TDR probe calibration methods for bulk electrical conductivity in two soil types

Abstract

Time Domain Reflectometry (TDR) stands out among the techniques used for determining volumetric soil water content (θ) and monitoring bulk electrical conductivity (ECb). However, the accuracy of TDR in measuring solute concentrations strongly depends on the adequacy of the calibration procedure employed. Therefore, this study adjusted models to determine the bulk electrical conductivity (ECb) of the soil solution as a function of the TDR-measured bulk electrical conductivity (ECb). The study was carried out at Embrapa Cassava and Tropical Fruits, located in Cruz das Almas, Bahia, Brazil, using a PCTDR version 4.0® (Campbell Scientific) and two calibration methodologies. The Giese–Tiemann constant was determined through readings of different levels of electrical conductivity using potassium nitrate as the solute in: (i) soil samples with sandy loam and loam–sandy clay textures saturated with potassium nitrate solution, and (ii) pure and commercial potassium nitrate solutions. ECb measurements were obtained either using the TDR probe directly connected to the device or coupled via electromechanical multiplexers. To verify the accuracy and validate the constants obtained from linear regression, the following statistical indices were applied: R² (coefficient of determination), RMSE (root mean square error), d (Willmott’s index of agreement), and GSD (general standard deviation). The technique used to measure electrical conductivity did not directly affect the determination of the Giese–Tiemann constant. However, soil type, solute solution, and calibration methodology influenced the determination of this constant.

Keywords: Geise and Tieman constant, salinity monitoring, time domain reflectometry.