Spatially characterizing apparent electrical conductivity and water content of surface soils with time domain reflectometry

Abstract

Unlike other measurement methods discussed in this special issue, time domain reflectometry (TDR) has the ability to measure both water content ( θ) and apparent electrical conductivity (EC a) of soils. From simultaneous knowledge of θ and EC a, the soil solution electrical conductivity (EC s) and even the concentration of specific ionic constituents such as NO 3–nitrogen may be estimated through soil-specific calibration. This paper provides an introduction, some theoretical background, and a practical review of the TDR method for spatially characterizing θ, EC a, and related attributes in soils. Time domain reflectometry measurement principles for determining θ, EC a, and EC s, along with suggestions for inferring matric potential from dielectric measurements, are addressed. We discuss point, handheld, and vehicle-based measurement methods. Applications of TDR to spatially characterize θ in a hilly agricultural field using TDR and gravimetric methods, and to monitor θ and nitrate concentrations at three depths under peppermint production, are presented. A pickup-mounted TDR measured θ at 100 m × 100 m grid spacing in two wheat fields in north-central Montana. Soil θ, as well as NO 3–N, grain yield, and grain protein increased from upper to lower slopes. Soil θ early in the growing seasons appeared critical to final yields in this rainfed system. An array of fixed TDR probes was monitored over two growing seasons under peppermint in northwest Montana, to estimate θ, EC s, and NO 3–N every 6 h at 12 field locations. Although the field soils appeared uniform, measured spatial patterns of θ, EC s, and NO 3–N were highly space- and time-variant. These results indicate that TDR is a potentially useful tool for precision agriculture, and that fixed TDR arrays could serve as real-time monitoring systems for water and fertilizer salts in soil profiles. The primary limitation of the TDR method for spatially characterizing θ and EC at soil management scales using fixed arrays is the cable length limitation of about 20–30 m. Mobile platforms are of high interest, and prototype designs have been reported in the literature. Truly ‘on-the-fly’ TDR measurements for large-scale applications may be feasible in the near future.