🎓 Lesson 8 D4

Time-Varying Demand Patterns: Diurnal, Weekly, Seasonal

Time-varying demand patterns are changes in how much water people use throughout the day, week, or year — like higher use in the morning or during summer.

🎯 Learning Objectives

✓ Analyze diurnal demand curves to identify peak, minimum, and coincidence factors
✓ Calculate hourly demand multipliers from measured consumption data
✓ Apply seasonal adjustment factors to forecast annual demand profiles
✓ Explain how weekly patterns influence reservoir sizing and pump operation strategies
✓ Design a time-series demand input for hydraulic simulation software (e.g., EPANET)

📖 Why This Matters

Imagine designing a water pipe that only meets average daily demand — it would fail every morning when thousands of residents shower, flush, and brew coffee simultaneously. Real-world water systems don’t experience steady flow; demand pulses hourly, dips on Sundays, and surges in summer. Ignoring these patterns leads to undersized pumps, pressure failures, wasted energy, and non-compliance with regulatory reliability standards (e.g., AWWA D100). Mastering time-varying demand isn’t just theory — it’s the difference between a resilient system and one that breaks at 7:15 a.m.

📘 Core Principles

Demand variability arises from three nested temporal scales: (1) Diurnal — driven by human routines (e.g., morning peak ~6–9 a.m., evening peak ~5–8 p.m.), typically modeled as hourly multipliers relative to average daily demand (ADD); (2) Weekly — reflects workweek vs. weekend behavior, where weekend demand may be 10–25% lower than weekdays due to reduced commercial/industrial use; (3) Seasonal — influenced by temperature, irrigation, tourism, and school calendars, causing summer demand to exceed winter by 30–100% in arid or tourist-heavy regions. These patterns are not independent: a hot Saturday in July combines all three effects multiplicatively. Engineers must deconstruct observed metered data into base demand + time-varying components using normalization and statistical decomposition (e.g., moving averages, Fourier analysis for cyclical trends).

📐 Hourly Demand Multiplier

The hourly demand multiplier (HDM) scales average daily demand to estimate flow in a given hour. It is derived from empirical data and applied in hydraulic models to simulate realistic transient loading.

Hourly Demand Multiplier (HDM)

HDM_h = Q_h / (ADD / 24)

Scales average daily demand to estimate flow in hour h.

Variables:

Symbol	Name	Unit	Description
HDM_h	Hourly demand multiplier for hour h	dimensionless	Factor applied to ADD to obtain hourly demand
Q_h	Measured hourly demand in hour h	MG/h or L/s	Actual volume consumed in that hour
ADD	Average daily demand	MG/day or L/s	Total monthly or annual demand divided by number of days

Typical Ranges:

Residential-only system (U.S.): 0.4 – 2.2

Mixed-use system with irrigation: 0.3 – 3.5

💡 Worked Example

Problem: A municipal utility records total monthly water use of 12.4 million gallons (MG) in June. Its service area has 8,200 residential connections. Metered data shows 6 a.m. consumption was 0.85 MG on a typical weekday. Calculate the HDM for 6 a.m. assuming uniform per-capita ADD and 30-day month.

1. Step 1: Compute average daily demand (ADD) = 12.4 MG ÷ 30 days = 0.4133 MG/day

2. Step 2: Compute hourly demand at 6 a.m. = 0.85 MG ÷ 30 days = 0.0283 MG/hour

3. Step 3: Compute HDM = (0.0283 MG/hour) ÷ (0.4133 MG/day ÷ 24 h/day) = 0.0283 ÷ 0.01722 ≈ 1.64

Answer: The 6 a.m. HDM is 1.64, meaning demand at that hour is 64% above the uniform hourly average — consistent with typical morning peaks (range: 1.4–1.8).

🏗️ Real-World Application

In Tucson, AZ, the City Water Department uses 15-minute SCADA data from 42 pressure zones to calibrate EPANET models. During summer (June–August), they apply a composite multiplier: diurnal (e.g., 2.1 at 7 a.m.) × weekly (1.05 for weekdays vs. 0.88 weekends) × seasonal (1.75 vs. winter baseline). This enabled them to right-size a $12M variable-speed pump station — reducing energy use by 22% while maintaining >55 psi residual pressure during peak irrigation hours, per AWWA M17 requirements.