Evaluating Runoff Generation in a Humid Bamboo Watershed Using Isotopic and Hydrochemical Tracer

Abstract

Most of the studies in humid catchments have discovered a similar conclusion that runoff generation was dominated by overland flow and interflow. However, the Hemuqiao watershed with long-term hydrological data which is located in the lower reach of Yangtze River shows a different runoff generation mechanism by comparing different tracers on the hydrograph separation analysis. The mechanisms of the runoff generation process from rainfall to streams during the typhoon events are studied by using a mixed method in the Hemuqiao watershed. The oxygen-18 (δO18) isotope ratios and electrical conductivity (EC) of precipitation in four storm events are significantly different from those in the river and the baseflow, allowing two-component hydrological separation to determine the contribution of the event and pre-event water. Both EC and stable isotopes give qualitatively similar results that the pre-event water component accounts for the majority of the hydrograph during both typhoon events (≥60%). This is primarily affected by rain intensity and the antecedent moisture condition. The EC method may slightly overestimate the proportion of pre-event water than the δO18 method. This may be due to the different factors that affect the variation of EC and stable isotopes and influenced by neglecting the soil water recharge to the stream. The value of the isotopic composition of streamflow and surface water are generally distributed along the local meteoric water line, which indicates that the main source of water in the Hemuqiao watershed is supplied by precipitation. The EC value of water samples collected from precipitation, streamflow, surface water, soil water, and groundwater is shown to be different because of the water–soil contact time and characteristics of soil layers. According to isotopic–hydrochemical tracer analysis, we clearly see that the water which is stored in soil and groundwater controls the value, lag time, and the composition of EC and isotopic of the streamflow in typhoon events.