The Large High Altitude Air Shower Observatory (LHAASO) is to be built at Daocheng, Sichuan Province, China. As one of the major components of the LHAASO project, a Water Cherenkov Detector Array (WCDA), with an area of 78,000 m 2 , contains 350,000 tons of purified water. The water transparency and its stability are critical for successful long-term operation of this project. To gain full knowledge of the water Cherenkov technique and investigate the engineering issues, a 9-cell detector array has been built at the Yangbajing site, Tibet, China. With the help of the distribution of single cosmic muon signals, the monitoring and measurement of water transparency are studied. The results show that a precision of several percent can be obtained for the attenuation length measurement, which satisfies the requirements of the experiment. In the near future, this method could be applied to the LHAASO-WCDA project.

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Introduction The Large High Altitude Air Shower Observatory (LHAASO) will be constructed at Mt. Haizishan, Sichuan Province, China. Among several detector components of the LHAASO, the Water Cherenkov Detector Array (WCDA) is of great importance for gamma-ray astronomy from 100 GeV to 30 TeV. The single-channel counting rate of a photo-multiplier tube can reach as high as 30 -35 kHz, most of them are background noise hits from the low energy cosmic ray showers, bringing a big challenge on data transferring, data storage and event reconstruction. Methods In this paper, a dedicated trigger scheme and a fast noise filtering method aiming to deal with these high rate background noise hits are introduced. These methods are tested with some Monte Carlo simulation data, showing a fair efficiency in filtering background noise hits, while most of the real shower signals are kept. Conclusion This method is proposed to be applied in a stage of the online processing just after the data are acquired in LHAASO-WCDA.