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Soil Physics

A Laboratory Method for Determining the Unsaturated Hydraulic Properties of Soil Peds

^a Desert Research Institute, Univ. and Community College System of Nevada, Las Vegas, NV 89119
^b Hydrologic Sciences Program, Univ. of Nevada, Reno, NV 89532
^c Desert Research Institute, Univ. and Community College System of Nevada, Reno, NV 89512

^* Corresponding author (darren{at}dri.edu)

The ability to estimate soil hydraulic properties of undisturbed soil peds is limited. Most laboratory methods repack samples into soil cores, thus destroying the structure. Most field methods average over multiple peds precluding measurements of individual peds. This article details a method for determining unsaturated soil hydraulic properties of individual highly structured soil peds. The procedure is based on the evaporation method. The peds were first coated in paraffin wax, with the top surface left open to the atmosphere, and then encased in expandable foam to provide extra support. After saturating, both mass (from digital balance) and soil water potential (from tensiometer) data were recorded every 5 min as the ped dried until the soil water potential reached approximately –70 kPa. The van Genuchten parameters (, n) and saturated hydraulic conductivity (K_s) were estimated with inversion modeling, using the soil water potential data and final water content. Residual and saturated water contents, _r and _s, respectively, were measured independently. A mini-permeameter provided independent estimates of K_s. We compared the areally weighted average of our optimized hydraulic properties with results from a field infiltrometer test conducted at the exact location from where the peds were collected. The two methods compared well, though some disparity existed between the estimates of n and K_s, likely indicating the effects of interped cracks sampled with the tension infiltrometer. The method is potentially valuable for geostatistical and scaling studies because the smallest structural unit, the soil ped, can be sampled and analyzed.

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