Crop water requirements are crucial for food production, especially in regions experiencing population growth, water resources depletion and climate change effects. After an explanation about the connection between water and energy balances, the evapotranspiration (ET) concept and the crop coefficient (Kc) approach are introduced. The book then explores a literature review study, designing an extensive Kc database for initial, mid- and end-season growing phases. This step allowed finding out that FAO 56 equation for local climate adjustment gave inaccurate corrections for a crop having the same characteristics as the reference crop surface, and it is likely to be inaccurate also for other crop surfaces. Consequently, the final aim was to develop and test a new method to correct midseason Kc for local weather. The goal of this book is to help irrigation managers to: make progress in water use efficiency by decreasing the ratio of evaporation to transpiration; improve both irrigation scheduling and water resource planning; and doing so, increase the effectiveness of irrigation on crop yields, in response to actual and future climate change.