To explain a food-supply constraint, you show how one limiting factor reduces the amount of food that reaches people. Good answers are chains: constraint, then effect on production, storage or delivery, then effect on supply.
This lesson follows reading a water-balance dataset and belongs to food, water and energy systems. The food-web and energy-flow explorer can help you picture how an input limit passes through a system.
How do you build the chain?
- Name the constraint and where it applies.
- Say what it changes: yield per hectare, area farmed, losses after harvest, or delivery.
- Quantify if you can, using the data you are given.
- Link to the outcome: total supply compared with what people need.
- Add a limit: who could still be affected, or what you would need to know.
Worked example
The figures are invented for practice. Island K grows rice on 40,000 hectares.
In a normal year the yield is 3 tonnes per hectare. The island has 600,000 people who each eat 150 kg of rice a year. About 15% of the harvest is lost in storage and transport.
Normal year. Production is 40,000 × 3 = 120,000 tonnes. After 15% loss, 120,000 × 0.85 = 102,000 tonnes remain. Demand is 600,000 × 150 kg = 90,000,000 kg = 90,000 tonnes. So there is a surplus of 102,000 − 90,000 = 12,000 tonnes.
Drought year. The yield falls to 2.2 tonnes per hectare. Production is 40,000 × 2.2 = 88,000 tonnes. After the same loss, 88,000 × 0.85 = 74,800 tonnes remain. Against demand of 90,000 tonnes, the shortfall is 90,000 − 74,800 = 15,200 tonnes.
Written explanation. Low rainfall in the drought year reduces the water available to rice, so yield falls from 3 to 2.2 tonnes per hectare. With the same land area, production drops by 32,000 tonnes. After storage losses, supply is 15,200 tonnes below what the population needs. The island would need to import rice or draw on stored grain, which depends on income and stocks the data does not show.
What mistake does this lesson prevent?
Mistaken answer: “Drought is a problem for food. People go hungry.”
This names a cause and an outcome but skips the middle. It does not say how drought changes production, and it assumes hunger without showing that supply fell below demand.
The correction adds the middle link and a figure: drought lowers yield, which lowers production, which makes supply less than need. Keep “go hungry” for when you can justify access problems.
Check yourself
1. A farm area of 20,000 hectares produces 50,000 tonnes of crop. What is the yield?
Show answer
50,000 ÷ 20,000 = 2.5 tonnes per hectare.
2. A village of 10,000 people eats 120 kg of grain each per year. How many tonnes does it need?
Show answer
10,000 × 120 = 1,200,000 kg, which is 1,200 tonnes.
3. Write a three-link chain for high fertiliser prices reducing food supply.
Show answer
High fertiliser prices make farmers apply less, so yield per hectare falls, so total production and local food supply are lower than in a year with cheaper fertiliser. A fourth link could note that small farms are affected most.
Where does this lead next?
Next, switch resource and trace an energy-system input and output. The practice set mixes both skills.
Chains are easier to write when someone checks each link. Our teachers do that in online one-to-one Geography tuition.