Assigning Demand in Water Distribution Networks: Methods and Calculations

Demand allocation in water distribution networks is a critical step in hydraulic modeling, as it helps determine the quantity of water that must be supplied to different areas. This task requires careful consideration of available data and the characteristics of the network. Several methods can be employed to assign demand at junctions within the network. This article provides an in-depth exploration of these methods, complete with an example and calculations. However, you can also watch the details of it in our lecture series.

Importance of Demand Allocation in Water Distribution Systems

Accurate demand allocation is essential for ensuring that water supply systems are designed to meet the actual needs of the population. Improper allocation can lead to issues such as low pressure, inadequate supply, and inefficiencies in the network. Engineers must consider factors such as population density, land use, and available data when assigning demand to junctions in a water distribution network.

Common Methods of Demand Allocation

1. Direct Assignment Method

   • Overview: This is the most straightforward method, where demand is assigned directly to junctions based on actual water consumption data. This data can come from metered connections or consumption records.
   • Application: This method is highly accurate but requires detailed data for each node.
   • Example: If a node represents a residential block with 50 houses, and each house uses 500 liters of water per day, the total demand at that node is $50 \times 500 = 25,000$ liters per day.

2. Linear Method (Proportional Allocation)

   • Overview: In the linear method, demand is distributed proportionally along the length of the pipes. This method assumes that water usage is evenly distributed.
   • Application: Suitable when demand data is not available for individual connections but can be estimated based on pipe lengths.
   • Example Calculation:
     • Assume a pipeline segment between Junction A and Junction B is 200 meters long, with a total demand of 10,000 liters per day.
     • If Junction A is 50 meters from the start, the proportional demand at Junction A is: $$\text{Demand at A} = \frac{50}{200} \times 10,000 = 2,500 \text{ liters per day}$$

3. Areal Method

   • Overview: This method allocates demand based on the area served by each junction. Thiessen polygons (Voronoi diagrams) are often used to divide the area, and demand is distributed proportionally to the area of each polygon.
   • Application: This method is used when population or land area data is available for different zones.
   • Example Calculation:
     • Suppose Junction A serves an area of 2 hectares, and the total demand for the region is 50,000 liters per day.
     • If the total area is 10 hectares, the demand at Junction A is: $$\text{Demand at A} = \frac{2}{10} \times 50,000 = 10,000 \text{ liters per day}$$

4. Population-Based Method

   • Overview: In this method, demand is allocated based on the population served by each junction. Population data is linked to spatial data, such as census blocks.
   • Application: Useful in urban and rural areas where population data is readily available.
   • Example Calculation:
     • Assume Junction A serves a population of 500 people, with an average water consumption of 150 liters per person per day.
     • The demand at Junction A is: $$\text{Demand at A} = 500 \times 150 = 75,000 \text{ liters per day}$$

5. Land Use-Based Method

   • Overview: Demand is assigned based on land use types (e.g., residential, commercial, industrial). Different demand rates are applied to each land use type.
   • Application: Ideal for regions with diverse land use patterns.
   • Example Calculation:
     • Assume a junction serves 3 hectares of residential land and 1 hectare of commercial land. The demand rates are 5,000 liters per hectare per day for residential and 10,000 liters per hectare per day for commercial.
     • The total demand at the junction is: $$\text{Demand} = (3 \times 5,000) + (1 \times 10,000) = 25,000 \text{ liters per day}$$

6. Cluster-Based Method

   • Overview: Junctions are grouped into clusters based on proximity, and demand is distributed across the junctions in each cluster.
   • Application: Useful for large networks where demand data is aggregated.
   • Example: If a cluster of junctions has a total demand of 100,000 liters per day and contains 10 junctions, demand can be allocated equally or based on other criteria like pipe lengths.

7. Zoning Method

   • Overview: The network is divided into zones, and demand is distributed across junctions within each zone. This method is often used in large municipal water systems.
   • Application: Suitable for urban water supply systems where demand is aggregated by administrative zones.
   • Example Calculation:
     • Assume a zone has a total demand of 500,000 liters per day and contains 20 junctions.
     • If the demand is distributed equally, each junction will have: $$\text{Demand per junction} = \frac{500,000}{20} = 25,000 \text{ liters per day}$$

8. Uniform Distribution Method

   • Overview: The total demand is equally distributed among all junctions, regardless of their location or population.
   • Application: This method is a last resort when no detailed data is available, and it is the simplest but often lacks accuracy.
   • Example: If a network has 50 junctions and the total demand is 1,000,000 liters per day, each junction will be assigned: $$\text{Demand per junction} = \frac{1,000,000}{50} = 20,000 \text{ liters per day}$$

9. Demand Density Method

   • Overview: This method calculates demand density (e.g., liters per second per hectare) based on land use or population density and then distributes this demand to junctions accordingly.
   • Application: Useful in areas with varying demand densities, allowing for more detailed distribution.
   • Example Calculation:
     • Assume a junction serves an area with a demand density of 100 liters per hectare per second, and the junction serves 5 hectares.
     • The demand at the junction is: $$\text{Demand at junction} = 100 \times 5 = 500 \text{ liters per second}$$

10. Consumption Pattern Method

    • Overview: This method distributes demand based on observed consumption patterns, which may vary between residential, commercial, and industrial areas.
    • Application: Effective when detailed consumption data is available for different consumer categories.
    • Example: If residential areas consume 70% of the total demand and commercial areas consume 30%, the demand is distributed to the corresponding junctions accordingly.

Example: Demand Allocation for a Small Town

Let’s consider a hypothetical small town with a population of 5,000 people and a water distribution network with 10 junctions. The total water demand for the town is estimated to be 750,000 liters per day. Here’s how the demand can be allocated using different methods:

Step 1: Direct Assignment Method

If the actual water consumption at each junction is known (e.g., from metered data), demand can be directly assigned to each junction. For instance, if Junction 1 has 100 houses consuming 500 liters per house per day, the demand at Junction 1 is:

$$\text{Demand at Junction 1} = 100 \times 500 = 50,000 \text{ liters per day}$$

Step 2: Population-Based Method

Assume each junction serves approximately the same population. The demand at each junction can be calculated as:

$$\text{Demand per junction} = \frac{750,000 \text{ liters per day}}{10 \text{ junctions}} = 75,000 \text{ liters per day}$$

This assumes an even distribution of the population across the network.

Step 3: Land Use-Based Method

If the town is divided into residential, commercial, and industrial zones, and each zone has a different demand rate, the demand can be allocated accordingly. For example, if Junction 1 serves a residential area with a demand rate of 5,000 liters per hectare per day, and it covers 3 hectares, the demand at Junction 1 would be:

$$\text{Demand at Junction 1} = 3 \times 5,000 = 15,000 \text{ liters per day}$$

Conclusion

The method of demand allocation in a water distribution network depends on the available data, the size of the network, and the specific requirements of the system. While direct assignment provides the highest accuracy, other methods like linear, areal, and population-based approaches can be used when detailed data is not available. By carefully choosing the appropriate method, engineers can ensure that the water distribution system is designed to meet the demand efficiently.

To learn more about these methods and master the design of water distribution systems, you can enroll in our specialized courses at [this link]