Non-invasive monitoring of soil static characteristics and dynamic states: a case study highlighting vegetationeffects

In this paper we present the results of long term monitoring and irrigation tests performed on an experimental farm in Southern Sardinia, an area of semi-arid climate. The goal of the study is to understand the soil-vegetation interactions and how they can affect the soil water balance, particularly in view of possible climatic changes. We used long-term electromagnetic induction (EMI) time lapse monitoring and short-term irrigation experiments monitored using ERT (electrical resistivity tomography) and EMI, supported by TDR soil moisture measurements. Mapping of natural gamma-ray emission, texture analysis and laboratory calibration of an electrical constitutive relationship on soil samples complete the dataset. We observe that the growth of vegetation, with the associated below ground allocation of biomass, has a significant impact on the soil moisture dynamics. It is well known that vegetation extracts a large amount of water from the soil particularly in the hot season, but it also reduces evaporation by shadowing the soil surface. Vegetation represents a screen for rainfall and prevents light rainfall infiltration but enhances the wetting process by facilitating the infiltration and the ground water recharge. In many cases, the vegetation creates a positive feedback system. In our study, this mechanisms are well highlighted by the use of non-invasive techniques, that provide data at the scale and resolution necessary to understand the hydrological processes of the topsoil, also in their lateral and depth spatial variability. Unlike remote sensing techniques, non invasive geophysics penetrates the soil subsurface and can effectively image moisture content in the root zone. We also developed a simple conceptual model capable of representing the vegetation-soil interaction with a simple enough parameterization that can be fulfilled by measurements of a non invasive nature, available at a large scale and evidences possible relevant developments of our research.