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DIVISION S-1-SOIL PHYSICS

Comparing Field Methods that Estimate Mobile–Immobile Model Parameters

^a Dep. of Agronomy, Iowa State Univ., Ames, IA 50011 USA
^b USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011 USA

fxcasey{at}iastate.edu

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 (_im) 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 _im and from data obtained on the same field soil location. Hydraulic conductivity (K(h₀)) 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 _im and estimated by the multiple tracer method compared well with previously measured values using the same technique on the same field. The _im 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 _im and the mobile water domain (_m). The values were less variable for the multiple tracer method than for the single tracer-variance method. Values of immobile water fraction (_im/) 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 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 using the single tracer-variance method; thus, the assumptions of this technique were compromised. The measured values of K(h₀) at the five sites ranged from 0.47 to 1.66 µm s^-1. There was evidence that the basic single tracer method underestimated _im and overestimated _m, because this method considers = 0 during the tracer application.

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