A water-balance dataset compares the water arriving in a place, mainly rainfall, with the water leaving it through evapotranspiration. In exam questions you subtract month by month, spot surplus and deficit periods, and write a conclusion that is supported by the figures.
This lesson opens food, water and energy systems. The statistics and distribution explorer helps you check totals and averages as you practise.
How do you read the data, step by step?
- Check the units. Rainfall and evapotranspiration are both in millimetres (mm), so they can be compared directly.
- Work out the balance. For each month, balance = rainfall − evapotranspiration.
- Mark the sign. A positive balance is a surplus. A negative balance is a deficit.
- Look for the pattern. Which months run together? Is the deficit seasonal?
- Total the year. Add the monthly balances, or add each column first and subtract.
- Say what it means, and what it cannot show.
Worked example
All figures are invented for practice. Valley A has the monthly data below, in mm.
| Month | Rain | Evap. | Balance |
|---|---|---|---|
| Jan | 120 | 50 | +70 |
| Feb | 100 | 55 | +45 |
| Mar | 90 | 70 | +20 |
| Apr | 60 | 80 | −20 |
| May | 40 | 90 | −50 |
| Jun | 20 | 95 | −75 |
| Jul | 15 | 100 | −85 |
| Aug | 25 | 95 | −70 |
| Sep | 50 | 80 | −30 |
| Oct | 80 | 65 | +15 |
| Nov | 110 | 55 | +55 |
| Dec | 130 | 50 | +80 |
Annual totals. Rain adds to 840 mm and evaporation to 885 mm. The annual balance is 840 − 885 = −45 mm.
Check by the other route. Surplus months add to 70 + 45 + 20 + 15 + 55 + 80 = 285 mm. Deficit months add to 20 + 50 + 75 + 85 + 70 + 30 = 330 mm. So 285 − 330 = −45 mm, which agrees.
Conclusion. Valley A has a deficit from April to September, six months, with the largest shortfall of 85 mm in July. Surplus months are October to March. Over the year the balance is slightly negative, 45 mm, so the valley may need stored water or a river from elsewhere to cover the dry season.
What mistake does this lesson prevent?
Mistaken answer: “Rainfall is 840 mm, so Valley A has plenty of water.”
This quotes one column and ignores what leaves the system. Evaporation is 885 mm, higher than rain. Even without that, the monthly pattern shows six months of deficit.
The correction is to compare both columns, month by month, before drawing a conclusion. Use phrases such as “exceeds” or “is less than”, and always quote a figure.
Check yourself
1. In one month, rain is 30 mm and evapotranspiration is 45 mm. What is the balance?
Show answer
30 − 45 = −15 mm, a deficit.
2. Three months have rain of 200, 150 and 100 mm and evapotranspiration of 80, 90 and 110 mm. Find the total balance.
Show answer
Monthly balances: +120, +60 and −10. Total = +170 mm. Check: rain 450 − evaporation 280 = 170.
3. Why can a positive annual balance still hide a water problem?
Show answer
The surplus may fall in a few wet months while other months have a deficit. People need water all year, so the timing matters as well as the total.
Where does this lead next?
Next, use the same careful reading to explain a food-supply constraint. Then test yourself in the mixed practice set.
Students often lose marks here by describing the table instead of interpreting it. That is a habit our teachers can work on in online one-to-one Geography tuition.