A Whole-Soil Modeling Approach for Field Capacity Assessment.

Field capacity (FC) has been defined as the amount of water held in the soil after the rate of downward movement of water has decreased to a negligible value. As such, FC is a property of a soil profile or of a part of it. The purpose of defining or determining FC for agricultural management is mostly related to irrigation. Another use of FC is in hydrologic modeling, where a well-established FC allows the use of tipping bucket models as a substitute for data-intensive Richards’ equation based models. Recent publications on the subject focused on the establishment of relations between FC and hydraulic properties determined in single soil layers or soil samples, without considering the vertical variability natural to a soil profile. The importance of the considered profile depth, especially for use in irrigation management, is also typically overlooked or simplified.

A numerical Richards’ equation modeling approach was used to analyze the importance of profile depth and layering on FC assessment. Results are compared to other proposed methods to assess FC. It is shown that the unit hydraulic gradient allowing computational and experimental simplifications in the analysis and design of internal drainage studies may be questionable especially when considering shallower soil depths. Furthermore, the unsaturated hydraulic conductivity at the bottom of the considered soil profile is shown to determine FC, in obvious dependence of the criterion to establish a negligible bottom flux, which is set by economic or environmental boundary conditions. The effect of layering and of the presence of compacted, less permeable layers is specifically addressed.