---
title: "Impounded reservoir, 4.4 million m3 draft"
subject: "Water Supply Engineering"
chapter: 2
key: "semester_5/water_supply_engineering:impounded-reservoir-bk68"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/impounded-reservoir-bk68
site: BCT Engineering
---

# Impounded reservoir, 4.4 million m3 draft

Water Supply Engineering · Chapter 2: Sources of Water

- Asked in: 2068 Baisakh (old course), 8 marks
- 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-bk68
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-2

## Question

The yield of water from a catchment area is given below. Determine analytically the minimum storage capacity of the impounded reservoir to maintain a constant draft of 4.4 million $\mathrm{m^3}$ of water per month. Neglect all losses and wastage.

| Month | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Inflow (million $\mathrm{m^3}$) | 1.5 | 2.0 | 2.5 | 5.0 | 6.0 | 8.2 | 9 | 7.5 | 5.0 | 3.5 | 3.1 | 2.0 |

## Short answer

The mass curve method with a constant draft of 4.4 million m³/month gives a maximum cumulative deficit of 11.8 million m³ at the end of March. Minimum storage capacity = 11.8 million m³.

## Answer

The minimum storage is the maximum cumulative deficit of inflow against the constant draft (analytical mass curve method). Losses are neglected. Draft $D = 4.4$ million m$^3$/month; total inflow $= 55.3$ million m$^3$ and total draft $= 12\times4.4 = 52.8$ million m$^3$, so the yearly yield is sufficient.

The deficit period starts after the reservoir has been filled by the wet-season surplus. September is the last month with a surplus, so the reservoir is full at the end of September.

| Month | Inflow (Mm3) | Demand (Mm3) | Surplus/deficit | Cumulative deficit |
|---|---|---|---|---|
| Oct | 3.50 | 4.40 | -0.90 | -0.90 |
| Nov | 3.10 | 4.40 | -1.30 | -2.20 |
| Dec | 2.00 | 4.40 | -2.40 | -4.60 |
| Jan | 1.50 | 4.40 | -2.90 | -7.50 |
| Feb | 2.00 | 4.40 | -2.40 | -9.90 |
| Mar | 2.50 | 4.40 | -1.90 | -11.80 |

The inflow is less than the draft from October to March, so the cumulative deficit increases throughout this period; the reservoir is then refilled from April onwards.

The largest cumulative deficit is 11.8 million m$^3$.

**Answer: Minimum storage capacity = 11.8 million m$^3$.**

## Check yourself

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

1. What gives the minimum storage capacity in the analytical mass curve method?
   A. Maximum cumulative deficit of inflow against the constant draft
   B. Sum of all monthly draft
   C. Average inflow minus draft
   D. Largest monthly surplus
   Answer: A. The reservoir must cover the longest and largest cumulative shortfall. (Key point: Minimum storage)

2. Why does the deficit table start in October?
   A. October has the highest inflow
   B. The reservoir is full at the end of September after the wet season surplus
   C. The year begins in October
   D. Draft starts in October
   Answer: B. The reservoir is filled by the wet season surplus and the deficit period begins after that. (Key point: Starting condition)

3. For a draft of 4.4 Mm3/month, inflow is below the draft in:
   A. April to August
   B. Jan to Mar only
   C. Oct to Mar
   D. Nov to Dec only
   Answer: C. Inflows from Oct to Mar (3.5, 3.1, 2.0, 1.5, 2.0, 2.5) are all below 4.4. (Key point: Deficit months)

4. What is the minimum storage?
   A. 9.9 Mm3
   B. 11.8 Mm3
   C. 7.5 Mm3
   D. 12.0 Mm3
   Answer: B. 0.9+1.3+2.4+2.9+2.4+1.9 = 11.8 Mm3. (Key point: Result)

## Close variants set in other exams

- [Impounded reservoir, 4.78 million m3 draft](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aimpounded-reservoir-as67) — Water Supply Engineering, chapter 2, asked 2×

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