*3.2. Management of Stored Water*

In Mykonos Island, two open top tanks were constructed for storing rainwater from a sub-surface rainwater collection system (Figure 9).

The collected water was used for the irrigation of a 0.4 ha oregano field using the rainwater stored into the open tanks. As the intention was to minimize electrical power consumption, one small pressure booster pump in combination with electrovalves controlled by the IoT nodes was placed for controlling the water flow between the two tanks and for enabling irrigation (Figure 10a), while to determine the level of stored water into the tanks, ultrasonic sensors (SR04T, AGENSO, Athens, Greece) were used (Figure 10b).

**Figure 9.** (**a**) Open top water tanks; (**b**) Sub-surface rainwater collection system.

**Figure 10.** (**a**) Electrovalve for controlling the water flow; (**b**) Level sensor installed in one of the tanks.

Depending on the water quantity monitored in each tank, and according to the thresholds defined by the user, the appropriate electrovalve is opened to irrigate the crop using the water stored in one of the two tanks. Figure 11 projects the sum of the water quantity stored in both tanks during the period from 22 July 2021 to 23 September 2021. The small differences that were observed during the monitoring (±0.5 m3) come from the effect of sunlight on the accuracy of the level measured by the sensors.

**Figure 11.** Water quantity monitoring.

Additionally, on a nearby house, a tank was constructed on its terrace to provide water for domestic use and to irrigate the 0.2 ha lavender field adjacent to it. To monitor its quantity, a node with an ultrasonic sensor was installed (Figure 12).

**Figure 12.** Measurement of the water level in a roof tank.

This tank can be refilled with rainwater collected on the rooftop or by pumping water from a nearby well. When the tank level is lower than the threshold defined by the user, the pump of the well is activated by another node. Figure 13 presents the water level of the aforementioned tank on a daily basis.

**Figure 13.** Monitoring of the water level in the roof tank.

In the case of rain, the excess water on the rooftop is directed after slow sand filtration to recharge water into a nearby confined aquifer, mitigating the long-encountered problem of saline water intrusion. To monitor the water in the aquifer, a well was constructed (Figure 14a) and its water depth was monitored using a submersible pressure transducer (SR05W, AGENSO, Athens, Greece) (Figure 14b). The measurements showed that the water depth reduced day by day in an almost steady rate, leading to the discovery of a fracture on the selected aquifer, which caused this water loss.

**Figure 14.** (**a**) Nodes for monitoring the well depth. (**b**) Water depth in the well (depth of the aquifer).
