S07 - Session P1 - Suitability of dielectric soil moisture sensors for measuring the water supply status of green roofs

Authors: Heinz-Josef Schmitz, Ivonne Jüttner, Dieter Lohr *, Elke Meinken

Extensive green roofs are a key component of urban water management in the future. They shall mitigate urban heat islands, for which evapotranspiration has to be maximized, on the one hand, and on the other shall reduce the risk of urban floods after heavy precipitation events. To achieve these goals, exact measurement of water supply is necessary. In arable soils as well as in organic growing media, dielectric sensors are widely common. However, there is only little knowledge about the suitability of this kind of sensors for measures in mineral and coarse-textured substrates for extensive green roofs. In the current research four dielectric sensors (EC-5, 10-HS, SMT 100 and Aquaflex) were tested using five different green roof substrates. The five substrates were filled in plastic boxes of 80 x 60 cm. Substrate height was 15 cm and the four sensors were placed at half height. Afterwards white lupine was sown and plants cultivated up to a height of about 30 cm. For testing the sensors, the substrates were saturated with water and then let become dry until plants show severe sign of wilt. During the dry out the sensor signals as well as the weight of the boxes were recorded automatically every five minutes. For each substrate six consecutive drying cycles were done. With exception of the Aquaflex probe, for all five substrates raw signals were closely linear correlated to weight loss and signal ranges were comparable for the six drying cycles. However, significant differences between the raw signals of the sensors in the different substrates were observed. This indicates a need for substrate specific calibrations. As mentioned before, the relation between the raw signal of the Aquaflex probe and the weight loss was not linear, but also reproducible and suitable to assess water supply status of plants.