Electrical resistivity of the soil can be considered as a proxy for the spatial and temporal variability of many other soil physical properties (i.e. structure, water content, or fluid composition). Because the method is non-destructive and very sensitive, it offers a very attractive tool for describing the subsurface properties without digging. It has been already applied in various contexts like: groundwater exploration, landfill and solute transfer delineation, agronomical management by identifying areas of excessive compaction or soil horizon thickness and bedrock depth, and at least assessing the soil hydrological properties. The surveys, depending on the areas heterogeneities can be performed in one-, two- or three-dimensions and also at different scales resolution from the centimetric scale to the regional scale. In this review, based on many electrical resistivity surveys, we expose the theory and the basic principles of the method, we overview the variation of electrical resistivity as a function of soil properties, we listed the main electrical device to performed one-, two- or three-dimensional surveys, and explain the basic principles of the data interpretation. At least, we discuss the main advantages and limits of the method.

/pdf/electrical-resistivity-survey-in-soil-science-a-review-52naoumd0b.pdf

Electrical resistivity survey in soil science: a review .

At the watershed scale, soil moisture is the major control for rainfall–runoff response, especially where saturation excess runoff processes dominate. From the ecological point of view, the pools of soil moisture are fundamental ecosystem resources providing the transpirable water for plants. In drylands particularly, soil moisture is one of the major controls on the structure, function, and diversity in ecosystems. In terms of the global hydrological cycle, the overall quantity of soil moisture is small, ∼0.05%; however, its importance to the global energy balance and the distribution of precipitation far outweighs its physical amount. In soils it governs microbial activity that affects important biogeochemical processes such as nitrification and CO2 production via respiration. During the past 20 years, technology has advanced considerably, with the development of different electrical sensors for determining soil moisture at a point. However, modeling of watersheds requires areal averages. As a result, point measurements and modeling grid cell data requirements are generally incommensurate. We review advances in sensor technology, particularly emerging geophysical methods and distributed sensors, aimed at bridging this gap. We consider some of the data analysis methods for upscaling from a point to give an areal average. Finally, we conclude by offering a vision for future research, listing many of the current scientific and technical challenges.

/pdf/soil-moisture-measurement-for-ecological-and-hydrological-1jak2uhu52.pdf

Soil Moisture Measurement for Ecological and Hydrological Watershed-Scale Observatories: A Review

Recent research has demonstrated that field soil salinity can be inferred from four-electrode soil electrical conductivity (ECa), if the soil profile is near "field capacity" and calibration curves, based on saturation extract salinity (ECe), are available. To extend the use of this field method to arbitrary water contents, we studied EC in the laboratory as a function of water content and in situ soil water conductivity (ECw). We collected undisturbed cores of four soil types (fsl, vfsl, 1, cl) using Lucite column inserts, which were tapped for later insertion of electrodes. The cells were equilibrated with waters of a desired EC,r and, using a pressure membrane apparatus, adjusted to a desired Values of ECa were calculated for each ECW equilibration from measured four-electrode resistances and an appropriate cell constant. Our results indicated that over the -range of practical concern, ECa = (transmission coefficient) x x ECw + surface conductivity. This relationship is derived using a simple capillary model, which assumes that liquid phase and surface conductivities (via exchangeable cations) behave as resistors in parallel. Additional Index Words: soil electrical conductivity, soil salinity, surface conductance. View complete article To view this complete article, insert Disc 3 then click button8

Effects of Liquid-phase Electrical Conductivity, Water Content, and Surface Conductivity on Bulk Soil Electrical Conductivity

A conceptual model is proposed for the prediction of the electrical conductivity of bulk soil in saturated and in unsaturated states. The model is based on the hypothesis that the tortuosity factor affecting the bulk soil electrical conductivity is identical to that defined for prediction of the soil hydraulic conductivity. Simple mathematical functions are derived to forecast the soil electrical conductivity as a function of the water content. Experimental results of seven previous studies carried out on 26 soils were used for validation tests. The model's efficiency as a predictive tool, in cases where limited experimental data are available, is found to be generally good.

Theoretical Prediction of Electrical Conductivity in Saturated and Unsaturated Soil

Summary Traditional methods for studying tree roots are destructive and labor intensive, but available nondestructive techniques are applicable only to small scale studies or are strongly limited by soil conditions and root size. Soil electrical resistivity measured by geoelectrical methods has the potential to detect belowground plant structures, but quantitative relationships of these measurements with root traits have not been assessed. We tested the ability of two-dimensional (2-D) DC resistivity tomography to detect the spatial variability of roots and to quantify their biomass in a tree stand. A high-resolution resistivity tomogram was generated along a 11.75 m transect under an Alnus glutinosa (L.) Gaertn. stand based on an alpha-Wenner configuration with 48 electrodes spaced 0.25 m apart. Data were processed by a 2-D finite-element inversion algorithm, and corrected for soil temperature. Data acquisition, inversion and imaging were completed in the field within 60 min. Root dry mass per unit soil volume (root mass density, RMD) was measured destructively on soil samples collected to a depth of 1.05 m. Soil sand, silt, clay and organic matter contents, electrical conductivity, water content and pH were measured on a subset of samples. The spatial pattern of soil resistivity closely matched the spatial distribution of RMD. Multiple linear regression showed that only RMD and soil water content were related to soil resistivity along the transect. Regression analysis of RMD against soil resistivity revealed a highly significant logistic relationship (n = 97), which was confirmed on a separate dataset (n = 67), showing that soil resistivity was quantitatively related to belowground tree root biomass. This relationship provides a basis for developing quick nondestructive methods for detecting root distribution and quantifying root biomass, as well as for optimizing sampling strategies for studying root-driven phenomena.

/pdf/in-situ-detection-of-tree-root-distribution-and-biomass-by-31wxs6lp8h.pdf

In situ detection of tree root distribution and biomass by multi-electrode resistivity imaging

MEASUREMENT OF DYNAMIC SALINITY AND WATER CONTENT PROFILES UNDER LABORATORY CONDITIONS WERE NEEDED TO TEST A MODEL DEVELOPED TO CHARACTERIZE THE SIMULTANEOUS FLOW OF WATER AND SALTS IN SOILS. METHODS WERE NEEDED FOR MEASURING SALINITY IN PLACE IN A SYSTEM WHERE RAPID WATER MOVEMENT OCCURRED. BECAUSE OF THE TIME LAG IN SALINITY SENSORS, OTHER METHODS WERE CONSIDERED FOR EVALUATING SALINITY. THE FOUR-PROBE SYSTEM OF MEASURING ELECTRICAL CONDUCTIVITY OF SOIL ELIMINATES THE TIME LAG ERROR, BUT INTRODUCES THE PROBLEM OF CORRECTING THE CHANGES IN CONDUCTIVITY CAUSED BY WATER CONTENT. UNITS WERE CONSTRUCTED AND SAMPLES PREPARED TO EVALUATE THE INFLUENCE OF WATER CONTENT, AND SALT CONCENTRATIONS OF THE FOUR-PROBE CONDUCTIVITY. TWO SOILS WERE STUDIED. REGRESSION EQUATIONS DEVELOPED TO ESTIMATE SATURATION OR 1:5 EXTRACT ELECTRICAL CONDUCTIVITY FROM FOUR- PROBE CONDUCTIVITY AND WATER CONTENT GAVE CORRELATION COEFFICIENTS OF 0.75 AND 0.90, RESPECTIVELY. CORRELATIONS FOR INDIVIDUAL SOILS WERE GENERALLY HIGHER THAN THE COMBINED SOILS.

S. C. Gupta

Papers

Influence of water content on electrical conductivity of the soil