, 60, 50, 50, 40, 40 mm. ETo 180, 150, 120, 80, 50, 40, 40, 60, 80, 120, 150, 170 mm. Growing envelope 207.8 m². These are round on purpose, and no conclusion drawn from them is about anybody’s home.

The words this week uses#

• ETo — reference evapotranspiration, in mm. The depth of water a reference grass surface would lose in a month. A property of the weather, not of your crop.
• ETc — crop evapotranspiration. ETo multiplied by a crop coefficient Kc. The course uses Kc 0.95 for a mixed vegetable garden.
• Effective rain — the part of the rain that reaches the roots. Always less than total rain.
• Deficit — ETc minus effective rain, in mm, floored at zero. A wet month does not bank water for a dry one.
• The demand curve — those twelve deficits converted to kilolitres for your chosen planted area. “Curve” because you will plot it; a twelve-row table is the same object.

Do this#

1. One month by hand, first (20 minutes). Pick January. Compute Ra from your site’s latitude and the day of year, then ETo from Equation 52, then ETc at Kc 0.95, then effective rain, then the deficit. It takes about fifteen minutes and it is the only way to find out whether your spreadsheet is computing what you think it is. Write the hand answer down before you build the sheet.

2. Build the twelve rows (30 minutes). Add these columns to sheet 2:

That row is Canberra, from its sourced climate file. Check it: 186 × 0.95 = 176.7, and 176.7 − 25.1 = 151.6 mm. The commonest error in the room is using total rainfall instead of effective — it overstates what the sky delivers by 20 to 40 % and makes every later tank look bigger than it is.

3. Then the decision (25 minutes). Multiply your deficit by candidate planted areas until the annual irrigation figure stops being absurd:

irrigation (kL) = deficit (mm) × planted area (m²) ÷ 1000

Alice Springs students will land somewhere well under their 400 m². That is the correct answer, not a compromise. Check your chosen area against your home’s retained-area constraint before you commit to it: Canberra must leave at least 100 m² of the back lawn as lawn, and Brisbane’s 40 m² allocation is common property outside the lot.

4. Write it in kilolitres (15 minutes). Millimetres are a depth and kilolitres are a volume; weeks 7 and 9 need the volume. Convert all twelve and put them on sheet 2 beside the deficits.

The output#

A twelve-row irrigation demand curve in kilolitres, and a planted area you can defend on water rather than on ambition, both on sheet 2 of your workbook.

Check your own before you submit#

1. Does your hand-computed January match your spreadsheet to within a few per cent?
2. Is every deficit computed from effective rain?
3. Is any deficit negative? It should be floored at zero.
4. Is the final column in kL, and does it have “kL” written on it?
5. Does your planted area fit inside the growing envelope and respect the retained area?
6. Is your chosen Kc written down, with a note that it is a course assumption?

Where it goes next#

The demand curve is fixed from here. Weeks 7 to 9 size supply against it. If you later want to plant more, change the curve deliberately and say why — do not let it drift to match a tank.

Back to week 2 overview