Estimating the lengths of crop growth stages to define the crop coefficient curves using growing degree days (GDD): Application of the revised FAO56 guidelines

Growing Degree Days (GDD) are a fundamental tool for estimating crop growth stage lengths. Their importance in irrigated agriculture is major since the new FAO56rev guidelines have adopted GDD to estimate the length of the four crop growth stages of the crop coefficient curves, thus replacing the fixed time lengths. GDD are useful for supporting crop and water management by quantifying temperature accumulation over time. They can be used to estimate the dates of crop stages required for cropping operations and irrigation scheduling, as well as predict such dates in real time when appropriate tools are used. This study focuses on the defining the base and upper temperatures (Tbase and Tupper) used to estimate the start and end of the crop growth stages. These values are crop specific and are herein tabulated for several vegetable, field, grass and woody fruit crops. Indicative cumulative GDD values, obtained from a literature review and from own computations, are provided for each crop growth stage of the FAO segmented crop coefficient curve, i.e. the initial, development, mid-season, and late season stages. Examples of application demonstrate their usage graphically and show how cumulative GDD results in different durations in different locations.