Correction of siphoning error in processed historical rainfall intensity data, a case study of data measured by Hellmann-Fuess type rainfall recorder

The Hellmann rainfall recorders have been one of the primary instruments of the rainfall intensity measurement, mainly in the countries of the central part of Europe, during the 20th century. These water level measurement-based rainfall recorders ensure the continuity of measurement by periodically emptying siphoned measurement cylinder. During the emptying, the measurement is paused, resulting in under-measurement. The duration and number of emptying can be known, and the correction of under-measurement can be adjusted if the registration ribbon is available. In the case of historical data, the registration ribbon is often unavailable, and only extracted values of the most intensive periods of rainfalls can be gained from databases. A procedure for correcting such kinds of data is shown, and also the influence of the correction of Hellmann-Fuess recorder’s data on the IDF curves.

The procedure is based on the method published in 2002 by Luyckx and Berlamont, which was compiled based on the physical process of the siphoning, and it assumed the existence of the original records (ribbons), with the exact time of siphoning. The extracted data tables nor time nor number of siphoning was registered, so the method of Luyckx and Berlamont for the data correction cannot be used. The proposed procedure’s principal is the estimation of the number and length of pauses in the extracted measurement interval. These estimated data make passible to fix the rainfall quantity and intensity for the given interval. The measurement cylinder of the device is generally not empty at the beginning of the most intensive period of rainfall. The water level is assumed as a uniformly distributed probability variable what can be estimated with its expected value. The raw data comprise underestimation in all cases, so the raw data represent only the possible minima of the plausible intensity values. The fixed data result in the expected value of the plausible intensities, which are sometimes higher and sometimes lower than the actual intensity values, with the same probability; so, if there are a high number of fixed data, the positive and negative deviations diminish the errors statistically. The use of the correction formula is presented with the parameters of a Hellmann-Fuess rainfall writer. The correction range has been the highest over ten years of average recurrence, and its measure was 10%. For 30 minutes and longer sampling intervals, the magnitude of the correction is not relevant. After that, the correction is based on statistical estimation, the exact value of the rainfall intensity cannot be retrieved, but the underestimation can be decreased significantly. The fixed data modify the IDF curves as well, and in this way, the effect of climate change can be investigated more appropriately.