| In this study, the seasonal variation of isotopes of rain water were studied by using stable isotope techniques, illustrated by the case of Dagouhe and Yanwahe watersheds in Xiaolangdi Reservoir of Yellow River, Jiyuan, Henan. The precipitation redistribution and the feature of soil water movement of typical dominant trees species of Quercus variabilis and Platycladus orientalis were tested at the stand scale. According to the principle of water balance, the rainfall threshold of surface runoff in different forest stands were discussed. We also tried to explain the relationship among rain water-surface water-groundwater, and evaluate the recharge sources of the surface runoff at watershed scale. The aim of this study was to reveal the regulatory mechanism of forest vegetation on rainfall runoff and to evaluate quantitatively the conversion relationship of watershed water resource and recharge sources of the runoff in Xiaolangdi Reservoir of Yellow River, Jiyuan, Henan. The main findings are as follows:(1) The ?D and ?18O of rain water exhibited an extremely wide range and decreased from spring through fall. The ?D values showed a significant negative correlation with the mean air temperature from April to October, but showed no obvious correlation with seasonal rainfall. The ?D and ?18O values showed significant negative correlations with the precipitation from April to October and at autumn as well. The Local Meteoric Water Line(LMWL) was: ?D=7.19?18O-1.29, the slope of LMWL and d value of precipitation were low in summer, and they were the maximum in autumn. According to the air mass back trajectories model of HYSPLIT, water vapor source in summer were from southeast and southwest maritime monsoon air masses, and the precipitation in spring and autumn were influenced by the continental and oceanic monsoons air masses. Therefore, identifying the characteristics of the seasonal precipitation fluctuations was the important prerequisite for the study of the isotope hydrologic process.(2) There were no significant differences between the ?DS value of stemflow from Platycladus orientalis and Quercus variabilis of different densities, it was the same with the ?DT value of throughfall from different forest stands. The ?D value of rain water had significant correlations with both ?DS and ?DT, moreover, it correlated better with the latter. When the precipitation was equal to or greater than 14.2 mm, the difference values( ?D) between ?D and ?DS, ?D and ?DT were more than 0. On the contrary, when the precipitation was less than 14.2 mm, the value of ?D were less than 0. This results showed that the ?D value of stemflow and throughfall were not only influenced by evaporation, but also by the synthetic action of vegetation structure and all sorts of environmental factors(temperature, humidity, evaporation and so on).In the Platycladus orientalis stand and medium and high density stands of Quercus variabilis, the rainfall rapidly infiltrated to the deeper soil in the rainy season and the early dry season with previously higher soil moisture. An exception was that the water infiltrated to the middle soil layer in the high density stand in the middle of rainy season with high soil moisture. However, in the prior low soil moisture condition, the residence time of rainfall was shorter in the litter and the soil surface. In the low density stand, the residence time of rainfall were shorter in the litter and the soil surface, then the conserved water in upper layer slowly infiltrated to the deep soil through the piston flow. Furthermore, the mixed water can form interflow through the lateral flow after previous plentiful rainfall events. Therefore, the Platycladus orientalis stand and the medium density Quercus variabilis stand can store more soil water in the low humidity conditions, and the high density stand can regulate the soil moisture in the low or high environment, but the low density stand had poor ability to store water. Moreover, after prior plentiful or high intensity rainfall events, the preferential flows can rapidly infiltrated and supplied to deeper soil water in the low and medium density of the Quercus variabilis stand. The ?D of spring and ground water did not change obviously, it was affected slightly by short-term precipitation.(3) The total canopy interception of Platycladus orientalis was higher than Quercus variabilis. And it increased with the stand density of Quercus variabilis, the values were 109.5, 114.8 and 117.4 mm in the low, middle and high density, respectively. The interception rate decreased with the increase of precipitation and became stable when the rainfall came to a certain amount. The interception had significant positive correlation with precipitation, and the relationship fitted to power function.The total litter interception of Platycladus orientalis was lower than Quercus variabilis, and the values increased with the increase of stand density(57.5, 78.5 and 82.9 mm, respectively). The interception had significant positive correlation with precipitation, and the relationship were logarithmic function.The ability of maximum water absorption in Platycladus orientalis and low density Quercus variabilis stands were weak, while they were strong in the middle and high density Quercus variabilis