S1 Geography | Hong Kong Water Supply and Water Resources: Reservoirs, Dongjiang Water and Conservation

S1 Geography | Hong Kong Water Supply and Water Resources: Reservoirs, Dongjiang Water and Conservation – Tutor Prep Guide

2026-11-15

Tutor lesson-prep guides · Junior Secondary Geography

Every student knows Hong Kong buys water from the Dongjiang, but almost none can answer “if that supply stopped, how long would the reservoirs last?” This lesson turns water supply from a fact they have heard into a number they can calculate.

Objectives — Students can: (1) name the three sources of water in Hong Kong (local catchments, Dongjiang water, seawater for flushing) with approximate shares and one limitation each; (2) divide total reservoir capacity by daily consumption to find how many days the stored water would last, and explain why reservoirs alone are not enough; (3) propose three conservation measures and estimate how much water each saves per day.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
The water cycle	Where does rainwater go after it lands?	Some flows into streams and reservoirs, some soaks into the ground, some evaporates
Volume units	How many litres are in one cubic metre?	1,000 litres
Percentages	A value is seven tenths of another. What percentage is that?	70%

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Three sources	Write “reservoirs, Dongjiang, seawater” and ask which one supplies flushing water	Seawater for flushing; fresh water from reservoirs and Dongjiang
5-15 min	Shares and risks	Use the 70% Dongjiang, 30% local split, then ask what the risk of an outside source is	Name the risk of relying on a single outside source
15-27 min	Days of storage	Model it: 2.6 million m3 per day against 586 million m3 of storage, so 586 / 2.6	Get about 225 days and add that losses are excluded
27-37 min	Conservation numbers	A five-minute shower uses about 50 litres against 150 litres per person per day	Get about 33% and name one saving action
37-41 min	Sanity check	Put “58.6 billion” and “586 million” side by side and ask which is the reservoir capacity	Spot that the first is ten times too large
41-45 min	Exit check	Ask whether reservoirs alone could cover a full year without Dongjiang water	No, because annual use exceeds total storage

03 Teaching script (read it out)

Scene 1: Three sources: separate drinking from non-drinking first

Start by splitting water into two layers, otherwise students list seawater among the sources and stop thinking. The first layer is fresh water, from rain collected in local catchments plus Dongjiang water. The second layer is non-potable water, mainly seawater used for flushing. Only then give the shares: about 70% of fresh water comes from the Dongjiang and about 30% from local catchments. The second term to pin down is catchment area: a reservoir does not make water, it collects rain from a catchment through catchwaters, so reservoir capacity and catchment size are two different things and enlarging catchwaters is cheaper than raising a dam. The third checkpoint is external risk: the Dongjiang lies in Guangdong, so low rainfall, growing upstream demand or a pollution incident all affect supply, which is why Hong Kong keeps reservoirs, desalination and seawater flushing as three separate supports.

Ask students to name the two main sources of fresh water in one sentence and one risk of either

Scene 2: Days of storage: turning capacity into days

The commonest error here is units, so write both numbers on the board: daily use about 2.6 million m3 and total reservoir capacity about 586 million m3. The calculation is one step: 586 / 2.6, about 225 days. Then immediately add three corrections: (1) this is a theoretical ceiling that ignores evaporation, leakage and the minimum operating level; (2) summer demand is higher, so the real figure is shorter; (3) the conclusion is therefore not “we can last 225 days” but “reservoirs alone cover a bit over half a year, so a second source is essential”. That step turns arithmetic into an argument; without the corrections it is just a division.

Ask students to run 586 / 2.6 themselves and name the three corrections

Scene 3: Conservation: persuade with numbers, not slogans

Telling students to cherish water achieves nothing. Give a contrast instead: domestic use is about 150 litres per person per day, a shower runs at about 10 litres per minute, so a five-minute shower is 50 litres, a third of the daily total. Cutting two minutes saves 20 litres, about 13% of the day. Then scale it up: a household of four saving 80 litres a day saves 29,200 litres a year, roughly 29 cubic metres. “29 cubic metres a year” lands far better than “use less water”. Close with the frame: conservation reduces demand, not supply, so a drought has to be met from both sides.

