| The launching of the project of 'Grain for Green' Project in Yanhe watershed makes the near-surface characters changed significantly,and caused the hydrological and sediment features variation consequently.The study of the response of runoff and sediment characteristics to the changed near-surface characteristics plays an important role in revealing the ecological significance of the‘Grain for Green'Project.This study analyzed the changes of rainfall,runoff and sediment transport characteristics before and after‘Grain for Green'Project,and the characteristics of surface vegetation cover and near-surface soil properties driven by the project.The surface hydrological process and erosion and sediment yield process were analyzed,and the response law of the variation of water and sediment in the watershed to the near surface characteristics through system analysis and research were revealed.The conclusions were drawn as follows:(1)The cultivated area of Yanhe watershed increased slightly between 1995 and 2000,and decreased partly after 2000,but it still accounted for about 40%of the total area of the basin.From 1995 to 2015,the land use types with large changes in the area of the Yanhe watershed were farmland and forest.The proportion of farmland decreased from 42.98%in 1995 to 39.93%in 2015,with the reduction of 3.05%.Furthermore,the proportion of forest land increased from 10.96%in 1995 to 13.95%in 2015,with an increase of 2.99%.The NDVI values of vegetation in Yanhe watershed experienced significant increase and decrease successively around 1991.After 1999,vegetation NDVI increased significantly except for 2002,2010 and 2014,and the increasing trend was particularly obvious during 2006-2008 and 2011-2014,indicating that the vegetation coverage has been significantly improved.After returning farmland,the high NDVI value area expanded from the southern edge of the basin to the central and southeastern parts,accounting for 28.83%and 18.61%more than that from 1982 to 1999.(2)Several years' measurement of conversion of farmland to forest had significantly changed the soil properties.After returning farmland to forest,the soil bulk density reduced by about 30%compared with the land abandoned for 35 years.The soil moisture content increased gradually,which reached to 1.2 times in 35 years of abandoned land.PH values first increased and then decreased with the prolongation of returning farmland.The effects of returning tillage on the available nutrients were different,the values of nitrate nitrogen and available phosphorus decreased first and then increased,while the effects of returning tillage on ammonium nitrogen were not significant.The contents of soil organic carbon and total nitrogen increased with the age of conversion,while total phosphorus decreased gradually,and the contents of C:P and N:P also increased significantly,but the contents of C:N did not change significantly.(3)With the increase of abandoned time,the infiltration capacity of the soil enhanced obviously,and was improved over time,especially after being abandoned for 30 and 40 years.Moreover,the infiltration capacity was improved but not significantly at the early stage.The vegetation surface canopy coverage increased,the root system increased,and the soil separation rate decreased with the increase of returning time.Soil separation rate is positively correlated with soil bulk density and negatively correlated with organic matter content,aggregate content,root system and other parameters,which ultimately depended on soil organic matter content.Slope erosion experiments under different vegetation coverage indicated that when coverage increased,the overland flow velocity decreased,the resistance coefficient and roughness coefficient increased,the runoff shear stress increased,the runoff kinetic energy and the unit flow power decreased,and the erosion sediment yield decreased.(4)Between 1960 and 2017,the annual flood season precipitation,runoff,sediment transport and runoff coefficient of Yanhe watershed showed a downward trend,but the variation trend of precipitation and runoff coefficient was small,while the runoff and sediment transport presented an obvious downward trend,especially the sediment transport varies greatly.Sediment transport in flood season accounts for 97.5%of annual sediment transport,and sediment transport in the watershed mainly came from rainfall in flood season.Compared with the period before farmland converted to forests(1960-1999),the average annual rainfall and flood season precipitation changed little,the mean runoff,the mean transporting sediment and the average runoff coefficient of the annual flood season decreased significantly,the average flood season runoff decreased 31%and 41%respectively,and the average sediment discharge of annual flood season reduced by 75%and 75%,and the average flood season runoff coefficient decreased 30%and 42%respectively after returning farmland(2000-2017).The returning farmland has a significant sediment reduction effect on sediment transport of Yanhe watershed.(5)The change of surface cover obviously influenced the total runoff and river runoff.The change of land use in Yanhe watershed basin,especially the expansion of farmland and forest land area year by year,increases the change of runoff in the basin.After 1990,the river basin runoff showed a trend of significant decrease due to a sharp rise in vegetation cover,while in 2010,it showed a trend of temporary increase due to a temporary decrease in vegetation cover,and in 2018,the whole basin runoff was relatively small.The spatial distribution of runoff in the basin is affected by the spatial variation of vegetation cover,which shows a trend of gradual decrease in the middle and southeast.The hydrological process in the river basin can be significantly changed by measures such as returning farmland to forest(grass).Since the implementation of the 'Grain for Green' project in 1999,human activities have significantly reduced the sediment yield in the watershed.Based on the study of surface hydrological process,erosion and sediment yield process and water and sediment change of typical hydrological stations in Yanhe watershed basin before and after returning farmland to farmland,the paper interpreted the potential impact of near surface characteristic change on watershed and sediment change of Yanhe watershed,and provided a scientific reference for objectively understanding the ecological effect of 'Grain for Green' Project in Loess hilly and gully region. |
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