Understanding of Storm Runoff Generation in a Weathered, Fractured Granitoid Headwater Catchment in Northern China

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.

Climatic and anthropogenic drivers of a drying Himalayan river

“…In this case, seepage would be increasingly likely to occur via slow drainage processes rather than macropore flow activated in large storms. Such changes have been observed in other karst catchments (Zhao et al, 2019) and are supported by the evidence that the quickflow declined and the baseflow index increased in the most upstream gauge in the Upper Jhelum watershed (Fig. 9).…”

“…For instance, a reduction in precipitation would decrease the water balance inputs and reduce the water available to generate streamflow. Alternatively, an increase in storm frequency and a reduction in average storm size could produce an increase in vadose zone water storage, greater evapotranspiration (ET), and reduced streamflow (Zhao et al, 2019), even if aggregate precipitation volumes remain unchanged. We, therefore, pose the following hypotheses:…”

Groundwater connectivity of a sheared gneiss aquifer in the Cauvery River basin, India

“…Where groundwater is not exploited, crystalline-rock aquifers can contribute significantly to baseflow (e.g. Kosugi et al 2006;Cai and Ofterdinger 2016;Comte et al 2019;Zhao et al 2019); however, as groundwater is increasingly abstracted and water levels fall, flow can become compartmentalised, depending on the presence and nature of fractures at depth (Perrin et al 2011b;Guihéneuf et al 2014;Alazard et al 2016).…”

Field-Based Analysis of Runoff Generation Processes in Humid Lowlands of the Taihu Basin, China

“…These cases in the upper left part (Figure 7a) show that shallo er table conditions meet extreme rainfall events, which would result in great rapid flows ke this area vulnerable to flashfloods [60]. The value of the soil water storage capacity was considered as the realistic antecedent isture condition, which is usually obtained from a proxy by other data like the antece cipitation index [61] for the difficulties of direct measurements in large scales. Good li tionships are presented in Figure 8 between the SWSC and initial groundwater depth in H BTQ, with the R 2 reaching 0.85 and 0.84, respectively, which reflected the strong interact een the shallow groundwater and soil water.…”