stands, the average values were 22.4, 22.4, 32.8 and 33.0 mm, respectively. In addition, the water absorption was higher in dry season, and was lower in rainy season.The seasonal variation ranges of rainfall threshold of surface runoff in Platycladus orientalis stand and low, medium and high Quercus variabilis stands were 11.7-39.1, 14.3-35.9, 29.3-49.5 and 30.2-49.6 mm, and the average values were 30.7, 29.7, 43.6 and 44.2 mm, respectively. The rainfall threshold of Platycladus orientalis and low density Quercus variabilis stands were lower than that in the middle and high density Quercus variabilis stands, which showed that the middle and high density Quercus variabilis stands was better on conserving water and soil resources. It required more precipitation to runoff in dry season, while less in the rainy season.(4) The p H of the surface water, shallow and deep groundwater were stable in different seasons, the average value were 8.4, 7.6 and 7.9, respectively. In addition, the surface water and shallow groundwater can interchange after a heavy rain. The seasonal change of the TDS of surface water was significant, and the TDS decreased along upstream to downstream of the river. Whereas the TDS of shallow and the deep groundwater differed slightly among seasons and they had minor change along the river. The mean TDS values of the surface water, shallow and deep groundwater were 408, 471 and 444 mg/L, respectively. The surface water and groundwater interchanged frequently after heavy rains. The EC value of different water bodies had significant difference among seasons, and the mean value of the surface water, shallow and deep groundwater were 602, 656 and 672 ?s/cm, respectively. The EC and TDS in the same water body had positive correlation, while the relationship was not obvious in different water bodies, which indicated that different water bodies had different water resources. The EC value of surface water varied significantly along the river, on the contrary, it varied slightly in the deep groundwater. According to TDS and EC values of different water bodies, it can be concluded that the surface water were recharged by both precipitation and groundwater.The ?D value of rain water decreased with seasonal turnover, and the change trend of the surface water and shallow groundwater were the same as the precipitation. It indicated that they were recharged by precipitation. The ?D value of surface water increased along downward the river in spring and summer, while it was almost unchanged during autumn. The fluctuation range of ?D value in shallow groundwater was lower than that of surface water, and the former fluctuated with the latter, which showed that there was a close hydraulic relationship with each other. The fluctuation range of ?D value in deep groundwater was small, which suggested that the ?D value of deep groundwater was influenced slightly by precipitation and was almost constant along the river.The relationship line of ?D-?18O in the surface water, shallow and deep groundwater were ?D=5.21?18O-15.40, ?D=5.52?18O-13.96 and ?D=5.77?18O-11.59, respectively. The ?D and ?18O of the surface water and shallow groundwater were scattered distributed on both sides of the LMWL, which indicated that they were recharged by precipitation. While the distribution of ?D and ?18O of the deep groundwater was concentrated. The variation range of the surface water was less than precipitation, which indicated that the surface water were recharged not only by precipitation but also by other water resources. The slope of ?D-?18O relationship line of different water bodies increased in order and all lower than that of LMWL, which indicated that the precipitation may go through evaporation before it recharged other water bodies, and deep groundwater was less affected by evaporation. The slope of surface water was close to the shallow groundwater, indicating a significant relationship between them.(5) The contribution rate of precipitation and shallow groundwater to the river had large fluctuations, which were 20.1%-81.2% and 8.3%-47.6%, respectively. Moreover, they showed a negative relationship. The rate of deep groundwater had little fluctuation, and the range was 10.6%-35.0%. The contribution rate of precipitation to the river had no significant correlation with precipitation, which suggested that the rate was caused multiply by the precipitation, river runoff and environmental factors. During the spring, because of small runoff and drought, the precipitation contribution rate was 24.6% even if there were heavy rainfall. As rainwater had to firstly recharge the dry soil. Whereas the rate of shallow and deep groundwater were 45.7% and 29.7%, respectively. In summer, the rate of precipitation rose to 47.0% because of the increase of precipitation and runoff, while the rate of the shallow and deep groundwater were 37.6% and 15.4%, respectively. In autumn, the precipitation was 266.3 mm in September which made the runoff rapidly increased, which made the rate of precipitation up to 81.2%. In October, the deep groundwater was sufficiently recharged by the previous plentiful precipitation, so the rate of the groundwater was 34.7% and that of the precipitation reduced to 30.6%. |
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