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Rainfall-runoff relation is the basic and critical issue in hydrology, as well as in the hydrologic experiments. Supported by Chuzhou Hydrology Laboratory, this study conducted experimental investigation of the rainfall-runoff relation of Hydrohill experimental catchment during its historical (1982-1995) and modern (2013-2018) practice periods. Hydrohill is equipped with a separate-runoff system which can measure the runoffs in different layers: the uppermost trough collects rain; the next lower trough collects surface runoff (RS); the three lower troughs collect subsurface low from soil layers of the depths of 0–30, 30–60, and 60–100 cm (inferred as R30, R60, and R100). A network of 21 aluminum alloy access tubes for neutron moisture gauges were constructed in the historical period. Since the last month of 2012, all aluminum alloy access tubes have been displaced with 31 profile soil moisture sensors (PR2) with six sensor points located at the 10, 20, 30, 40, 60, and 100 cm. Eight profiles with another kind of soil moisture sensors (SM-1) were installed as well since 2017. This type of SM-1 has nine sensor points located at the 10, 20, 30, 40, 50, 60, 70, 80, and 90 cm. An array of 22 galvanized tube wells intersect through the soil till the concrete aquiclude. Water table measurement is performed with pressure-type sensors (LEV1) located at the bottom of each well. According the experiment results, conclusions are determined as following: (1) The proportion of RS decreased from 57.3% during the historical period to 35.3% during the modern period, and thus the runoff structure became subsurface runoff dominated from surface runoff dominated. During the historical period, the proportion of RS accounted for 54.4%, 57.3% and 70.3% under the conditions of 10≤P<25 mm, P≥25 mm, and P<10mm (P represents rainfall amount of events), respectively. (2) the synthetic runoff coefficient of RS decreased while that of R100 increased from historical period to modern period; the synthetic runoff coefficient of R30 decreased but R60 increased under the condition of P≥25 mm. (3) The maximum discharges of all separate runoffs during historical period were larger than those during the modern period. (4) The average rising times of RS, R30 and R60 during historical period were longer than that during the modern period, but R100 was just the opposite. The average peak times of RS during historical period were shorter than those during the modern period. The duration of all separate runoffs in both catchments were during historical period were longer than those during the modern period. (5) The depths of soil distinction layers were located at the range of 30-40 cm based on the data of profile soil moisture. (6) Saturated overland flow just occurred in the unchanneled upstream based on the analysis of the dynamics of water table in Hydrohill. This investigation can enhance the in-depth understanding of rainfall-runoff relation in the small headwater catchments. [ABSTRACT FROM AUTHOR]

Hydrology, to date, has confronted its developmental bottleneck due to lack of newmonitoring and experimental data for hydrological processes in watersheds. Fortunately, newconcepts and techniques have been emerging to better meet the demands in the acquisition ofnew monitoring and experimental data. In addition, some artificial experimentalcatchments or watersheds, whose boundaries and inner structures can be designed anddefined in advance, have been established to investigate hydrological processes underwell-controlled conditions. In order to clarify the mechanism and regularity ofwatershed hydrological cycle, this study designs hydrological experiments withcoupled natural and artificial system using two-direction multi-scale approach, i.e.upward and downward. Thus, hydrological experiments at Huashan Watershed inChuzhou City, representative of the low-altitude warm region in China, are designedand being performed by adopting new and advanced monitoring techniques andequipments. Within Huashan Watershed with a drainage area of 80.0 km2, two representativewatersheds are selected: Sanchahe Watershed (17.6 km2) and Huangwa Watershed (2.6 km2).Four smaller catchments act as critical zone experimental blocks, including WangyingCatchment (0.27 km2), Gaochong Catchment (0.08 km2), Nandadish Catchment (0.008 km2)and Hydrohill Catchment (artificial, 0.0005km2). These catchments or watersheds jointogether to form an excellent multi-scale experimental system, including artificial and naturalexperimental blocks. At the outlets of Wangying, Gaochong, Huangwa, Sanchahe, andHuashan watersheds, V-notch weirs grooves and hydrological cables have been combined toobserve total stream discharges. Hydrohill and Nandadish catchments have been wellinstrumented to automatically measure hydrological elements such as precipitation,discharge, soil moisture, groundwater level, and evapotranspiration with high precision. Inaddition to the above measurements of water quantity related hydrological elements,precipitation, stream water, soil water, and groundwater in Hydrohill Catchment,Nandadish Catchment, Sanchahe Watershed and Huashan Watershed are sampled atdifferent location through water sampling systems designed and manufactured byourselves to determine the δ2H, δ18O, EC, pH, DO, and concentration of mainions. Hopefully, the experimental work and results in this site may push forward the emergenceof new data, new discoveries and new ideas for the development of hydrology, especiallyhydrological models. Key words: hydrological experiment; Nandadish; Hydrohill; Chuzhou HydrologicalExperimental Site; coupled natural and artificial system Acknowledgements: This study is supported by the National Natural Science Foundationof China (No. 91647203, 91647111, 41330854 and 41371063) and the Science Foundation ofNanjing Hydraulic Research Institute (No. Y516027, Y516032 and Y517016). [ABSTRACT FROM AUTHOR]