Responses Of Stormflow To Rainfall In A Typical Karst Catchment Based On Stable Isotopes And Hydrogeochemistry

The special dual geological background in the karst area leads to hydrological processes significantly differs from the other types. Studying rainfall-runoff response is helpful to understand the supply mechanism of surface runoff and the characteristics of nutrient losses in order to provide a necessary basics for the small watershed water resources utilization and contaminant controlling. This paper takes typical karst small watershed in Huanjiang, Guangxi as an example. Based on in situ observation and tracing sampling, using the method of hydrogen and oxygen stable isotope and hydrogeochemistry, we have studied the seasonal dynamic variation on stable isotopic composition and hydrochemical process of streamflow during the rain, meanwhile, the influencing factors were analyzed. The main conclusions are as follows:(1) It existed uneven distribution of rainfall, and mainly concentrated on the rainy season (from 5 to 9 month), which accounts for more than about 75% of rainfall amount throughout the year. The streamflow existed positive correlation with rainfall amount. Monthly runoff coefficient of variation existed a large amplitude(0.05-0.45), the average accounting for 0.26. The stream did response to the rainfall promptly, however, it also exists certain hysteresis (1-2 h).(2) During a heavy rainy process, the responses of water stage (H), (EC), (T) and pH were not obvious during the first rainfall event (8.5 mm). The stream did not response to the rainfall until the rainfall amount reached to 35.5 mm. After that, all hydrochemical indexes, except for the nitrate nitrogen, began to rapidly change. Conductivity,?18O, and Ca2+ quickly dropping to the lowest values because of the dilution effect. The stream was mainly recharged by the supply of old water during the early and late rainfalls. However, the proportion of new water was up to 34% when focused only on the rapidly rising stage. During the two rainfalls, the concentration of nitrate nitrogen (NO3--N) still showed higher than its value before the rainfall event.(3) From a time scale perspective (diurnal scale, storm-scale, and seasonal scale), hydrochemical characteristics of a typical karst watershed including water stage (H), electrical conductivity (EC), temperature (T) and pH were studied based on in situ observation and indoor test from June 2013 to March 2014. Their influencing factors were analyzed. Diurnal variations were particularly obvious, which was probably attributed to the alteration of the main physiological activity of vegetation (shifting from photosynthesis in the day to respiration in the night). At storm-scales, the hydrochemical process was mainly controlled by number of consecutive rainless days and rainfall intensity, while diurnal scale effects were covered up. Seasonal hydrochemical changes (higher H, EC and lower pH in summer; lower H, EC and higher pH in winter) tended to co-vary with the trends of temperature and yearly rainfall distribution, both of which were the main factors influencing the stream hydrochemical changes. In addition, hydrochemical characteristics showed quicker response to rainfall in rainy season than in dry season(4) The meteoric water line in the study area revealed the equation: ?D=8.29518O+13.68(n=111, R2=0.9596), and hydrogen and oxygen isotope composition of precipitation showed seasonal variation, suggesting its values greater in dry season than the rainy season. Precipitation in dry season accounts for 31.29% in total rainfall amount.(5) Atmospheric precipitation was the only source of supply water in the study area, and the hydrogen and oxygen isotopic compositions of spring, streams and baseflow also showed obviously seasonal change, However, the variations happened reversely, the fluctuation range being larger. Analysis of the streamflow seasonal recharge characteristics showed, in addition to the direct recharge of precipitation, it was supplied by both spring and baseflow in the rainy season. However, it was mainly affected by the shallow groundwater which came from the rainy precipitation in the dry season.