If the rainfall useful is for the growth of crops, then more the rainfall less will be the requirments of water for the growth of crops and more will be the duty of irrigation water.

As water flows from river to subsequent parts of the canal system some water losses takes place due to evaporation, and percolation. Due to this, the duty of water losses takes pace due to evaporation, and percolation. Due to this, the duty of water goes on increasing from starting point to canal system till its end point. Therefore, the duty of water is more at the last point of the system, i.e head of the water-course.