Comparing field methods that estimate mobile-immobile model parameters

Recent studies have used field techniques that estimate soil hydraulic and solute transport parameters. These methods utilize a tension infiltrometer to infiltrate either a single tracer or a series of tracers in order to estimate immobile water content and mass exchange coefficient of the mobile-immobile solute transport model. The objective of this study was to compare two single tracer methods (basic and variance) with one multiple tracer method for estimating immobile water content and mass exchange coefficient from data obtained on the same field soil location. Hydraulic conductivity was also estimated using these methods. Research was done at five interrow sites in a ridge-tilled corn (Zea mays L.) field, and the soil was mapped as a Nicollet series (fine-loamy, mixed, superactive, mesic, Aquic Hapludoll). The values of immobile water content and mass exchange coefficient estimated by the multiple tracer method compared well with previously measured values using the same technique on the same field. The immobile water content values for the multiple tracer technique were larger than values derived from the basic single tracer technique. The basic single tracer technique did not take into consideration a mass exchange between immobile water content and the mobile water domain. The mass exchange coefficient values were less variable for the multiple tracer method than for the single tracer-variance method. Values of immobile water fraction for the multiple and basic single tracer techniques ranged from 0.30 to 0.52 and from 0.24 to 0.35, respectively. The values of the mass exchange coefficient for the multiple and single tracer-variance techniques ranged from 0.06 to 0.9 d(-1) and from 0.03 to 60 d(-1), respectively. The volumetric water content changed considerably over the course of the experiment for the estimation of the mass exchange coefficient using the single tracer-variance method; thus, the assumptions of this technique were compromised. The measured values of hydraulic conductivity at the five sites ranged from 0.47 to 1.66 micrometer s(-1). There was evidence that the basic single tracer method underestimated the immobile water content and overestimated the mobile water domain, because this method considers the mass exchange coefficient = 0 during the tracer application.