*3.2. Salinity Spatial Distribution*

Two quantities were used to describe the long-term accumulation of salinity: (i) electrical conductivity (EC)—representing the total salinity, and (ii) chloride concentration—as a soil native conservative tracer. The EC distribution is shown for the transect along (Figure 7a) and perpendicular (Figure 7b) to the drip line. The chloride distribution is shown in Figure 7c and d for transects along and perpendicular to the drip line, respectively. For both transects, salinity and chloride distribution exhibited similar patterns. Specifically, both quantities demonstrated a leaching zone above and below the emitters and salt accumulations zone between nearby emitters. For the transect along the drip-line, the highest salinity prevailed above the drip line in the middle of two nearby emitters. Nevertheless, the salinity values below the drip-line are also very high and may reduce water uptake by the olive trees' roots, due to the high osmotic pressure, even if a high water content is maintained. Regarding the perpendicular transect, a distinct, uneven distribution could be observed. Specifically, the salt accumulation away from the tree (−100 to 0 cm) is significantly higher than the one obtained toward the tree line (0 to 100 cm). The lowest salinity obtained near the tree line may be explained by a reduced evaporation and capillary rise toward the soil surface, due to the surface shading by the olive trees. However, the entire zone exhibited very-high to extreme values of EC, which indicates the salinization of the olive plantation, as clearly illustrated from the three dimensional visualization of the entire 2 m2 view of the spatial EC distribution at six individual depths (Figure 8). The representation of the entire domain emphasizes the extreme values of salinity above the drip-line and away from the tree-line. A clear pattern could not be observed, due to the large salinity spectrum that was considered in this counter map. Nevertheless, the leaching zones above and below the emitter is clearly demonstrated, indicating moderate to high salinity levels.

**Figure 7.** Electrical conductivity (dS m<sup>−</sup>1) distribution (**a**) along the drip line, (**b**) perpendicular to the drip line, and soil chloride (mg L<sup>−</sup>1) distribution (**c**) along the drip line and (**d**) perpendicular to the drip line. The black and white stars indicate the location of the drippers.

**Figure 8.** An overall distribution of the electrical conductivity (dS m<sup>−</sup>1) at all six depths, 0–5, 5–10, 10–15, 15–30, 30–45, and 45–60 cm. The black and white stars indicate the location of the drippers.

As mentioned, the mean annual rainfall in this region is below 125 mm and may be distributed over ten low-rain events (Figure 3). Under these conditions, there is insufficient rain to leach the accumulated salts from the soil surface below the active root zone. This minimal rainfall can also exacerbate the salinity problem by bringing surface salts (0–15 cm) to the root zone (30–60 cm) after one or several rainfall events. The salinity observed in the orchard soils is far above the normal threshold salinity level for olive growth, i.e., a soil EC value of 4 to 6 dS m−<sup>1</sup> is the accepted critical limit for normal olive growth [12–15]. To leach the excess salts from the root zone, high rainfall events >600 mm are required [48,49] or sprinkler irrigation has to be implemented in order to leach salts, but the long-term economic sustainability of this system is questionable [30].
