Soil cores of 5 cm diameter were taken from a subset of sample sites for laboratory analysis.  Tests for hydraulic conductivity KH, thermal conductivity KT, porosity f, and bulk density (r b) were conducted on each core.  For a further subset of sites, we measured moisture-dependent KT, KH and soil water tension, i.e., we developed soil moisture retention curves.  Using these observations, we analyzed for the parameters α, n, residual water content qr, and saturated water content qs (which equals f) of the van Genuchten model [van Genuchten, 1980].

KH was measured using a constant-head test implemented with a KSAT Benchtop Hydraulic Conductivity instrument (UMS Corp., Berlin, DE).  KT was measured at both full water saturation and total dryness using a KD2 Thermal Analyzer (METER Group, Pullman, WA).  Dryness was achieved during the measurement of f: the cores, which were a known volume, were weighed after saturation, placed in an oven at 105°C to dry for 24 hours, and then re-weighed; this measurement also provided rb.

Soil water retention properties were determined through the evaporation method at relatively low soil suction pressure, and the dew point technique at relatively high soil suction pressure.  We performed the evaporation method [Schindler et al., 2010] using the HYPROP instrument (UMS Corp., Berlin, DE) that generates a soil water retention curve by simultaneously measuring the pressure gradient between two tensiometers at different heights in a soil core and the soil water weight.  The evaporation method does not work at relatively high soil suctions.  Thus, to complete the soil water retention curve at high suctions, we used a WP4C Dew Point Potentiometer (METER Group, Pullman, WA) that measured the humidity of the air in a small chamber in equilibrium with a sample of dried soil, as described in Leong et al. [2003].  Three WP4C points were included for each sample, providing the information necessary to complete the soil water retention curve up to high suction pressure.  The van Genuchten equation [van Genuchten, 1980] was fitted to the experimental curves.

References:

1.  Leong, E. C., S. Tripathy, and H. Rahardjo (2003), Total suction measurement of unsaturated soils with a device using the chilled-mirror dew-point technique, Geotechnique, 53(2), 173-182.

2.  Schindler, U., W. Durner, G. von Unold, L. Mueller, and R. Wieland (2010), The evaporation method: Extending the measurement range of soil hydraulic properties using the air-entry pressure of the ceramic cup, Journal of Plant Nutrition and Soil Science, 173(4), 563-572.

3.  van Genuchten, M. T. (1980), A closed-form equation for predicting the hydraulic conductivity of unsaturated soils, Soil Science Society of America Journal, 44(5), 892-898.