FAO crop coefficient and plant‐available water: Do soil and plant hydraulic properties matter?

How to ensure that “the availability of soil water does not limit plant growth or transpiration?” Determining the so‐called crop coefficient ( K c ) relies on this assumption of optimal or non‐limiting soil water conditions, which, in turn, is understood as the soil water content at field capacity, a parameter with, de facto, no relation to plants or transpiration. Many studies on crop evapotranspiration ignore the dynamic nature of plant available water (PAW) driven by the soil hydraulic properties (water retention and hydraulic conductivity) and the characteristics of the plant root system. This raises important questions: How can we guarantee that a K c value is transferable if soil and plant hydraulics are not explicitly considered? And how can we reconcile the practical determination of crop evapotranspiration with the theoretical one using complex water transfer models? While addressing these complexities remains a challenge, recent advances in process‐based modeling offer new opportunities to represent soil–plant–atmosphere interactions more accurately, and to refine the criteria underlying K c and PAW. Integrating such physically based understanding with established approaches may help bridge the gap between theoretical knowledge and practical applications, ultimately supporting more reliable and adaptable methods for estimating crop evapotranspiration.