Ask students to use the 150-litre figure to estimate their own household saving

04 Question types and worked steps

Type	Example	Steps	Answer
Share calculation	About 70% of Hong Kong fresh water comes from the Dongjiang. If annual fresh water use is 949 million m3, how much is Dongjiang water?	(1) Confirm the 70% refers to fresh water (2) 949 x 0.7 (3) keep the unit in million m3	About 664 million m3
Storage-days calculation	Total reservoir capacity is 586 million m3 and daily use is 2.6 million m3. How many days could storage cover?	(1) Match the units (2) 586 / 2.6 (3) answer about 225 days and note losses are excluded	About 225 days, fewer in practice
Risk comparison	Compare enlarging reservoir capacity with expanding the Dongjiang supply.	(1) One is local, one is external (2) one benefit each (3) one risk each (4) a closing judgement	Local is controllable but limited by terrain; Dongjiang is large but externally exposed
Recommendation	Suggest two water-saving measures for a family of four and estimate the daily saving.	(1) Name concrete actions (2) give a number for each (3) add them up	Two minutes less showering saves 20 litres x 4 = 80 litres; full-load laundry saves about 30 litres a week

Sanity check (teach this): Always run a unit and magnitude check: (1) are both numbers in the same unit; (2) does the answer make sense (never 2 days, never 2,000 days); (3) annual use (949 million) is 1.62 times total storage (586 million), so reservoirs alone can never cover a year. That inequality is the best self-check.

05 Independent practice (3 questions)

Question	Answer	What to watch
Daily use 2.6 million m3 and capacity 586 million m3: how many days of storage?	About 225 days (586 / 2.6)	Did they match units first?
150 litres per person per day, 50 litres for a shower: what share of the day?	About 33%	They should divide 50 by 150, not the reverse
Water supply questions often stress the word “local”. Why?	Local sources are controllable; outside sources carry risk	Do they explain controllability?

06 Common mistakes and what to say

Mistake	Student writes	Say this
Counting seawater as drinking water	Hong Kong has three sources: reservoirs, Dongjiang and seawater, so seawater is drinking water too.	“Seawater is for flushing, it never enters the fresh water system. Split your three sources into fresh and non-fresh.”
Units off by a factor of 100	Capacity is 58.6 billion m3 and daily use is 2.6 million, so we can last 22,500 days.	“You added two zeros. Total capacity is 586 million. Redo it as 586 / 2.6.”
Answering without corrections	Answer: 225 days.	“225 days is the ceiling before evaporation, leakage and minimum level. Add one line saying the real figure is lower.”
Conservation as a slogan	We should cherish water and not waste it.	“Cherishing water is not a number. Say how many litres a two-minute shorter shower saves, using 150 litres as the denominator.”

07 Homework (paper, section and question numbers)

Weather and climate chapter, questions 1-3 (rainfall and seasons) — Rain is the first input of any water supply, so rainfall patterns come first
Map reading and scale chapter, questions 4-6 (catchment area) — Catchment area has to be measured from a map using the scale
Agriculture and fisheries chapter, question 2 (local self-sufficiency) — The same self-sufficiency idea applied to vegetables

08 Exit check (last 5 minutes)

Question	Answer	Decision
If Dongjiang supply stopped, how long would reservoirs last?	A bit over half a year, not a full year	Answering “yes, a year” means annual use was never compared with storage
What share of fresh water comes from local catchments?	About 30%	Saying 70% means Dongjiang water was confused
Give one conservation measure with a number.	Two minutes less showering saves 20 litres per person per day	“Use less water” alone is not quantified

09 Tutor FAQ

Do students need to memorise 586 and 2.6?

Not the exact figures, but the magnitudes: total storage is about 600 million m3 and daily use about 2.6 million, two orders of magnitude apart, which proves reservoirs alone cannot cover a year.

Why mention desalination?

It is the third support: unaffected by outside supply but expensive and power-hungry, so it runs as insurance during droughts or incidents rather than as a main source.

Should calculations include units?

Always. Most errors in this chapter come from units, and an answer of “225” with no “days” usually means the student just divided two numbers.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.