Derivation of the Field Drainage Depletion Curve from Volumetric Water Content Data for Quantitative Evaluation of Field Drainage Properties

Abstract

This study examines methods for deriving a "field drainage depletion curve" from volumetric water content (θ) data to evaluate field drainage properties in converted paddy fields quantitatively. Time-series θ data collected 2-3 cm above the plow pan were analyzed to construct the depletion curve, effectively capturing key drainage characteristics. The curve, approximated by two fitted lines, revealed that the initial slope reflects evapotranspiration and gravitational drainage, while the latter represents water loss due solely to evapotranspiration. The inflection point of the curve corresponded to field capacity. Additionally, the influence of drainage measures on the curve was investigated: topsoil-focused techniques increased the difference between θ at the curve's initial intercept and the inflection point, whereas subsoil-focused methods enhanced the initial slope, thereby accelerating soil water depletion. These findings highlight the importance of soil moisture sensor installation depth and drainage measures in evaluating field drainage dynamics.