Once water has been used, or has served for irrigation purposes, it is necessary to evacuate the used or excess water.

Once more, we need millions of kilometres of sewers, pipes, or drainage canals, firstly to assemble the water, then to carry it to treatment plants. After treatment, if necessary, different pipe systems transport the water towards the natural outlets capable of dealing with the corresponding flow of water.

Certain floodwater evacuation systems can be linked to these works.