Many studies have compared the flow measurements obtained from the clamp-on transit-time ultrasonic flowmeters with other measurements devices such as ultrasonic Doppler flowmeters (e.g., ultrasonic Doppler, Acoustic Doppler Current Profiler (ADCP), and Laser Doppler Anemometer (LDA)), electromagnetic flowmeters, and pressure transducer based current meters. Among these methods, ultrasonic transittime method appears most promising because it is non-intrusive, accurate, applicable to wide range of flow velocities, cost-effective, easy to calibrate and has high resolution. Its popularity can be gauged by the fact that it is considered one of the fastest growing flow measurement technologies (Trofatter, 2002). Cascetta (1994) compared the measurements obtained from a clamp-on transit-time portable ultrasonic flowmeter with an in-line (or wetted) electromagnetic meter and concluded that the measurements from both the meters are substantially close to each other and the transit-time ultrasonic device seems to be quite adequate for water industry applications. In addition, the clampon ultrasonic flowmeters are significantly economical in terms of installation and maintenance costs. Lozano and Mateos (2009) compared transit-time and Doppler ultrasonic flowmeters for measuring discharge in irrigation canal with the propeller current meter and found that both ultrasonic instruments provided measurements that were highly consistent with those obtained from the propeller meter. They also reported that the ultrasonic flowmeter based on transit-time principle operated without interruption during the entire irrigation season.