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Accurate and reliable water level measurement is a critical task in hydrology, water resource management, irrigation systems, and automated industrial processes. This paper presents a comprehensive study of modern ultrasonic methods and tools for non-contact water level measurement, supported by mathematical modeling of liquid level dynamics in reservoirs. Typical mathematical models describing ultrasonic level measurement systems are developed and analyzed. The principles of operation of ultrasonic sensors, the formation of the detection zone, and the influence of geometric and physical parameters on measurement accuracy are discussed in detail. Particular attention is paid to sensor installation rules, operational constraints, and signal reflection characteristics. As a practical case study, the technical and metrological parameters of the UDM-110 ultrasonic level meter are analyzed. Mathematical models based on mass balance equations are derived to describe transient and steady-state variations of liquid level in reservoirs. The obtained models enable analytical evaluation of system dynamics and provide a foundation for the design and optimization of automated level control systems. The results confirm that ultrasonic level measurement combined with mathematical modeling ensures reliable, accurate, and cost-effective monitoring of water levels in open and closed reservoirs.