Population forecasting for water supply:\r For a rapidly growing city, which projection technique is generally preferred to estimate future population for design horizons?

Introduction / Context:
Urban water systems are designed for multi decade horizons. Selecting an appropriate population forecast method is crucial for credible demand projections. Different empirical methods suit different growth patterns and historical trends.

Given Data / Assumptions:

• City growth is described as rapidly increasing.
• Historical data presumably show compounding growth rather than linear increments.
• Objective: choose a method matching exponential growth tendencies.

Concept / Approach:
The geometrical method assumes a constant percentage growth rate, leading to exponential population increase over time. It is more suitable than arithmetic methods when recent decades exhibit strong compounding growth. Incremental increase blends linear growth with changing increments and tends to be moderate, while graphical comparison benchmarks against similar cities but does not ensure conservative sizing for fast growth.

Step-by-Step Solution:
Define methods: arithmetic equals constant absolute increase; geometric equals constant percentage increase.Match method to trend: rapid growth indicates percentage compounding dominates.Select geometrical method as the preferred approach for planning.

Verification / Alternative check:
Where strong policy or land availability constraints exist, planners may temper geometric projections with scenario analysis, but the baseline choice for rapid growth remains the geometric method.

Why Other Options Are Wrong:

• Arithmetical mean underestimates compounding growth.
• Incremental increase provides intermediate results and may still understate aggressive growth.
• Graphical comparison is useful for context but not a primary projection for fast growth without corroborating data.

Common Pitfalls:

• Over projecting by using a short term high percentage without economic validation.
• Under projecting by defaulting to arithmetic growth in a boom corridor.

Final Answer:
Geometrical method