The long-term research Orgeval observatory (https://gisoracle.inrae.fr/) is one of the oldest French observatories of the critical zone. The study site, of ~100km² surface area, is located 70 km east of Paris. To understand the structure, functions and the reactivity of the critical zone (soil, surface, groundwater and atmosphere), measurement of water quality and quantity are carried out at different points and time steps. The observatory implements a multi-scale observation strategy (from 1 km² to 100 km²), thanks to the nested instrumented sub-catchments, intensively instrumented, to quantify the response of the critical zone system to human activities and metrological events. The meteorological, hydrological or chemical measurements relate to more than 80 measurement stations. They allow long-term observations at different time intervals (hourly, daily or weekly) of precipitation, surface water and groundwater. The reference measurement allows continuous quantification of water and solutes from the soil surface, river stations and groundwater. These measurements include: - River discharge, river temperature and water sampling stations, for quality analysis purpose, at the outlet of six sub-catchments; - Precipitation measurement stations supplemented by those of Météo-France; - Meteorological stations for air temperature, humidity and radiation measurements; - Soil moisture and soil temperature measurement stations from the soil surface to 1.5 m depth; - Piezometers along the plateau-valley cross section to monitor the temperature and the hydraulic head variations in the aquifers; - Local monitoring stations measurements of the stream-aquifer exchanges including: two shallow piezometers, two hyporheic zone temperature profiles located close to each river bank and one water level and temperature monitoring system in the river; - Geophysical survey (high-resolution time-lapse seismic acquisitions, ERT, optic fiber) to characterize the sedimentary heterogeneities of the critical zone and water table fluctuation. The monitoring includes a RiverLab prototype installed in June 2015, allowing real time and high frequency (every 30 minutes) measurement of the major ions comprising water quality. A complement ad hoc monitoring, event-based measurement campaign and measurement specific for each research are available. The data collected is available free of charge to the scientific community, with a simple registration on the website (https://bdoh.irstea.fr/). The observatory is representative of sedimentary basins and artificially drained hydromorphic soils, in the context of intensive agriculture, changes in agricultural policy and climate change. Since its creation in 1962, for floods and erosion issues, the observatory was able to adapt to other societal and environmental issues. The problem of diffuse pollution of agricultural origin, initiated in 1975, has contributed to a better understanding of the interactions between agricultural activities and the chemical quality of surface water and groundwater. Since the 2000s, the problem of pesticides and scientific questions linked to biology and biodiversity have been added. It is also an exploratory platform for technological innovations, such as high-frequency measurement in biogeochemistry and hydrogeophysics. The observational data support the development and use of numerical models for impact studies and territorial management. This basin is used to test experimental protocols as well as numerical methodologies to be used on a larger scale (e.g. Seine basin).