In the case of gravity-fed systems, only gravity is used to transport water (or other fluid) from a source to a final application. Manning's Formula for Pipe Flow Calculations | PDF - Scribd
Q = (1/n) * A * R^2/3 * S^1/2
Below is a breakdown of the essential features, the underlying physics, and a conceptual tool you can use. gravity pipe flow calculator
ΔP = f * L * ρ * v^2 / (2 * D)