Evaluation of soil water content in tilled and cover-cropped olive orchards by the geoelectrical technique

Abstract

Electrical Resistivity Imaging (ERI) was performed in southern Italy, under semi-arid climate, on a Haplic Calcisol soil of two rainfed olive orchards managed according to different soil management (Tillage – T, and Cover Cropping – CC), applied for 8 years. The main aim was to test the usefulness of such geoelectrical method for the assessment of spatial and temporal variability of soil water content for water balance studies. For that purpose, the quality of univariate relationships between soil electrical resistivity (ρ) and volumetric soil water content (θ v) was tested. Three geoelectrical measurement campaigns were performed in the autumn–spring period of 2006–2007. For each campaign, 3 two-dimensional (2-D) geoelectrical tomographies were carried out in the pole–dipole array using a fixed electrode spacing of 0.25 m. 2-D apparent resistivity pseudosection data sets have been inverted using the software TomoLab®. Each covered soil section was 3 m in depth and 11.5 m long. On 14 November 2006 and 18 April 2007 campaigns the ERI were followed by soil samplings carried out with auger directly below the respective geoelectrical profiles. Direct measurements of soil electrical conductivity and soil water content were conducted on the soil samples in situ and in the laboratory. Volumetric soil water content, θ v (mm mm −1), was significantly related to soil electrical resistivity data, ρ (Ω m), by the exponential model θ v = 1.641 ρ − 0.599 (R = 0.866, n = 84). This relation allowed to assess the change of soil water content in the two orchards during the autumn–spring period and to reveal that the CC plot was more efficient to intercept and store rainwater. ERI technique allowed to highlight differences between the systems in the dynamics of water distribution along the soil profile as an effect of the adopted soil management. In particular, such technique consented to easily evaluate the extent of water content of the deepest soil layers (> 1.0 m) evidencing a significant water reserve in the CC plot, convenient for the root system of rainfed olive trees in the driest months. Finally, the soil water content evaluated by means of the ERI technique was used for the short-term application of soil water balance which showed interesting data on the progress of water consumption during the autumn–spring period, particularly useful as operative indications for cover crops management.