| This study selects Heihe River Basin in the tableland-gully region of Loess Plateau as the study area, based on hydrogen and oxygen stable isotopes and hydrochemistry method, the different kinds of water samples were collected to research the conversion relationship between precipitation, surface water, groundwater and soil water. Variations in isotopic composition of precipitation were analyzed to investigate the vapor source of the Heihe River Basin, which can lay the foundation for the mechanism of water cycle. Combined with isotope analysis and chloride balance method, the mechanism of groundwater recharge in Loess Tableland was explored. Through the analysis of water chemistry and isotope composition characteristics of surface water and groundwater, the transformation relationship between surface water and groundwater was discussed, in order to analyze the water cycle mechanism in the tableland-gully region of the Loess Plateau futher. The main conclusions are as follows:1. The hydrogen and oxygen isotopic composition characteristics and vapor source were studied in detail.Both seasonal and inter-annual variations were detected for precipitation isotopic compositions. The isotopic compositions were depleted from July to November, while enriched from February to June. δD and δ18O had a significant relationship with precipitation amount and air temperature for June to September and for October to May, respectively.However, inter-annual variations existed due to the disturbance of climate variability.According to hydrogen and oxygen isotopic composition, meteoric water line, D-excess and17 O excess analysis, the marine air mass was the dominant vapor source of precipitation from June to September, and precipitation from October to May were significantly affected by local evaporation and continental air mass. In rainy season, A few precipitation events were directly from marine air mass, and the other events occurred only when the relative humidity exceeded90% due to the reallocation of marine air mass. At least 30% precipitation events have experienced serious secondary evaporation.2. The isotopic composition and chloride concentration of soil profile was studied indetail.Under different geographical location, land use pattern and sampling time, soil water contents were highly variable within the 0-4 m depth, but fluctuated around a constant value thereafter. The apple orchard had substantially smaller soil water contents(0.13 g g-1) at depth than those(>0.18 g g-1) of farmland and grassland. In some soil profile, the δ18O in deep soil water were richer than the up layer, but the other were poorer. Meanwhile, δ18O values in groundwater was lower than that in 10 m depth layer, which meant soil water was not the only source of the groundwater, episodic pulse of new water through macropore flow will likely be an important source of groundwater.3. The isotopic composition and hydrochemistry of surface water and groundwater were studied in detail.Stable isotope content of different water follows the precipitation, groundwater and surface water from low to high. The isotopic variation range of spring and well were basically the same, with a certain homogeneity. The isotopic composition of groundwater did not change with the temporal and spatial variation, and were poorer than that in surface water.Before the rainy season, water belonged to Na+-K+-Cl--SO42-and Na+-K+-HCO3-,but Na+-K+-HCO3- and Ca2+-Mg2+- HCO3- after the rainy season. Meanwhile, after the rainy season, HCO3- concentration in the river springs, wells and runoff increased significantly,which may cause by the precipitation recharge4. The mechanism of water circulation in the Loess Plateau gully regions was explored.Precipitation was the main source of soil water and groundwater, precipitation flows into deep soil by piston flow or preferential flow, and the preferential flow dominated. Meanwhile,the precipitation in August and September were the main source. The main source of the channel groundwater was the precipitation in rainy season, which mainly flowed by the form of preferential flow andthe groundwater played an extremely important role in the recharge of surface water in whole year.The main innovation of this research is to study interactions between precipitation,surface water groundwater and soil water systematically in tableland-gully region of Loess Plateau. Meanwhile, 17 O was used to analyze the characteristics of samples. On the basis of which, the chloride mass balance(CMB) was applied to estimate the soil water recharge from precipitation. |
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