---
title: "Storage for uniform inflow over 24 hours"
subject: "Water Supply Engineering"
chapter: 7
key: "semester_5/water_supply_engineering:res-a74"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/res-a74
site: BCT Engineering
---

# Storage for uniform inflow over 24 hours

Water Supply Engineering · Chapter 7: Reservoirs and Distribution System

- Asked in: 2074 Asoj, 8 marks
- Video: [Capacity of Balancing Reservoir | Example 10](https://www.youtube.com/watch?v=vXO9fdtUN0s) — EngiNerd
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/res-a74
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-7

## Question

Calculate the storage required to supply the demand shown in the following table if the inflow of water to the reservoir is maintained at a uniform rate throughout 24 hours.

| Time (hours) | 00-04 | 04-08 | 08-12 | 12-16 | 16-20 | 20-24 |
|---|---|---|---|---|---|---|
| Demand (million litres) | 0.48 | 0.[?]7 | 1.[?]3 | 1.00 | 0.82 | 0.54 |

## Short answer

Assuming the unclear demand values are 0.67 and 1.33 ML, total demand is 4.84 ML and uniform inflow is 0.2017 ML/h. Mass-curve gives maximum surplus 0.46 ML and maximum deficit 0.27 ML, so storage required ≈ 0.73 ML (730 m³).

## Answer

#### Given data
- Demand in million litres for six 4-hour periods: 0.48, 0.67, 1.33, 1.00, 0.82, 0.54
- Two digits are unclear in the question paper; they are read as 0.67 and 1.33. Total demand = 4.84 ML
- Inflow is uniform over 24 h: 4.84/24 = 0.2017 ML/h = 0.8067 ML per 4 h

#### Mass-curve table (analytical method)
Inflow: 0.807 ML per 4-hour period. Time is counted from the start of the first demand period; a positive cumulative value means surplus in the tank.

| Time | Demand (ML) | Inflow (ML) | Inflow − demand | Cumulative |
|---|---|---|---|---|
| 00:00–04:00 | 0.48 | 0.81 | +0.33 | +0.33 |
| 04:00–08:00 | 0.67 | 0.81 | +0.14 | +0.46 |
| 08:00–12:00 | 1.33 | 0.81 | -0.52 | -0.06 |
| 12:00–16:00 | 1.00 | 0.81 | -0.19 | -0.25 |
| 16:00–20:00 | 0.82 | 0.81 | -0.01 | -0.27 |
| 20:00–00:00 | 0.54 | 0.81 | +0.27 | -0.00 |

Total demand = 4.84 ML; total inflow = 4.84 ML.

#### Capacity
- Maximum cumulative surplus = 0.46 ML
- Maximum cumulative deficit = 0.27 ML
- Balancing capacity = max. surplus + max. deficit = 0.46 + 0.27 = **0.73 ML**

**Answer: Storage required ≈ 0.73 ML (730 m³).** If the unclear digits are different, repeat the same table with the correct values: capacity = maximum surplus + maximum deficit of the cumulative column.

## Check yourself

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

1. Total demand is 4.84 ML over 24 h in six 4-h periods. What is the inflow per 4-h period?
   A. 0.48 ML
   B. 0.807 ML
   C. 1.21 ML
   D. 4.84 ML
   Answer: B. 4.84/6 = 0.807 ML per period (0.2017 ML/h). (Key point: Uniform inflow)

2. In the table, what does a positive cumulative value mean?
   A. Tank is short of water
   B. Surplus water stored in the tank
   C. Demand has stopped
   D. Inflow has stopped
   Answer: B. Positive means inflow has exceeded demand so far, so there is a surplus in the tank. (Key point: Sign of cumulative)

3. Max surplus is 0.46 ML and max deficit is 0.27 ML. What storage is required?
   A. 0.19 ML
   B. 0.46 ML
   C. 0.73 ML
   D. 0.27 ML
   Answer: C. 0.46 + 0.27 = 0.73 ML (730 m3). (Key point: Capacity formula)

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