---
title: "Impounded reservoir, 4.78 million m3 draft"
subject: "Water Supply Engineering"
chapter: 2
key: "semester_5/water_supply_engineering:impounded-reservoir-as67"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/impounded-reservoir-as67
site: BCT Engineering
---

# Impounded reservoir, 4.78 million m3 draft

Water Supply Engineering · Chapter 2: Sources of Water

- Asked in: 2067 Asar (old course)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/impounded-reservoir-as67
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-2

## Question

The yield of water from a catchment area during each successive month is given below. Determine analytically the storage capacity of the impounded reservoir to maintain a constant draft of 4.78 million $\mathrm{m^3}$ of water per month.

| Month | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Inflow (million $\mathrm{m^3}$) | 1.40 | 2.10 | 2.80 | 2.80 | 8.40 | 11.90 | 11.90 | 7.70 | 2.52 | 2.25 | 1.96 | 1.68 |

## Short answer

With a constant draft of 4.78 million m³/month, the mass curve method shows a maximum cumulative deficit of 20.73 million m³ at the end of April. Reservoir storage capacity = 20.73 million m³.

## Answer

Analytical (mass curve) method: the storage required equals the maximum cumulative deficit of the yield compared with the constant draft of $4.78$ million m$^3$/month.

Total yield $= 57.41$ million m$^3$ and total draft $= 12\times4.78 = 57.36$ million m$^3$, so the draft is within the yearly yield. The reservoir is assumed full at the start of the dry period (end of Aug), when the last surplus ends.

| Month | Inflow (Mm3) | Demand (Mm3) | Surplus/deficit | Cumulative deficit |
|---|---|---|---|---|
| Sep | 2.52 | 4.78 | -2.26 | -2.26 |
| Oct | 2.25 | 4.78 | -2.53 | -4.79 |
| Nov | 1.96 | 4.78 | -2.82 | -7.61 |
| Dec | 1.68 | 4.78 | -3.10 | -10.71 |
| Jan | 1.40 | 4.78 | -3.38 | -14.09 |
| Feb | 2.10 | 4.78 | -2.68 | -16.77 |
| Mar | 2.80 | 4.78 | -1.98 | -18.75 |
| Apr | 2.80 | 4.78 | -1.98 | -20.73 |

The largest cumulative deficit occurs at the end of Apr and equals 20.73 million m$^3$.

**Answer: Storage capacity of the impounded reservoir = 20.73 million m$^3$.**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does total yield 57.41 versus total draft 57.36 show?
   A. Reservoir is not needed
   B. The yearly yield is just enough for the draft
   C. The draft is far above yield
   D. Losses are large
   Answer: B. Yield is slightly above the draft, so the draft can be met with enough storage. (Key point: Yearly sufficiency)

2. Where does the cumulative deficit start?
   A. End of August, when the last surplus ends and the reservoir is full
   B. January
   C. End of April
   D. Start of May
   Answer: A. Inflow exceeds draft from May to Aug, so the reservoir is full at the end of Aug and deficits start in Sep. (Key point: Start of the dry period)

3. When does the cumulative deficit peak?
   A. End of Dec
   B. End of Jan
   C. End of Mar
   D. End of April
   Answer: D. Inflow stays below 4.78 until April, so deficit keeps growing to the end of April. (Key point: Maximum deficit)

4. What is the storage capacity?
   A. 18.75 Mm3
   B. 16.77 Mm3
   C. 20.73 Mm3
   D. 14.09 Mm3
   Answer: C. Cumulative deficit at the end of April is 20.73 Mm3. (Key point: Result)

## Close variants set in other exams

- [Impounded reservoir, 4.4 million m3 draft](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aimpounded-reservoir-bk68) — Water Supply Engineering, chapter 2, asked 2×

## Related in Water Supply Engineering

- [Impounded reservoir, 6202 million L per month](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aimpounded-reservoir-c69) — Water Supply Engineering, chapter 2, asked 2×
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