An analytic description of field capacity and its application in crop production

Abstract

It has been established for decades that soil water field capacity (FC) includes four main components: drainage rate or flux (q), drainage time from saturation (t), soil water content in the drained zone (θ), and drainage depth (Z). Despite this, rather little work has been done to incorporate these components into analytic FC expressions, or to determine how such expressions are best used in applications of the FC concept. The primary objectives of this study were: i) develop analytic FC expressions that account for the four components of FC; ii) determine the accuracy of the FC expressions using numerical simulations of soil profile drainage from saturation; and iii) illustrate how analytic FC expressions might be used to optimize air, water and nutrient dynamics in field crop production.Using a range of representative soil types, the analytic FC expressions predicted q and θ within 36% of numerically determined values for t ≥ 0.01 days and Z = 0.3 m or 1.0 m, which is considered sufficiently accurate for most FC applications. The FC expressions showed that all four FC components are important, and that most FC applications have both agronomic and environmental implications. Static FC definitions based on specified matric head or retention curve shape were frequently inaccurate. It was concluded that the FC concept is most appropriately applied in crop production studies by using integrative FC expressions to optimize air and water storage, drainage flux, drainage time, and potential nutrient leaching rate in the crop root zone.