Irrigation has been practised in India from ancient time. Even now, India has the largest area under irrigation. Tamilnadu state in India stands one among the few states in successfully harnessing and utilising the available water. Future development and management of available water resources can be achieved through optimisation techniques. Water management at farm level is one of the most important activities to be carried out for optimal use of water. Evapotranspiration is the most important variable next to rainfall in the context of irrigation to crops. In this paper, the results of the comparative study on crop water requirements estimation by two methods namely FAO-Penman method and Duty-delta method are reported. It is found that duty-delta method underestimates the crop water requirements.