Knowledge depends on the existence of data. Modern hydrology has been primarily based on quantitative data from gauging stations. However, other sources may provide data relevant to hydrology progress as demonstrated by researchers since the 1970s. The general picture on the subject highlights the lack of standard terminologies for data types and sources, which can lead to misunderstandings. Besides, there is still no consensus on the use of data from sources other than monitoring stations. In this context, in the present study, we suggested standard denomination for data and their sources. We classified from the recorded characteristics as systematic data and non systematic data. We also classified the data sources according to the origin: instrumental in-situ evidence, orbital instrumental evidence, physical evidence and documentary evidence. Considering the relevance of non-systematic data, mainly concerning extreme hydrological events, we suggested the use of both data – systematic and non systematic - applying the data triangulation method.
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