GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.2 673 APPLICATION OF ELECTRICAL RESISTIVITY TOMOGRAPHY TO MONITOR THE SOIL-ROOT INTERACTIONS UNDER DEFICIT IRRIGATION D. Vanella 1 , G. Cassiani 2 , L. Busato 2 , J. Boaga 2 , S. Consoli 1 1 Dipartimento di Agricoltura, Alimentazione, Ambiente, Università degli Studi di Catania, Italy 2 Dipartimento di Geoscienze, Università degli Studi di Padova, Italy Introduction. Geophysical methods can provide indirect high-resolution information on soil water content (SWC) distribution, which is fundamental during irrigation to avoid excessive water depletion conditions, especially when water deficit treatments are imposed [e.g. using partial root-zone drying technique -PRD- as described by Romero-Conde et al. (2014)]. Recent studies (Cassiani et al. , 2015; Consoli et al. , 2017; Satriani et al. , 2015) have shown that near-surface observing technologies (e.g. geophysical methods) can support and improve the irrigation operations in terms of both applied water amounts and irrigation timing. Furthermore, results have demonstrated the match between SWC variations and temporal changes in electrical resistivity (ER). However, the effects of other governing factors (e.g. pore water electrical conductivity and soil temperature) must also be considered. In this study, we present and discuss the results of a 3-D ERT time-lapse monitoring applied in an orange orchard with the following main goals: i) verify, with different time resolutions, the reliability of a small scale ERT setup to qualitatively monitor the soil-root interactions, in the context of two irrigation treatments (i.e. full drip irrigation and PRD); ii) identify, for each treatment, the active root water uptake (RWU) patterns and their time evolution, by integrating time-lapse ERT with ancillary measurements. Materials and methods. Experimental site and irrigation scheduling. We conducted small scale 3-D ERT monitoring in an orange orchard located in Eastern Sicily (Italy) and belonging to the Italian Council for Agricultural Research and Agricultural Economics Analyses (CREA). Here, 8-year old orange trees spaced are 4 m within the tree rows and 6 m between rows. The soil is fairly uniform with a sandy-loam texture, while the mean SWC at field capacity (pF = 2.5) and wilting point (pF = 4.2) are 28% and 14%, respectively. Irrigation rates were fixed on the basis of crop evapotranspiration (ET c ). Two different irrigation regimes were tested: (i) a control treatment (called T1), with trees irrigated with enough water to replace 100% of the ET c , and (ii) a partial root-zone drying (PRD) treatment (called T2), with trees irrigated at 50% of the ET c level. In both cases, the drip irrigation took place using two surface lateral pipes per tree row. More in detail, in T1 the pipes were located on the same side, while in T2 they were placed on opposite sides with respect to the tree trunk and irrigation was applied only to one lateral pipe. Moreover, ancillary measurements were performed during the study period and comprised SWC at both treatments, soil temperature and electrical conductivity of soil pore water (in order to evaluate their effect on the soil ER variability and add the necessary correction if needed), and tree transpiration rate, by means of heat pulse velocity sap flow technique. 3-D ERT time-lapse monitoring. ERT acquisition scheme. Small scale 3-D ERT monitoring was conducted around two selected orange trees irrigated at full level (T1) and by PRD (T2), respectively. The ERT set-up (Fig. 1) comprised both superficial and buried electrodes (204 in total) and consisted of 9 boreholes (1.2 m deep) each housing 12 electrodes (vertically spaced 0.1 m), plus 96 surface electrodes (spaced 0.26 m on a regular square grid). In both treatments, the boreholes are spaced 1.3 m on a square grid, thus delimiting 4 quarters (named q1, q2, q3, q4), only one of which is centred around the tree (q4). Each quarter represents the minimal unit of ERT acquisition, with 72 electrodes, surrounding a soil volume of about 1.3 m×1.3 m, and 1.2 m thickness. The ERT monitoring was performed in an attempt to capture long-term variations (along the irrigation season) as well as short-term changes (during a day), within the monitored irrigation season. The 3-D ERT long-term monitoring was conducted at the following time steps: i) when no irrigation was supplied (June); ii) 1 month after the irrigation start (July); iii) at the end