TR-29 : Estimation of soil moisture variation using resistivity technique

Abstract

Relation between bulk resistivity and amount of pore water as well as its properties has been known in form of empirical formulae. The electrical properties of soils in unsaturated zone above water table are dependent on a number of parameters. Important among these are porosity, degree of saturation, grain size and shape, and conductivity of water/ electrolyte saturating the soil. The reported study aims at using the surface resistivity measurements in determination of soil moisture status and its temporal variation.

Introductory section includes general description of soil properties, especially electrical properties. Some recent developments in non-conventional methods of soil-moisture measurements, e.g. time domain reflectometry, optical transmission, geophysical methods etc., have been reviewed in order to familiarize readers with the latest state-of-art techniques available in the subject field.

Various factors affecting resistivity of soils have been briefly described. It is observed that soil water content and its salinity has largest effect on soil resistivity. Effects of porosity and pore water conductivity along with variation of soil moisture content on soil resistivity have been computed. Resistivities were computed at various porosities (30-50%), pore water conductivities (16-26 ohm-m) ,and soil water content (10-60%). Relative changes in bulk resistivity and computed soil moisture show a distinct pattern of relation between soil resistivity and water content.

The study shows that higher soil moisture content (50% and above) has marginal effect on soil resistivity, especially for medium porosity ranges (30-40%). Effect is appreciably large for low soil moisture contents (5-50%) and for higher porosities (40% and above). It is, therefore, possible to use field resistivity measurements to monitor soil water variation for irrigation scheduling etc