---
title: "Balancing reservoir: 500 population, spring 0.28 lps"
subject: "Water Supply Engineering"
chapter: 7
key: "semester_5/water_supply_engineering:res-a75"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/res-a75
site: BCT Engineering
---

# Balancing reservoir: 500 population, spring 0.28 lps

Water Supply Engineering · Chapter 7: Reservoirs and Distribution System

- Asked in: 2075 Asoj, 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-a75
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-7

## Question

A village has design year population of 500 nos and water demand of 45 lpcd. The demand is met by a continuous system of supply from a spring source with safe yield of 0.28 lps. The consumption pattern is as follows:

| Time (Hours) | 05 - 07 | 07 - 12 | 12 - 17 | 17 - 19 | 19 - 05 |
|---|---|---|---|---|---|
| Consumption (%) | 15 | 45 | 10 | 20 | 10 |

Is balancing reservoir necessary? Calculate its capacity if necessary and justify your answer.

## Short answer

Daily demand is 22.5 m³ and spring yield is 24.19 m³/day, but peak demand rates exceed the inflow, so a balancing reservoir is necessary. Mass-curve gives maximum deficit 6.44 m³ and surplus 1.69 m³; capacity ≈ 8.14 m³, adopt about 9–10 m³.

## Answer

#### Given data
- Population 500 × 45 lpcd = 22,500 L/day = 22.5 m³/day
- Spring yield = 0.28 lps = 24,192 L/day = 24.19 m³/day = 1.008 m³/h
- Demand: 5–7 h 15%, 7–12 h 45%, 12–17 h 10%, 17–19 h 20%, 19–5 h 10%

#### Check of source and need of storage
The yield (24.19 m³/day) is more than the demand (22.5 m³/day), so the source is adequate. The demand rate in 7–12 h is 0.45 × 22.5/5 = 2.03 m³/h, and in 5–7 h it is 1.69 m³/h, both more than the inflow rate of 1.008 m³/h. So a balancing reservoir **is necessary**.


#### Mass-curve table (analytical method)
Inflow: 1.008 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 | 3.38 | 2.02 | -1.36 | -1.36 |
| 07:00–12:00 | 10.12 | 5.04 | -5.08 | -6.44 |
| 12:00–17:00 | 2.25 | 5.04 | +2.79 | -3.65 |
| 17:00–19:00 | 4.50 | 2.02 | -2.48 | -6.14 |
| 19:00–05:00 | 2.25 | 10.08 | +7.83 | +1.69 |

Total demand = 22.50 m³; total inflow = 24.19 m³.

#### Capacity
- Maximum cumulative surplus = 1.69 m³
- Maximum cumulative deficit = 6.44 m³
- Balancing capacity = max. surplus + max. deficit = 1.69 + 6.44 = **8.14 m³**
- Because the inflow in a day (24.19 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, 6.44 m³. The textbook value 8.14 m³ is on the safe side.

**Answer: Balancing reservoir is necessary; capacity ≈ 8.1 m³ (adopt 9 m³ or 10 m³).** It is needed because the peak demand in the morning is about twice the continuous spring flow.

## Check yourself

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

1. Spring yield (24.19 m3/day) exceeds demand (22.5 m3/day). Why is a balancing reservoir still needed?
   A. Because the spring may dry up
   B. Because peak-hour demand rates (e.g. 2.03 m3/h) exceed the uniform inflow of 1.008 m3/h
   C. Because the yield is too high
   D. Because the population is small
   Answer: B. A daily surplus is not enough; hourly demand in the morning exceeds the continuous inflow, so storage is needed. (Key point: Need of reservoir)

2. What is the spring inflow rate used in the table?
   A. 0.28 m3/h
   B. 1.008 m3/h
   C. 0.94 m3/h
   D. 2.03 m3/h
   Answer: B. 0.28 lps x 3600 = 1,008 L/h = 1.008 m3/h. (Key point: Inflow rate)

3. Max surplus 1.69 m3 and max deficit 6.44 m3. What capacity does the standard method give?
   A. 6.44 m3
   B. 1.69 m3
   C. 4.75 m3
   D. 8.14 m3
   Answer: D. Surplus + deficit = 8.14 m3; the minimum needed is the 6.44 m3 draw-down, so 8.14 is on the safe side. (Key point: Capacity value)

## Related in Water Supply Engineering

- [Balancing reservoir: village 600 population, 65 lpcd](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ares-as70) — 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
- [Is balancing tank necessary: 20000 L/day spring](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ares-k72) — Water Supply Engineering, chapter 7
- [Balancing reservoir, intermittent supply 7-10 and 17-20](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Ares-c74) — Water Supply Engineering, chapter 7
- [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