A look inside 'black box' hydrograph separation models: a study at the Hydrohill catchment

Runoff sources and dominant flowpaths are still poorly understood in most c atchments; consequently, most hydrograph separations are essentially 'black box' models where only external information is used. The well-instrumented 490 m(2) Hydrohill artificial grassland catchment located near Nanjing (Ch ina) was used to examine internal catchment processes. Since groundwater le vels never reach the soil surface at this site, two physically distinct flo wpaths can unambiguously be defined: surface and subsurface runoff. This st udy combines hydrometric, isotopic and geochemical approaches to investigat ing the relations between the chloride, silica, and oxygen isotopic composi tions of subsurface waters and rainfall. During a 120 mm storm over a 24 h period in 1989, 55% of event water input infiltrated and added to soil water storage; the remainder ran off as infil tration-excess overland flow. Only about 3-5% of the pre-event water was di splaced out of the catchment by in-storm rainfall. About 80% of the total f low was quickflow, and 10% of the total flow was pre-event water, mostly de rived from saturated Row from deeper soils. Rain water with high delta O-18 values from the beginning of the storm appeared to be preferentially store d in shallow soils. Groundwater at the end of the storm shows a wide range of isotopic and chemical compositions, primarily reflecting the heterogeneo us distribution of the new and mixed pore waters. High chloride and silica concentrations in quickflow runoff derived from event water indicate that t hese species are not suitable conservative tracers of either water sources or flowpaths in this catchment. Determining the proportion of event water a lone does not constrain the possible hydrologic mechanisms sufficiently to distinguish subsurface and surface flowpaths uniquely, even in this highly controlled artificial catchment. We reconcile these findings with a percept ual model of stormflow sources and flowpaths that explicitly accounts for w ater, isotopic, and chemical mass balance. Copyright (C) 2001 John Wiley & Sons, Ltd.