Long term measurement is crucial to understand the impact of human activities on hydrology. The Orgeval catchment (70 km East from Paris) is one of the oldest experimental catchment in France: streamflow and rainfall data are available since 1962 in a set of embedded watersheds. The main goal is to acquire and provide scientific knowledge to be used for the management of water resources and risks (drought and floods) along with the assessment of human impacts on water regime and quality. However studying one catchment behaviour along a long period requires a high confidence in the data. Do the changes in measurement device, in monitoring procedures or in people in charge of this monitoring have an impact on data quality? Are these effects negligible compared to those related to changes in land use, climate change, water management within the catchment? This study focuses on one of the Orgeval sub-catchment, Mélarchez (700 ha). 80% of the surface is artificially drained to allow intensive agriculture, and this drainage strongly impacts the hydrological behaviour. Four hydrological seasons can be distinguished: (i) intensive drainage season in winter (high rainfall restitution rate) (ii) groundwater contribution in summer season without drainage (low rainfall restitution rate) (iii) two transition periods in spring and autumn. Rainfall restitution rate during each season is chosen to detect catchment hydrology change in time. Significant increase in rainfall restitution rate is surprisingly observed during some drainage seasons after 1994: runoff amount becomes higher than rainfall amount, which never happened before... Are we faced with a hydrological monster ?