Study On Land Use Change And Soil Conservation Function In Yan'an City Based On InVEST Model Application

The soil erosion and the soil conservation function of the ecosystem are not only closely related to the rainfall intensity,soil properties,topographic conditions,but also affected by human activities and other natural factors.For Yan'an city,the implementation of large-scale Grain for Green Project has significantly affected local land use and vegetation cover conditions,thus causing changes in the amount of soil erosion and conservation in the region.This paper analyzes the land use change in Yan'an city in the past 15 years,and on this basis,adopts the"sediment delivery model"in The Integrate Valuation of Ecosystem Services and Tradeoffs Tool.The amount of soil erosion,the function of soil conservation service and the value of soil erosion in2000 and 2015 before and after Grain for Green Project in Yan'an City were evaluated.The effects of land use change on soil erosion and soil conservation service function were also discussed.The conclusions are as follows:(1)From 2000 to 2015,the main types of land use in Yan'an city are woodland,grassland and farmland,which account for more than 90%of the total area of the city.During the past 15 years,significant changes have taken place in all land use types,with the largest increase in the area of woodland and a significant decrease in the area of cultivated land.The forestland is mainly distributed on gentle slopes and steep slopes above 10°.In the past 15 years,the area of woodland has increased in all slope segments and has increased most significantly on steep slopes more than 25°.The distribution and variation of grassland on each slope is consistent with that of woodland,and the area of cultivated land in each slope is reduced,and almost all of them are returned to farmland on steep slopes larger than 25°.Most of the urban land is distributed below15°,the area ratio of each slope segment has not changed much in the past 15 years,but the area of each slope section is in an increasing state.The area of water area is concentrated in the downhill section of 10°,and the area of each slope section is increased.In the past 15 years,the conversion of cultivated land to forest was more frequent in Yan'an city.The conversion of farmland to forest and grassland to forest land were mainly carried out step by step.(2)The total soil erosion in Yan'an city in 2000 and 2015 was 220.06×10~6 t and138.37×10~6 t,respectively.The spatial distribution of soil erosion intensity is obvious.The intensity of micro-erosion and slight erosion are mainly distributed in the low hilly region of southern Yan'an city and the loess-gully landform area.Polar intensity and severe erosion are mainly distributed in the Loess Hilly and Gully region in the Northeast and Northwest of city.The intensity of soil erosion in Yan'an city increases with the increase of slope,but in the past 15 years,the intensity,extreme intensity and intense erosion have decreased in varying degrees and the area has been concentrated in steeper slope segments.It is shown that the key and difficulty of controlling soil erosion are mainly on steep slopes greater than 25°.During the past 15 years,the area of different soil erosion types of 18792.22km2 has changed,accounting for 51.21%of the regional area.As a whole,each erosion intensity type changed to low grade erosion intensity type in 15 years.The decrease of farmland area plays a leading role in the decrease of intensity,extreme intensity and intense erosion area,while the increase of woodland and grassland area is the main factor of increasing the area of micro-erosion and slight erosion in the region.The change of grassland area also plays a leading role in increasing the area of moderate erosion in the region.(3)The total soil conservation in Yan'an city in 2000 and 2015 was 772 million t and 850 million t respectively.The soil conservation intensities of each sub-basin are concentrated in the range of 100 t/hm~2~200 t/hm~2 and 200 t/hm~2~300 t/hm~2.The sum of the two accounts for about 80%.In addition,the area of higher soil conservation(>200 t/hm~2)increased in different degrees during 15 years of soil conservation.The distribution of soil conservation function in Yan'an city showed an increasing trend from northeast to southwest,and the higher values appeared in the regions with lower soil erosion intensity in the southwest and areas with higher soil erosion in the northwest of city.Soil conservation and soil conservation per unit area of Yan'an city increased with the increase of slope.In 2000 and 2015,the value of soil conservation function in Yan'an city was 86.163 billion yuan and 94.045 billion yuan respectively.The average value of soil conservation function per unit area of each district and county was 2.43yuan/square meter.Among the woodland types,grassland had the largest soil conservation value per unit area.The mean value was 2.73 yuan/square meter and 2.43yuan/square meter respectively.The soil conservation amount per unit area of each area is woodland>grassland>farmland>water area>town>bare land,and the modulus of soil conservation has increased to some extent in the past 15 years.The soil conservation effect of woodland and grassland is the best,all above 200t/hm~2.In the regions where land use types change,the transfer of grassland to woodland and farmland,and the transfer of cultivated land to forestland and grassland are the main driving forces and factors to increase the function of regional soil conservation.