---
title: "Balancing reservoir: village 600 population, 65 lpcd"
subject: "Water Supply Engineering"
chapter: 7
key: "semester_5/water_supply_engineering:res-as70"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/res-as70
site: BCT Engineering
---

# Balancing reservoir: village 600 population, 65 lpcd

Water Supply Engineering · Chapter 7: Reservoirs and Distribution System

- Asked in: 2070 Asar, 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-as70
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-7

## Question

A small village has design year population of 600 with 65 lpcd per capita demand. The demand is to be fulfilled by spring sources with safe yield 0.5 l/s. The consumption pattern in % of a day is as below.

| Time | 05-07 | 07-12 | 12-17 | 17-19 | 19-05 |
|---|---|---|---|---|---|
| Water consumption (%) | 30 | 30 | 15 | 20 | 5 |

Is balancing reservoir necessary? Calculate capacity of balancing reservoir if needed.

## Short answer

Daily demand is 39 m³ and spring yield is 43.2 m³/day, but peak demand rates exceed inflow, so a balancing reservoir is necessary. Mass-curve gives maximum deficit 11.85 m³ and surplus 4.20 m³; capacity ≈ 16.05 m³, adopt 16–17 m³.

## Answer

#### Given data
- Population 600 × 65 lpcd = 39,000 L/day = 39 m³/day
- Spring yield = 0.5 lps = 43,200 L/day = 43.2 m³/day = 1.8 m³/h
- Demand: 5–7 h 30%, 7–12 h 30%, 12–17 h 15%, 17–19 h 20%, 19–5 h 5%

#### Check of source and need of storage
The yield (43.2 m³/day) exceeds the demand (39 m³/day). But the demand in 5–7 h is 0.30 × 39/2 = 5.85 m³/h and in 7–12 h it is 2.34 m³/h, both more than the inflow of 1.8 m³/h. So the balancing reservoir **is necessary**.


#### Mass-curve table (analytical method)
Inflow: 1.8 m³/h (spring yield). 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 | 11.70 | 3.60 | -8.10 | -8.10 |
| 07:00–12:00 | 11.70 | 9.00 | -2.70 | -10.80 |
| 12:00–17:00 | 5.85 | 9.00 | +3.15 | -7.65 |
| 17:00–19:00 | 7.80 | 3.60 | -4.20 | -11.85 |
| 19:00–05:00 | 1.95 | 18.00 | +16.05 | +4.20 |

Total demand = 39.00 m³; total inflow = 43.20 m³.

#### Capacity
- Maximum cumulative surplus = 4.20 m³
- Maximum cumulative deficit = 11.85 m³
- Balancing capacity = max. surplus + max. deficit = 4.20 + 11.85 = **16.05 m³**
- Because the inflow in a day (43.20 m³) is more than the demand, the extra water simply overflows once the tank is full. The minimum storage that still meets every demand is the largest draw-down, 11.85 m³. The textbook value 16.05 m³ is on the safe side.

**Answer: Balancing reservoir is necessary; capacity ≈ 16.1 m³ (adopt 16–17 m³).**

## Check yourself

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

1. Demand 39 m3/day, yield 43.2 m3/day. How is the need for a balancing reservoir decided?
   A. Yield exceeds demand so none is needed
   B. By comparing hourly demand (5.85 m3/h in 5-7 h) with the inflow of 1.8 m3/h
   C. By the village population
   D. By the number of taps
   Answer: B. Hourly demand exceeds the continuous spring flow, so storage is necessary even though the daily yield is enough. (Key point: Need of reservoir)

2. 0.5 lps spring yield equals what hourly inflow?
   A. 0.5 m3/h
   B. 1.8 m3/h
   C. 30 m3/h
   D. 43.2 m3/h
   Answer: B. 0.5 x 3600 = 1,800 L/h = 1.8 m3/h. (Key point: Inflow rate)

3. Max surplus 4.20 m3, max deficit 11.85 m3. What capacity is obtained?
   A. 11.85 m3
   B. 16.05 m3
   C. 7.65 m3
   D. 43.2 m3
   Answer: B. 4.20 + 11.85 = 16.05 m3. (Key point: Capacity value)

## Related in Water Supply Engineering

- [Balancing reservoir: 500 population, spring 0.28 lps](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ares-a75) — Water Supply Engineering, chapter 7
- [Balancing tank: village 80000 L/day, spring 0.8 lps](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ares-b79) — Water Supply Engineering, chapter 7
- [Is reservoir necessary: village of 800, spring 0.5 L/s](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ares-b81) — Water Supply Engineering, chapter 7
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- [Population from spring yield of 5 L/s](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Apop-s71) — Water Supply Engineering, chapter 3
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