Urban population forecasting – key demographic components When predicting the future population of a city for water-supply design, which demographic factors must be considered together?

Introduction / Context:
Population forecasts drive source sizing, treatment-plant capacity, and distribution design. Errors in demographic assumptions can lead to under- or over-sizing with financial and service-level consequences.

Given Data / Assumptions:

• Standard municipal planning framework.
• Projection horizon typically 20–30 years.
• Multiple methods (arithmetical, geometrical, incremental, logistic) rely on demographic drivers.

Concept / Approach:
Net population change = natural increase (births − deaths) + net migration (inflow − outflow). Any robust prediction explicitly or implicitly accounts for all three. Even methods that extrapolate historical totals inherently embody these drivers.

Step-by-Step Solution:
Identify components of change → births, deaths, migrants.Recognize forecasting methods incorporate these directly (cohort) or indirectly (trend-based).Hence, the comprehensive correct choice is “All the above.”

Verification / Alternative check:
Planning manuals admonish designers to validate projections against known housing, industry, and migration trends, not merely rely on one historical curve.

Why Other Options Are Wrong:

• Births or deaths alone omit the critical migration term; migrants alone ignores natural increase.

Common Pitfalls:
Transposing national growth rates to a local city without accounting for migration shocks from new industries or transport links.

Final Answer:
All the above