Water is usually supplied as per requirement of different growing crops in the field. The water requirement of crops is not uniform over a large area and varies widely in accordance with variation in climate, particularly in rainfall and soil. India has vast differences in rainfall pattern from nearly 250mm in Western Rajasthan to over 5000mm in Northeast India and so also divergent types of soil. It is clear that water requirement of the same crop is different from one place of the country to the other. The optimum water requirement of a crop during its growth period results in maximum yield. The optimum quantity of water includes water supply by precipitation as well as water delivered by irrigation. Correct estimate of optimum requirement of water for a crop is essential for getting the maximum yield for economic use of water. The supply of water more than optimum requirement may not result in increase in yield of most crops, but also decreases the quality of product. It is established that excessive irrigation is as bad as without irrigation. The irrigation from the headwork is supplied to the cultivable field through the main canals, brunch canals, sub brunch canals and distributaries. From the main canal, water is generally not directly supplied to the command area. The outlets are fitted in branches, sub-branches and distributaries for supplying water to the crop field. The total discharge required at the head of the main canal is determined considering duty and delta.