FIELD DETERMINATION OF SOIL HYDRAULIC AND CHEMICAL TRANSPORT PROPERTIES 1

Abstract

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 (K s ) and the macroscopic capillary length (λ c ). The chemical transport properties were the immobile water content, expressed as a fraction of water content (θ im /θ) and the mass exchange coefficient (a). 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 K s values ranged from 7.5 to 79.0 cm h -1 , with a median of 27.4 cm h -1 (± 16.8). The λ c values ranged from 0.03 to 13.1 cm, with a median of 2.6 cm (± 3.6). The θ im /θ values ranged from 0.36 to 0.88, with a median of 0.57 (± 0.098). The a values ranged from 0.002 to 0.12 h -1 , with a median of0.034 h -1 (± 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 semi-variogram 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.