Impact Of Land-use And Vegetation Change On Hydrological Connectivity In Xinshui River Basin

Since the implementation of small watershed comprehensive management and returning farmland to forest(grass)project in the Loess Plateau,significant changes have occurred in runoff sediment transport in the basin.The change of vegetation cover largely explains the change of sediment in the basin.However,more studies only focus on the changes in the area and distribution of vegetation cover,while ignoring the changes in the hydrological path caused by changes in vegetation cover.Index of 1 connectivity is an important way to explain the possibility of sediment transport in the inner diameter of the entire basin.Understanding the spatial distribution and dynamic changes of index of connectivity in the basin can provide a basis for predicting the potential for runoff and sediment transport in the basin.In this paper,the Xinshui River Basin in the hilly and gully loess region is taken as the research object.Based on the analysis of land use and cover change in different time periods(1986,1990,1995,2001,2005.2009.2015),with the watershed Slope-HRU index(the "source-sink" index established by taking the hydrological response unit as the landscape unit,taking into account the influence of the slope unit size,land use type and soil erodibility of the landscape unit on the process of water and soil loss)and IC(index of connectivity),explores the ffects of land use/cover change and soil factors on the index of connectivity of the basin;and combines the observation of export runoff and sediment during the observation period to further explore the impact of changes of IC on runoff and sediment yield in the basin:and compares the role and ability of Slope-HRU index and IC in quantitatively revealing the water and sediment transport in the basin.The research results are helpful to understand the impact mechanism of land cover change on runoff erosion and sediment yield,and provide a basis for basin land use planning and vegetation restoration project construction,and effectively promote the ecological restoration process of' hilly and gully regions on the Loess Plateau.The main findings are as follows:(I)During the 29 years from 1986 to 2015,the structure of various land use types in the Xinshui River Basin changed significantly.The most significant land types were shrub land,residential and construction land and cultivated land.Residents' living and construction land and shrub land increased significantly,from 153.82km2 to 452.59km2 and 191.64km2 increased by 417.61km2 from 1986 to 2015 respectively;the cultivated land decreased year by year,from 272.14km2 to 23.75km2.The cultivated land in the Xinshui River Basin is mainly converted into grassland and shrub land;the grassland is transformed into shrub land and broad-leaved forest land;the shrub land and broad-leaved forest land transform each other;cultivated land and grassland are the main sources of growth of residents' living and construction land.(2)Between 1986 and 2015,the connectivity of the basin changed drastically with the change of land use cover.The area of IC increased in the basin exceeded 50%,and area of IC decreased in the basin was only 30%.The overall connectivity of the basin increased,but there are still 58%of area where IC is less than zero.Coniferous forest,broad-leaved forest,shrub forest and grassland area have more than 45%of area where IC is less than 0,having better ability of water and sand interception,and the area where IC is less than 0 in the broad-leaved forest coniferous forest area is more than 75%.It plays a vital role in water and soil conservation in the basin.(3)Both Slope-HRU index and IC can preferably characterize the water and sediment transport capacity of the basin.The Slope-HRU index can only collectively characterize the water-sand transport capacity of the basin.Although it is more accurate,it can not reveal the role of various land use and cover in water and soil conservation in the basin.IC not only has better characterization effect on small scale,but also can show the role of each landscape played in the process of water and sediment transport,and it can also collectively characterize the capacity of water and sediment transport of the basin.(4)Studies have shown that coniferous forests,broad-leaved forests,shrubs,and grasslands have good effects on soil and water conservation in various soil types.The IC of coniferous forest in the soil types of loessial soil,fluvo-aquic soil and alluvial soil,is generally lower than that of broad-leaved forest land,and is obviously lower than that of grassland and shrub land.Broad-leaved forest has the best water-sand interception effect in the soil types of calcareous coarse soil,neutral coarse soil,neutral stony soil,new soil and coarse soil.Shrubs have the best water-sand interception effect in the soil type of neutral stony soil.The grassland has the best water-sand interception effect in the soil types of coarse soil and new soil,and is better than shrub forest.In this paper,by analyzing the influence of soil on the interception ability of different vegetation cover in water and sediment,and the correlation between the IC of different vegetation types and runoff and sediment yield,it is concluded that the correlation between the IC of bush and broad-leaved forest and river basin runoff and sediment yield is the best,and the areas of bush and broad-leaved forest have lower IC and have good ability in water and sand interception under various soil conditions.