## 1 Introduction
Moreover, an UAV-assisted millimeter wave wireless system is proposed in [8], where the flight altitude, beam number and transmit power of the UAV are jointly optimized. Similarly, the transmit power and the position of the monitored UAV are jointly optimized in [9] to maximize the covert rate for legitimate receivers on the ground. To ensure covertness, the transmit power and trajectory of the UAV are jointly optimized in [10]. In addition, stationary UAV-aided CCSs are designed in [8; 9], where the UAVs hover at a fixed position. However, in these works, the length transmission message block (LTMB) is assumed to be infinite, which is impractical. In practice, the LTMB transmitted via a channel limits transmission efficiency, which thus requires the transmitted messages with short block lengths to be short for practical CCSs [11]. In this sense, the LTMB affects the performance of a CCS, i.e., the detection performance of Willie, covert rate [12]. For example, according to [12; 13], finite LTMB imposes a significant impact on the data rate of the channel from Alice to Bob. Therefore, not only the covertness, but also a specified finite LTMB play important roles in the optimization of the CCS to maximize the covert rate [14]. Since the collected parameters and instruments are small blocks with a few bits for a hydropower plant, we adopt the finite LTMB for the covert channels. To improve the performance of the system, we optimize not only the transmit power of the base station, but also the length of the block length. However, how to optimize the performance of a CCS with a finite LTMB remains a challenge. Recently, UAV-assisted covert communication with NOMA has been widely investigated. As a typical work, the authors of [15] proposed an optimized scheme of the covert communication of downlink NOMA systems by studying the behavior of the outage probability of NOMA users and the expected minimum detection error probability under the uncertain channel knowledge. Inspired by this idea, the optimization algorithm has been designed for the intelligent reflecting surface (IRS)-assisted NOMA systems to maximize the covert rates by jointly optimizing the transmit power and the IRS reflect beamforming [16]. Similarly, the covert communication scheme was developed for NOMA systems under uncertain channel distribution information to maximize the expected covert rate [17]. In addition, an iterative algorithm was proposed in [18] for UAV-assisted NOMA communication to jointly optimize time slots, transmit power, and trajectories, which ultimately improves the covert rate of UAV-assisted data transmission. The authors of [19] optimized a multiple inputs multiple outputs (MIMO)-NOMA system by minimizing the delay in UAV-assisted caching networks. In this paper, we propose an UAV-assisted CCS, in which the UAV provides a covert wireless communication service to terminals accessing hydropower IoT networks via NOMA. The UAV acts as a transmitter of covert messages and thus communicates with terminal devices with line of sight (LoS) without being blocked by the barrier in complex hydropower plants. Furthermore, transmit power and the flying altitude of the UAV are optimized to maximize the covert rate under the constraint of covertness and the length of the transmission block. In particular, the contributions of this paper are listed as follows. * To protect privacy in the hydropower IoT, we propose an UAV-assisted covert communication scheme for the hydropower IoT, where the UAV can communicate with terminals via covert communication channels. Furthermore, the edge terminals the hydropower IoT by using NOMA to reduce energy and spectrum consumption. Moreover, the proposed UAV-assisted covert communication scheme can also be applied to those IoT systems with complex communication environments and large cover ranges.