## 1 Introduction  
Recently, intelligent terminals have become popular in the hydropower Internet of Things (IoT), which can be used to collect boundary data and automatically analyze the collected data, thereby improving the efficiency of hydropower plants. According to [1], there is a scope of study for monitoring the performance under conditions in real-time in hydropower, as it is difficult to predict the behavior of the machine using the existing methods. To improve the real-time and reliability of information exchange of hydropower plants, an IoT-aided hydropower system was proposed in [2] based on message queuing telemetry transport. Furthermore, to enhance energy and spectrum efficiency of the hydropower IoT, the advanced multiple access technology known as non-orthogonal multiple access (NOMA) has been identified as a promising solution for facilitating terminal access to wireless networks due to its advantages, such as low latency, extremely dense coverage, and high fairness [3]. Especially, in hydropower IoT, NOMA can improve energy efficiency and spectrum utilization by providing lower power consumption and more efficient data transmission solutions for IoT devices to support various IoT applications [4]. However, developing a secure communication scheme based on NOMA is a major challenge in IoT networks for hydropower plants. The production of hydropower plants is characterized by corruptibility and sudden changes, which requires quick and close monitoring of hydropower equipment to ensure safe production. In this case, it is urgent to design the communication system to transmit the monitored data safely, reliably, and regularly to the control center. To ensure absolute security, traditional hydropower plants are not connected to a network or the Internet, and thus bring greater inconvenience and low productive efficiency. However, the amount of sensitive data is very small and can be easily cracked by using cryptographic methods with obtained cipher texts. To improve productive efficiency, covert communication is used for hydropower plants to collect parameters and hand out instructions. In this case, covert communication has attracted much attention as a key technology for network security. Traditionally, covert communication systems (CCS) are designed by using network protocols, such as transmission control protocols (TCP), Internet protocol (IP), and media access control (MAC) protocols [5]. However, these communication systems cannot be used for wireless sensors in IoT systems. In this scenario, wireless covert communication is adopted as an effective method for secure communication of intelligent terminals. However, wireless CCSs cannot achieve a high covert rate in complex environments due to the barriers between the transmitter and receiver. In a hydropower plant, there are volumes of power generation equipment, hydraulic mechanical equipment, electrical equipment and systems, flood gates, and hydraulic buildings, which form a complex communication environment. Moreover, the IoT network for a hydropower plant usually covers a larger range than traditional IoT networks. Owing to the barriers and the large range coverage, traditional wireless covert communication cannot be deployed in hydropower plants. In this case, the UAV-aided covert communication is used as the base station to receive the collected data from sensors and the handout instructions to the devices. To solve the problem, existing works have focused on secure communication with Unmanned Aerial Vehicle (UAV) communication networks, launching an UAV to avoid being blocked by barriers [6]. Furthermore, the transmission behavior of an UAV cannot be determined, and thus, the UAV-assisted covert communication cannot be easily detected by malicious users [4]. Accordingly, the authors of [7] propose a covert wireless communication method to transmit covert information from [PERSON] to [PERSON] under the monitor of [PERSON].