Climate change 2014 - Synthesis Report

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.

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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.

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

The Prediction in Ungauged Basins (PUB) initiative of the International Association of Hydrological Sciences (IAHS), launched in 2003 and concluded by the PUB Symposium 2012 held in Delft (23–25 October 2012), set out to shift the scientific culture of hydrology towards improved scientific understanding of hydrological processes, as well as associated uncertainties and the development of models with increasing realism and predictive power. This paper reviews the work that has been done under the six science themes of the PUB Decade and outlines the challenges ahead for the hydrological sciences community.Editor D. KoutsoyiannisCitation Hrachowitz, M., Savenije, H.H.G., Bloschl, G., McDonnell, J.J., Sivapalan, M., Pomeroy, J.W., Arheimer, B., Blume, T., Clark, M.P., Ehret, U., Fenicia, F., Freer, J.E., Gelfan, A., Gupta, H.V., Hughes, D.A., Hut, R.W., Montanari, A., Pande, S., Tetzlaff, D., Troch, P.A., Uhlenbrook, S., Wagener, T., Winsemius, H.C., Woods, R.A., Zehe, E., and Cudennec, C., 2013. A d...

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A decade of Predictions in Ungauged Basins (PUB)—a review

Recent developments in the direct-current geoelectrical imaging method

Electrical resistivity measurements at high resolution (1.5-cm electrode spacing) were performed to detect, from the soil surface, small cracks developing within the soil. We recorded a vertical electrical pseudo-section in a dedmetric undisturbed homogenous soil block (silt loam) for different artificial cracking stages. Because of the unusually reduced electrode spacing associated with an air-dried soil surface, a specific Cu/CuSO 4 electrode was designed for precision wet contact at given points. The apparent resistivity measurements of the pseudo-section and the interpreted data inverted by using the Res2Dinv software are discussed. The range of interpreted electrical resistivity associated with cracking is considerable, (from 168 to 2185 Ω m) because the cracks are filled with air that is an infinitely resistant medium. Results showed that even small structures cause perceptible changes in resistivity that can be detected by the electrical resistivity method. Results also showed that specific software is required to predict real crack depth.

Electrical Resistivity Imaging for Detecting Soil Cracking at the Centimetric Scale

Our research was aimed at analysing the possibilities of using a geophysical method, the electrical resistivity method, to describe the structure of a cultivated loamy soil. Soil electrical resistivity was measured in laboratory conditions on two soil blocks (0.30 m x 0.30 m x 0.20 m) with 2D Wenner configuration using an inter-electrode spacing of 0.015 m. The two soil blocs exhibited different structure: one with a compacted structure (bulk density equal to 1.59 Mg m -3 with a standard deviation of 0.05 Mg m -3 ) and the second with a porous structure (bulk density equal to 1.39 Mg m -3 with a standard deviation of 0.04 Mg m -3 ). The electrical resistivity results showed a significant 10 Ω m difference between the compacted block (30 Ω m) and the porous block (40 Ω m) due to the difference in their bulk density. This structural distinction by electrical resistivity needs temperature correction using the Campbell equation. The soil electrical resistivity was also measured in the field with a 2D Wenner configuration using an inter-electrode spacing of 0.10 m along a 3.20 m transect. After the electrical measurements, a pit was dug and the contours of porous and compacted zones in the ploughed layer were identified, the boundaries between the ploughed layer, the plough pan and the pedological horizons were defined. Comparisons between inverted electrical resistivity maps and visual morphological descriptions showed the ability of electrical resistivity to detect wheels tracks. However, electrical surveying in a heterogeneous field after ploughing did not correspond to the visual morphological description, the latter being 2D whereas the electrical resistivity map is a 2D projection of a 3D sensing. As a non-destructive method, the electrical resistivity method could improve the quantitative description of the tilled layer and permit a temporal survey.

Structural heterogeneity of the soil tilled layer as characterized by 2D electrical resistivity surveying

Lessivage as a major process of soil formation: A revisitation of existing data

Summary

Soil cracks formed by natural processes play a key role in water and gas transfer. Patterns of soil cracks are, however, difficult to characterize. Our aim here is to assess the effectiveness of three-dimensional electrical resistivity surveys in detecting soil crack networks. A three-dimensional electrical survey was carried out by a square array quadripole with Cu–CuSO4 electrodes (electrode spacing of 3 cm). The measurements were made with two orientations (0° and 90°) on a block (26 cm × 30 cm × 40 cm) of soil while it dried for 18 days under controlled conditions. Two indexes, calculated from the apparent resistivity values, were evaluated to detect the degree of soil heterogeneity: (i) an anisotropy index based on the ratio between the apparent resistivity at 0° and that at 90°; and (ii) the angle-array orientation corresponding to the preferential anisotropic orientation (maximum resistivity). The anisotropy index provided information on the presence of cracks and the orientation for crack width > 1 mm in the first pseudo-depth (i.e. depth of investigation), while the angle-array orientation provided information on crack extension for the whole pseudo-depth. Information about the presence, position, orientation and extension of cracks can be obtained from an analysis of apparent resistivity obtained by a three-dimensional electrical survey. Such direct analysis will help the resistivity inversion to detect the crack network.

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Anatja Samouëlian

Papers

Electrical resistivity survey in soil science: a review .

Electrical Resistivity Imaging for Detecting Soil Cracking at the Centimetric Scale

Structural heterogeneity of the soil tilled layer as characterized by 2D electrical resistivity surveying

Lessivage as a major process of soil formation: A revisitation of existing data

Three‐dimensional crack monitoring by electrical resistivity measurement