S. R. Green, I. Vogeler and B. E. Clothier - Environmental group, HortResearch, Private bag 11-030 Palmerston North, New Zealand 

Zealand

When time domain reflectometry (TDR) is used to measure soil water content (θ) and salinity, the probes can be installed either horizontally or vertically. We tested the common convention that θ values averaged from horizontal probes will be equal to a vertical direct measurement. In a laboratory experiment, a sandy loam soil, packed uniformly (0.02‐m layers to 0.18‐m depth) into a box, was gradually wetted to saturation by CaCl₂ solutions of 0, 0.5, 1.1, 2.1, 3.1, 4.5, 5.9, and 8.4 dS m⁻¹ Horizontal and vertical TDR probes for measuring θ and electrical conductivity of the bulk soil (σ_a) were installed during soil packing. A comparison between θ values averaged from three horizontal probes and from two vertical ones showed deviations of 0.02 (L L⁻¹). A simple water redistribution model was used to attribute this deviation to the process of averaging the horizontal results. The best practical attainable reproducibility, under our experimental conditions, were close to the theoretical limit 0.005 (L L⁻¹) but the average experimental reproducibility of θ was 0.01 to 0.02 (L L⁻¹). Width of the soil layer affecting the moisture measurement was reconfirmed to be close to 30 mm. The resistors‐in‐series model was found to be a good approximation to describe the soil profile σ_a from separately measured horizontal σ_a Final values of the electrical conductivity of the soil solution (σ_w) after sufficient leaching were in good agreement with σ_w values calculated by an empirical protocol that uses σ_a, θ, and a soil texture property.