## 2 Materials and Methods  
The study was conducted in the experimental field located at the Department of Agrotechnology--University of Thessaly, Larissa, Greece (39\({}^{\circ}\)37\({}^{\prime}\)34.0\({}^{\prime\prime}\) N, 22\({}^{\circ}\)22\({}^{\prime}\)52.8\({}^{\prime\prime}\) E, elevation of 80 m above sea level). The total area used for the experimental site was 520 m\({}^{2}\) (20 m \(\times\) 26 m). A hemp crop (_Cannabis sativa_ L.) was established on 5 May 2020 at the experimental site for the purposes of the study. The variety selected for the experiment was Futura 75. Its selection was based on climate adaptation requirements, as well as seed and fiber yields. The wireless sensor network (WSN) in the proposed remote soil water monitoring and logging system consists of 4 wireless soil moisture sensors and a weather station (Figure 1). Measurements of volumetric water content (VWC) are essential for assessing the status of the hemp crop-available water in soil. Wireless sensor nodes, which are distributed on the experimental site, collect, process, and transmit field sensing data using the GPRS communication protocol by making HTTP requests. The data are accessible to the end users (farmers) through a dedicate GUI that was developed for projecting the received data. In addition, the data were accessible to the users though an API that was developed using OpenAPI 3.0 specifications (OAS 3.0). The data can be exported from the API in XML, JSON, YAML, and CSV format. Wireless sensor nodes were developed by the Agricultural & Environmental Solutions Private Company (AGENSO), Athens, Greece. Each node plays the role of a base station independently transmitting its data through the Internet to a cloud server. A node can support a soil moisture sensor, a weather station, and an external energy harvesting unit (solar cell). The architecture of such node comprises analog sensor channels, an onboard digital signal processor (DSP), RAM and flash memory, a GSM modem, a global positioning system (GPS) receiver, an LCD display, and a power source (Figure 2). Each sensor node was developed to be power efficient and run by 1 rechargeable AA battery (average capacity of 1.940 mAh) for a long period of time. In addition, solar-based energy harvesting technology was adopted in the case of sensor node 1. The hardware components are enclosed in an IP68 box, as shown in Figure 3, to protect them from damage and environmental conditions. An ultralow power energy harvester and battery charger circuit applies the MPPT function and integrates the switching elements of a buck-boost converter. It allows the charge of battery by monitoring the end-of-charge and the minimum battery voltage to avoid overdischarge. Unregulated voltage output is available (e.g., to supply the microcontroller), while 2 fully independent LDOs are embedded for powering the sensor and wireless transceiver module. To extend the battery life, the microcontroller and wireless transceiver go to their own standby or sleep mode with a low current consumption when the sensor node is idling. By using the real-time clock, the microcontroller in the sleep mode is periodically awakened by the interrupt, which is set previously (e.g., 1 h). The microcontroller awakening releases the sleep request to the transceiver. As a result, the transceiver also wakes up. The microcontroller acquires the data from the sensor and sends it to the cloud server via the activated transceiver. After finishing the transmission on the cloud server, the  
Figure 1: Overall view of wireless sensor network (WSN). A 112 mm \(\times\) 84 mm \(\times\) 3 mm commercial PV panel was used for converting sunlight into usable energy. The PV panel features an open circuit voltage of 7.3 V and a short circuit current at 204 mA. The maximum power it can produce is around 1.1 W. Table 2 details the specifications of the used PV panel. The PV panel was placed on the steel tube, facing south with a tilt angle of 30\({}^{\circ}\) (see Figure 4).