Precipitation measurements by manual and automatic rain gauges and their influence on homogeneity of long-term precipitation series

The introduction of automatic instruments for precipitation measurements may have resulted in some differences in results from those taken by previous manual instruments. In the station network of the Czech Hydrometeorological Institute (CHMI) this replacement process began in the 1990s, when precipitation measurements taken by the METRA 886 rain gauge gave way to automatic tipping-bucket rain gauges, the MR3H and later the MR3H-FC. Continuous simultaneous measurement by both types of rain gauge at the Brno-Zabovresky and Ostrava-Poruba stations in the 2000-2019 period enable differences in recorded daily precipitation totals to be compared. Only those days upon which at least one of the two types of rain gauge recorded precipitation are analysed herein. Although the highest proportion of differences in daily precipitation totals lies between -0.1 and 0.1 mm, there is a distinct tendency towards higher totals in positive deviations as measured by the manual METRA 886, that is, tipping-bucket rain gauges generally record lower precipitation totals then the 'standard' manual rain gauge (the MR3H-FC undervalues totals to a greater extent than the MR3H). The design and construction of tipping-bucket rain gauges are reflected in overvaluation of precipitation days with smaller totals and undervaluation of days with high totals when compared with the METRA 886. Series combining measurements by METRA 886 and by tipping-bucket rain gauges (as held in the CHMI database) and series measured by the METRA 886 alone were created, homogenized and analysed in terms of long-term fluctuations and annual variations. Homogenisation of precipitation series based on combined measurements leads to generally undervalued totals compared with those measured only by the METRA 886. Results are discussed with respect to other similar articles, sources of errors in both types of precipitation measurement, and with regard to the homogenisation of precipitation series.