植被覆盖度和降雨侵蚀力变化对小流域泥沙连通性的影响

泥沙连通性可以反映泥沙源汇的潜在联系，识别流域水土流失热点区域及泥沙迁移路径。研究泥沙连通性的影响因素有助于更好地理解泥沙连通性的时空变化特征。该研究在已有泥沙连通性指数（Index of Connectivity，IC）模型基础上，考虑影响连通性的功能性因素，并采用修正泥沙连通性指数（Revised Index of Sediment Connectivity，ICr）探讨了植被覆盖度和降雨侵蚀力耦合作用下的季节与年际变化对天目湖中田舍河流域泥沙连通性特征的影响。结果表明：2019年夏冬季植被覆盖度分别为85%、57%，对应的泥沙连通性指数均值是-9.39、-6.85，植被覆盖度变化对泥沙连通性具有重要影响，利用NDVI值获取模型中的地表综合系数，可以动态反映地表植被和土地利用的区域及季节性变化；降雨影响泥沙的功能连通性，年尺度上的连通性指数均值同流域泥沙量的相关系数达0.91。说明在流域植被覆盖变化不明显时或者在林地为主的流域中，降雨因子具有主导作用；植被覆盖度升高28%，IC均值降低37%，而单独考虑降雨因子的IC-R均值则反映出雨量升高，指数值随之升高，修正连通性指数ICr综合考虑了植被与降雨因子，但在应用中要依据流域实际情况适当调整两者的权重。研究结果指出泥沙连通性指数在中国东南部区域运用中存在的问题，将对气候变化背景下我国湿润区湖泊小流域水土保持与水环境治理等提供科学参考。

Effects of vegetation coverage and rainfall erosivity changes on sediment connectivity in small watersheds

Abstract: Sediment connectivity can be an indicator to identify the erosion hotspots and sediment migration paths, particularly for the potential linkage of sediment sources and sinks in watersheds. It is a high demand to determine the influencing factors for the spatial and temporal variability of sediment connectivity. In previous studies, the functional factors are simply added to calculate the connectivity index, but it is still lacking the difference between the connectivity indexes rich in functional factors and the original ones. At the same time, the connectivity index is mainly concentrated in the Loess Plateau in China, where the time scale is also dominated by the annual scale. The vegetation, terrain, and climate conditions vary greatly in different regions. For instance, the Loess Plateau belongs to the continental monsoon climate, most of which are semi-arid areas dominated by bare land and gullies. By contrast, the study area of watersheds belongs to the subtropical monsoon climate with an average annual rainfall of 1291mm for many years rich in vegetation and surface types. Moreover, the slow change of vegetation can be mainly determined by human factors on the annual scale, while there are four distinct seasons for the vegetation on the seasonal scale. Therefore, it is necessary to clarify the sediment connectivity in the different regions on the various time scales. In this study, the revised index of sediment connectivity (ICr) was adduced to explore the influence of seasonal and inter-annual variation of vegetation coverage and rainfall erosivity on the sediment connectivity considering the functional factors. The study area was selected as the Zhongtianshe River watershed in the Tianmu Lake. The connectivity index was firstly calculated in the summer and winter of 2019, according to the formula of the original sediment connectivity index (IC) model. The data show that the fluctuation range of IC value was from -12.71 to 0.09, with the lowest IC value in summer and the highest in winter. The weighting coefficient (W) in the original IC was directly replaced by the Rainfall Erosivity Factor R to obtain the index IC-R, in order to verify the introduction of rainfall erosivity. There was a strong correlation with the measured sediment volume of the silver carp bridge sub watershed (P = 0.85). Finally, the improved index ICr was obtained to couple the vegetation and rainfall factors. The mean value of ICr and sediment demonstrated the same change trend (P = 0.91) for the inter-annual connectivity index in 2016, 2018, 2019, and 2020, although there was no outstanding change for the seasonal changes within the year. The results show that the vegetation coverage values were 85% and 57% in the summer and winter of 2019, respectively, where the mean values of sediment connectivity index were -9.39 and -6.85, respectively. It infers that there was an important influence of vegetation coverage on the sediment connectivity. The Normalized Difference Vegetation Index (NDVI) was used to obtain the surface integrated coefficient in the model, which dynamically represented the regional and seasonal changes in surface vegetation and land use. There was also a significant influence of rainfall on the functional connectivity of sediment, where the correlation coefficient was 0.91 between the mean connectivity index on an annual scale and the sediment yield in the basin. Especially, a dominant role of rainfall factor was found, when the less vegetation coverage in the watershed or woodland. The vegetation coverage increased by 28%, whereas, the IC mean value decreased by 37%, indicating a similar trend in the IC-R mean value and rainfall. when considering the rainfall factors alone. The weights of the vegetation and rainfall factors in the improved connectivity index ICr can also be expected to adjust appropriately in the application for the actual situation of the watershed. Consequently, the sediment connectivity index can be feasible in the southeastern region of China. The finding can provide a scientific reference for soil and water conservation and water environment management in the small watersheds of lakes in humid areas under the background of climate change.