GNGTS 2013 - Atti del 32° Convegno Nazionale

water with the new low salinity irrigation water (piston effect). This vey interesting aspect will be examined by the use of theoretical modeling. Venice Lagoon case studies. Venice salt marsh location was chosen to test the capability of tidal plants to generate a persisting aerated layer in the subsoil when flooded, due to root uptake (Marani et al. , 2006). We planned time lapse ERT measurements and tensiometers measurements before, during and after sea flooding of the marsh in a hot summer day (33°) of July 2012, with slow wind. This was expected to be the best conditions for salt marsh uptake process (potential evapotranspiration estimated to be approximately 8 mm/day, based on Penman-Monteith equation). Fig. 2 shows the inversion results at three time steps during the flooding of the marsh by seawater. Here we consider only the portion of subsoil between the instrumented boreholes, which presents the highest sensitivity. The three time steps are before the marsh is flooded (12:36 pm), when the marsh is flooded (2:40 pm), and after the marsh surface has been flooded and has started drying (3:11 pm) during ebb, the highest tidal level being of 0.34 m above MSL, at 1:52 pm. These ERT ratio images clearly show the development of a zone of increased electrical resistivity near a depth of 30-40 cm below soil surface. Note that the tide starts flooding the marsh shortly after noon (that corresponds to the reference time instant) but is not visible per se in the ERT images, confirming that the high pore water salinity makes the system poorly sensitive to the modest change in water saturation due to water level rise. The area which is getting dryer during marsh flooding is the expected root zone for the typical wetlands vegetation (Sarcocornia fruticosa, Juncus, and Limoniumn narbonense, etc.). Tensiometers measurements, installed at 20, 30 and 40 cm depth confirmed the micro ERT results (Fig. 3). Surface tensiometers shows in fact a wetter condition when sea flooded the marsh but tensiometer installed at root depth (30-40 cm) show a surprising dryer condition during the flooding. ERT and tensiometers results are summarized in Fig. 3. The geophysical techniques were able enhance the dynamics interaction within marsh plants and first subsoil. Discussion and Conclusion. We performed 3D and 2D micro-ERT measurements of root plant during controlled irrigation tests and natural flooding, in order to verify the potential of these non-invasive techniques to monitor the dynamic root zone activities. The advanced 3D micro-ERT methodology exposed proved to be able to monitor suction zones, in good agreement to the expected root positions. In dryer conditions micro 3D-Ert image the penetrating water plumes injected, while in wetter high growing seasons of the plants micro 3D-ERT points out how certain subsoil zones are interested by quick drying conditions. These drying suction areas are in good agreement with the expected depth of the apple tree root activities. 3D micro ERT through indirect estimations enhanced the root zone Fig. 2 – Time-lapse ERT results: resistivity ratio with respect to background at three time steps before (left panel), during (central panel) and after (right panel) marsh flooding by tide. Note the 140% value zone (drier zone) located at a depth of 20-40 cm below the marsh surface around 2.40 pm (central panel), when the marsh is completely flooded, (from Boaga et al. , 2013, modified). 94 GNGTS 2013 S essione 3.2