Hydrology has a long history due to its early origin, but it is still considered young due to lack of a solid scientific foundation as a natural science. To lay a solid foundation of hydrology, field experimentation is crucial for investigating hydrological processes and revealing hydrological mechanisms. Professor Wei-Zu Gu (1932–2022) was an internationally renowned scientist in the field of hydrology and is recognized as the greatest pioneer of experimental hydrology and isotope hydrology in China. He created the Hydrohill experimental catchment, which serves as both a great public works for experimental hydrology and a valuable legacy for future researchers to conduct hydrological experiments. This legacy represents an innovative infrastructure that bridges the gap between natural watershed experiments and artificial physical models. The Hydrohill is an intensively-instrumented experimental catchment, allowing for comprehensive measurement of elements of the hydrologic cycle and their tracing indicators in a sophisticated manner. To provide an in-depth understanding of the Hydrohill, this paper presents its short history, experimental objectives, site description (including location, construction, and instrumentation), site conditions (such as soil, hydrological and meteorological properties), and contributions to hydrologic science. We pay our respects to Professor Gu for his hard work in creating the Hydrohill for experimental hydrology and enhancing our understanding of hydrological processes and mechanisms. Finally, we hope that with healthy operation at Chuzhou Scientific Hydrology Laboratory (CSHL) along with support from Professor Gu’s friends, CSHL will enable the continued growth of the Hydrohill so that it can address some unsolved problems in hydrology.

The quantitative identification of nitrate sources is of great significance for the control of non-point source pollution and the comprehensive management of water resources in watersheds. δ 15N-NO 3 - and δ 18O-NO 3 - isotopes combined with the Bayesian isotope mixing model were widely used as effective methods to identify nitrogen sources. In this study, a total of 60 surface water samples and 82 groundwater samples were collected in study area from November 2021 to October 2022, and atmospheric deposition (AD), chemical nitrogen fertilizer (NF), soil nitrogen (SN), and manure and sewage (M&S) were determined as the potential nitrate sources. Source identification by SIAR indicated that in November 2021 the M&S was the main contributor of nitrate to surface water (mean 38.1%), while NF was the main contributor to groundwater (mean 39.8%). In April 2022, NF contributed the most to surface water (38.3%), while groundwater mainly originated from SN (29.4%) and MS (29.8%). The uncertainty analysis showed that the greatest uncertainties were in SN and NF, followed by M&S and AD. Sensitivity analysis showed that the changes in the nitrate isotopic composition of M&S had the greatest effect on the results for δ 15N, whereas only the mean values of oxygen isotope values of AD had a greater effect on the results for δ 18O. The sensitivity analysis results can optimize the sampling scheme and improve the accuracy of the model predictions. Additionally, the contributions of soil nitrogen and nitrogen fertilizer to nitrate in surface water and groundwater reached 58% and 64%, respectively. Therefore, optimizing fertilizer and irrigation management is necessary to improve nitrogen use efficiency in watershed management.