Reexamination of the Field Capacity Concept in a Brazilian Oxisol

"Field capacity" is the most frequently cited soil physical quantity and also the most ambiguous one. Its true assessment involves an internal drainage experiment in the absence of evaporation and transpiration, together with the establishment of a negligible drainage rate. For practical reasons however, estimating field capacity using an arbitrary value of pressure head is common practice. The objective of this study was to assess field capacity based on a fixed bottom flux or on a fixed time. This goal should be achieved through numerical simulations of internal drainage experiments using experimentally determined soil hydraulic properties on 46 locations on a 50-m transect in a layered soil. Flux-based estimates of field capacity exhibited high correlation to unsaturated hydraulic conductivity at the lower profile boundary, hydraulic gradients ranging from 0.25 to almost 1. Considering the 46 locations, bottom flux at a fixed time varied over one order of magnitude, whereas the time to reach a predetermined bottom flux associated to field capacity also showed a variation of about an order of magnitude. By setting hydraulic conductivity equal ( or slightly higher to compensate for a smaller hydraulic gradient) to a pre-established bottom flux, the pressure head or water content of field capacity was then assessed. Based on the flux criterion of 1 mm d(-1), field capacity corresponded to a pressure head of about -0.55 m in the evaluated soil; the corresponding time of drainage to establish this bottom flux was 4 d when considering only the top 0.15 m to almost 2 wk for a 0.75-m profile depth.