Use of a Dielectric Sensor for Salinity Determination on an Extensive Green Roof Substrate

The irrigation of extensive green roofs with recycled or saline water could contribute to the conservation of valuable drinking water supplies. In such cases, the continuous monitoring of substrate electrical conductivity (EC(sw)) is of immense importance for the sustainable growth of the plants growing on the green roof. The present study aimed to estimate the EC(sw) (pore water EC) of an extensive green roof substrate in lysimeters with the use of the WET-2 dielectric sensor. Half of the 48 lysimeters that simulated extensive green roofs had a substrate depth of 7.5 cm, while the other half had a 15 cm substrate depth. The warm season turfgrass Paspalum vaginatum ‘Platinum TE’ was established at the lysimeters, and during the summer period, it was irrigated every two days at a rate of 14 mm with NaCl solutions of various electrical conductivities (EC(i)): (a) 3 dS m(−1), (b) 6 dS m(−1), and (c) 12 dS m(−1), while potable water of 0.3 dS m(−1) EC(i) served as the control. The relation between bulk electrical conductivity, σ(b), and bulk dielectric permittivity, ε(b), of the substrate was observed to be linear for all EC(i) levels up to σ(b) values of 2–2.5 dS m(−1). The EC(sw) was predicted by employing the salinity index method which was modified to be applied to the particular case of a green roof substrate. Knowing the salinity index and organic portion (%, v/v) for a given green roof substrate, we could calculate the EC(sw). It was found that the use of the salinity index method predicts reliably the EC(sw) up to 10–11 dS m(−1), while the method overestimates EC(sw) at very low levels of electrical conductivity.