Abstract
Few of the classical field studies of streamflow generation in headwater watersheds have been conducted in catchments with thin soils and deeply weathered crystalline silicate bedrock. As such, the role of the (potentially very large) storage capacity of weathered, fractured rock in baseflow and storm event discharge remains poorly characterized. Here we present a study of streamflow generation in an upland semi-humid watershed (Xitaizi Experimental Watershed, XEW, 4.22 km2) dominated by baseflow feeding one of the main water supply reservoirs for the city of Beijing, China. This catchment is relatively dry (625 mm/yr precipitation, 480 mm/yr Evapotranspiration), but has strongly seasonal precipitation that varies in phase with strongly seasonal potential evapotranspiration. The catchment was instrumented with four weather stations and precipitation collectors, 11 deep wells drilled into the bedrock along three hillslopes, and additional soil moisture sensors and water samplers along one hillslope. In six storm events over two years, samples of rainfall, soil water (10–80 cm depth), groundwater, and stream water were collected with high frequency and analyzed for stable water isotopes (δ18O and δ2H). Tracer-based hydrograph separation showed that event water (precipitation) makes up the majority of the hydrograph peak above baseflow, and pre-event water contributions (on average) simply represent the steady release of groundwater. The quantity of event water corresponded to a very small effective contributing area (<0.2% of the catchment) that nevertheless showed a clear dependence on catchment wetness as measured by the streamflow. The streamflow itself was isotopically identical to the deep groundwater in wells. This suggests that the fractured, weathered, bedrock system dominates the production of streamflow in this catchment.
Highlights
A solid understanding of rainfall-runoff processes in headwater catchments is necessary for the appropriate management of surface water and groundwater resources [1]
On the basis of intensive hydrological and isotopic observations under base flow conditions and in six rainfall events, we examined streamflow generation in a baseflow-dominated semi-humid catchment underlain by deeply weathered granite and shallow soils
Stream water isotopic composition at baseflow is nearly identical to deep groundwater sampled from wells in the weathered, fractured granite
Summary
A solid understanding of rainfall-runoff processes in headwater catchments is necessary for the appropriate management of surface water and groundwater resources [1]. Isotope-based Hydrograph Separation (IHS) [2,3,4,5] and transit time distribution analyses [6,7,8] in order to characterize and conceptualize catchment behavior [9]. The results of such analyses must be interpreted in the light of other hydrologic and hydrometeorological data, as well as other information on the soils, bedrock, and morphology of the landscape [10,11,12,13,14,15] in order to test hypotheses about the nature of runoff generation in the watershed. Benchmark studies have been conducted in the Panola Mountain Research
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