Base and upper temperature thresholds to support the calculation of growing degree days aiming at their use with the FAO56rev crop coefficients curve: A review

Abstract

Growing Degree Days (GDD) are a fundamental tool for supporting crop and water management, estimating crop development and growth and, in particular, defining the crop coefficient curve, thus computing crop evapotranspiration and deciding on irrigation scheduling, a key issue in precision irrigated agriculture. GDD can provide a timely estimation of key crop growth stages such as germination, flowering, fruit setting, and maturity, which are relevant for decision making. This review article examines the principles and methods of calculation, focusing on the base and upper temperature thresholds (Tbase and Tupper), which are the basis for calculating GDD and then implementing it in irrigation scheduling. The review discusses the concepts and the procedures for determining Tbase and Tupper and presents the tabulation of their values for 117 crops. These include vegetable crops, herbs and spices, grasses, grain legumes, fiber, sugar and oil crops, cereals and pseudocereals, fruit trees and vines. The GDD estimation using the results of this review is dealt with in a companion paper (Paredes et al., (2025)), which focuses on supporting their applicability to compute the crop coefficient curves as now used in the new FAO56rev crop water requirements guidelines. The resulting computation of crop evapotranspiration, namely in real time, is expected to lead to improved performance of irrigated agriculture, efficient water use, and water savings.