---
title: "Impounded reservoir, 400 m3/day demand"
subject: "Water Supply Engineering"
chapter: 2
key: "semester_5/water_supply_engineering:impounded-reservoir-b66"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/impounded-reservoir-b66
site: BCT Engineering
---

# Impounded reservoir, 400 m3/day demand

Water Supply Engineering · Chapter 2: Sources of Water

- Asked in: 2066 Bhadra (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-b66
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-2

## Question

The water demand of a community is 400 $\mathrm{m^3}$/day. The flow in the river in various months of the year is given below. If the river is to be used as a source for the water supply system, calculate the capacity of the impounded reservoir.

| Month | January | February | March | April | May | June |
|---|---|---|---|---|---|---|
| Inflow (lps) | 2.40 | 4.00 | 5.60 | 4.80 | 5.60 | 6.40 |

| Month | July | August | September | October | November | December |
|---|---|---|---|---|---|---|
| Inflow (lps) | 7.20 | 6.40 | 1.12 [?] | 0.00 | 5.60 | 6.40 |

## Short answer

Converting flows to monthly volumes (30‑day months) and applying the mass curve method yields a maximum cumulative deficit of 21,404 m³ from September to February. Reservoir capacity ≈ 21,400 m³.

## Answer

Convert the river flow (lps) to monthly volumes using a 30-day month and compare them with the demand. Demand $= 400\ \mathrm{m^3/day}\times30 = 12{,}000\ \mathrm{m^3}$ per month. Volume $= Q(\text{l/s})\times10^{-3}\times86400\times30$ m$^3$. The September flow (1.12 lps) is used as printed.

Total inflow $= 143{,}908$ m$^3$ and total demand $= 144{,}000$ m$^3$. The yearly inflow is almost equal to the yearly demand (only 92 m$^3$ short), so the river can just about meet the demand with storage.

| Month | Inflow (m3) | Demand (m3) | Surplus/deficit | Cumulative deficit |
|---|---|---|---|---|
| Sep | 2903 | 12000 | -9097 | -9097 |
| Oct | 0 | 12000 | -12000 | -21097 |
| Nov | 14515 | 12000 | +2515 | -18582 |
| Dec | 16589 | 12000 | +4589 | -13993 |
| Jan | 6221 | 12000 | -5779 | -19772 |
| Feb | 10368 | 12000 | -1632 | -21404 |

The critical period is September to February. The inflow is below the demand in September, October, January and February, and the surpluses of November and December are not enough to refill the reservoir in between. The reservoir is full at the start of September (after the surplus months of March to August), and the largest cumulative deficit in this period is the capacity needed.

**Answer: Capacity of the impounded reservoir $\approx 21{,}404\ \mathrm{m^3}$ (about 21,400 m$^3$).**

## Check yourself

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

1. What is the monthly demand of 400 m3/day (30-day month)?
   A. 400 m3
   B. 1,200 m3
   C. 120,000 m3
   D. 12,000 m3
   Answer: D. 400 x 30 = 12,000 m3. (Key point: Monthly demand)

2. How is a monthly volume obtained from a flow in lps?
   A. Q x 86400 x 30 / 1000 m3
   B. Q x 30 m3
   C. Q x 24 x 30 m3
   D. Q x 86400 m3
   Answer: A. Litres per second x seconds in the month, divided by 1000 gives m3. (Key point: Flow to volume)

3. What is the reservoir capacity?
   A. 9,097 m3
   B. 18,582 m3
   C. 21,404 m3
   D. 12,000 m3
   Answer: C. The largest cumulative deficit at the end of Feb is about 21,404 m3. (Key point: Result)

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