The movement of water into sub-surface drains is influenced by the following main factors.
1. The hydraulic conductivity of the soil.
2. The configuration and location of free water surface and pressure and magnitude of the artesian pressure or the back pressure of the drains.
3. Depth of drain below ground surface and location of drain with respect to various soil horizons.
4. The spacing between individual drains.
5. The diameter of drains.
6. The title joint spacing or diameter and spacing of holes in case of PVC pipes.
7. Depth to impervious layer below the ground surface.