---
title: "Balancing reservoir, intermittent supply 7-10 and 17-20"
subject: "Water Supply Engineering"
chapter: 7
key: "semester_5/water_supply_engineering:res-c74"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/res-c74
site: BCT Engineering
---

# Balancing reservoir, intermittent supply 7-10 and 17-20

Water Supply Engineering · Chapter 7: Reservoirs and Distribution System

- Asked in: 2074 Chaitra, 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/res-c74
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-7

## Question

A rural area has a design year demand of water 20,000 litres per day. The demand is met by an intermittent system of supply two times a day at 7-10 and 17-20 (altogether 6 hrs).

| Time (Hour) | 5-7 | 7-12 | 12-17 | 17-20 | 20-5 |
|---|---|---|---|---|---|
| Consumption (%) | 20 | 35 | 10 | 25 | 10 |

Determine the balancing reservoir capacity for that rural area.

## Short answer

Daily demand is 20 m³ supplied intermittently 6 h/day at 3.33 m³/h. Mass-curve gives maximum surplus 2.00 m³ and maximum deficit 4.00 m³, so balancing capacity = 6.00 m³ (30% of daily demand).

## Answer

#### Given data
- Daily demand = 20,000 L = 20 m³/day
- Intermittent supply in 7–10 h and 17–20 h (6 h in all): inflow rate = 20/6 = 3.33 m³/h
- Demand: 5–7 h 20%, 7–12 h 35%, 12–17 h 10%, 17–20 h 25%, 20–5 h 10%

#### Mass-curve table (analytical method)
Inflow: 3.33 m³/h during 7–10 h and 17–20 h only. Time is counted from the start of the first demand period; a positive cumulative value means surplus in the tank.

| Time | Demand (m³) | Inflow (m³) | Inflow − demand | Cumulative |
|---|---|---|---|---|
| 05:00–07:00 | 4.00 | 0.00 | -4.00 | -4.00 |
| 07:00–10:00 | 4.20 | 10.00 | +5.80 | +1.80 |
| 10:00–12:00 | 2.80 | 0.00 | -2.80 | -1.00 |
| 12:00–17:00 | 2.00 | 0.00 | -2.00 | -3.00 |
| 17:00–20:00 | 5.00 | 10.00 | +5.00 | +2.00 |
| 20:00–05:00 | 2.00 | 0.00 | -2.00 | +0.00 |

Total demand = 20.00 m³; total inflow = 20.00 m³.

#### Capacity
- Maximum cumulative surplus = 2.00 m³
- Maximum cumulative deficit = 4.00 m³
- Balancing capacity = max. surplus + max. deficit = 2.00 + 4.00 = **6.00 m³**

**Answer: Balancing reservoir capacity = 6 m³ (30% of the daily demand).**

## Check yourself

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

1. Daily demand is 20 m3 and supply is only for 6 h a day. What is the inflow rate during supply hours?
   A. 0.83 m3/h
   B. 3.33 m3/h
   C. 6 m3/h
   D. 20 m3/h
   Answer: B. The whole 20 m3 comes in during 6 h: 20/6 = 3.33 m3/h. (Key point: Intermittent inflow rate)

2. In the 5-7 h period, supply is off. What is the inflow minus demand?
   A. +4.00 m3
   B. 0
   C. -4.00 m3
   D. -2.00 m3
   Answer: C. Demand is 20% x 20 = 4 m3 with zero inflow, so the difference is -4 m3. (Key point: Inflow outside supply hours)

3. Maximum surplus is 2 m3 and maximum deficit is 4 m3. What is the balancing capacity?
   A. 2 m3
   B. 4 m3
   C. 6 m3
   D. 8 m3
   Answer: C. 2 + 4 = 6 m3 (30% of daily demand). (Key point: Capacity formula)

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