Field determination of soil hydraulic and chemical transport properties 1

Hydraulic and chemical transport properties are the major inputs in predictive models that simulate the movement of water and chemicals through the vadose zone. However, there is a lack of field measurements of such properties to verify models describing water and chemical movement through the soil. One of the objectives of this study was to use a point source method to determine simultaneously the hydraulic and chemical transport properties at multiple field locations. A second objective was to determine the spatial distribution of such properties across a field. A total of 50 field locations within a 7 × 15-m area were rapidly and simultaneously evaluated for such properties. The hydraulic properties were the saturated hydraulic conductivity (Ks) and the macroscopic capillary length (λc). The chemical transport properties were the immobile water content, expressed as a fraction of water content (θim/θ) and the mass exchange coefficient (α). The hydraulic properties were determined by applying three discharge rates from irrigation dripper lines and measuring the resultant steady-state flux densities at the soil surface beneath each emitter. The chemical transport properties were determined by applying a sequence of three conservative tracers at a steady-state infiltration rate and measuring their resident concentration in the soil. The Ks values ranged from 7.5 to 79.0 cm h, with a median of 27.4 cm h (± 16.8). The λc values ranged from 0.03 to 13.1 cm, with a median of 2.6 cm (± 3.6). The θim/θ values ranged from 0.36 to 0.88, with a median of 0.57 (± 0.098). The α values ranged from 0.002 to 0.12 h, with a median of 0.034 h (± 0.027). The values of the hydraulic and chemical transport parameters were found to be comparable with values reported by studies conducted on nearby field locations on similar soil. Based on semivariogram analysis, the measured properties were not spatially correlated. Because the method required only 2 days to collect data it should prove useful for future studies that require extensive field measurements of hydraulic and chemical transport properties.