Vol. 125, No. 3 * Pages 367–519 * July - September 2021

Historical rainfall data registered by siphoned rainfall recorder (SRW) devices have been widely used for a long time in rainfall intensity investigations. A relatively known counting error of the SRW devices is the siphoning error, when the registration of rainfall is blocked temporarily, during the drainage of measure tank. This issue causes a systematic underestimation in the rainfall and rainfall intensity measurement results. To reduce its consequences, a data correction is crucial when SRW data are used, for example as a reference for climate comparison studies, or for proceeding of intensity-duration-frequency curves, etc.  In this paper, a formula is presented to fix the siphonage error of SRW devices for historical rainfall data. The early measures were processed in a significant percentage of cases, and sometimes the original measurement results (registration ribbon) have been lost. An essential advantage of the presented formula is that it can be applied for these processed data, which show only the intensity of a known length time interval. For this correction, the average rainfall intensity and the length of the time window are needed, over the physical parameters of the SRW device. The data correction can provide a fixed value of the rainfall intensity, which is undoubtedly closer to the real average rainfall intensity. The importance of this formula is in the reprocessing and validation of the historical rainfall intensity data, measured by siphoned rainfall recorders.