The water requirement for the Chulsar Rice Crop is becoming a more and more common practice in Cambodia research, as it secures productivity and ensures constant the amount of water use. Effective water planning and management require accurate determination of crop water requirement so it plays an important role for irrigation practice. Consumptive water requirement is the amount of water potentially required meeting the evapotranspiration needs of the paddy rice so that the rice does not suffer in its growth through short supply of water and it is the depth of irrigation water, exclusive of precipitation, stored moisture, or groundwater, which is required to meet evapotranspiration during the crop period. We have been preformed a sensitivity analysis by using and comparing the different technological methods where we employ Lysimeter method (direct method), Bowen Ratio Instrument is using two different equations: Bowen Ratio Energy Balance (BREB) & ASCE Short, and CROPWAT model which is indirect method. By the way, the observation was undertaken in a small experimental paddy field located in Cambodia Agriculture Research and Development Institute (CARDI), which have an area of 2650 meter square, particularly with Chulsar for 100 days and we found out that the amount of crop water requirement of BREB is 571 millimetres. Totally, compared to the other method we are apparently coincided get 15 percentages different, for overall, the result can be used to assist the user in order to obtain the products qualifiedly and much more than that previously in deflicit irrigation.