Response Of Soil And Water Loss To The Change Of Land Use Pattern In The Xunwu Watershed

Soil and water loss is one of China’s top environmental problems,and the southern red soil region is one of the serious soil and water loss region next to the loess plateau in China,a series of soil erosion,Soil and water loss and its induced related environmental problems have severely hindered the sustainable development of social and economy in the area,so it is an important task for soil and water conservation work to explore the law and mechanism of soil and water loss in the southern red soil region and to seek ways and methods to prevent and control soil erosion.The land use change caused by the combined action of human activities and natural factors has an important impact on soil and water loss.The research on the relationship between them is helpful to control soil and water loss and protect the ecological environment.The watershed,as a natural spatial complex representing the utilization of human soil and water resources and material migration,is the best unit to study the relationship between land use pattern and soil and water loss.Therefore,this paper takes the Xunwu watershed in the southern red soil hilly region as the research object,analyzed the land use change in the valley,the landscape pattern evolution and the characteristics of soil and water loss from 1990 to 2016,explored the soil and water loss and environmental effects caused by the change of land use pattern.Therefore,it provides a theoretical basis for soil and water conservation and environmental protection in the southern red soil region.The main conclusions are as follows:(1)During the study period,orchard land(increased by 41.33%)and forest land(decreased by 36.82%)in the Xunwu watershed changed the most,followed by cultivated land(decreased by 6.26%),water body(decreased by 0.18%)samllest.The “rising trend”of orchard land and “falling trend” of forest land are very obvious,the construction land grew relatively fast,and the cultivated land and unused land showed a small decline trend.Land use change is frequent with one-way characteristics and revealed as an unbalanced state.(2)The land use types in Xunwu watershed are mainly forest land,orchard land and cultivated land.The controlling effect of forest land landscape component on the watershed was weakened and the later orchard land became the dominant landscape component in the watershed.There was a high degree of agglomeration of landscape types,and the landscape ecological process was more active in the Xunwu watershed during the study period.With the increase of human disturbance intensity,the landscape diversity index increases,the fragmentation degree further strengthens,the landscape heterogeneity increases,and the complexity and variability degree of watershed landscape increases.In a word,the landscape pattern of Xunwu watershed is evolving towards the direction of low connectivity,fragmentation and high heterogeneity.(3)The average annual precipitation and runoff depth in Xunwu watershed are respectively 1585.93 mm and 862.20 mm,which show a downward trend in general and a more significant downward trend in runoff depth.Combined with the analysis of precipitation-runoff accumulation curve,the abrupt change points of precipitation and runoff were determined to be 1997,so the study period was divided into two periods: 1980-1997 and 1998-2016.Among them,1980-1997 is the base period,and 1998-2016 is the period that the change of land use pattern has a great impact on human activities.By analyzing the influence of precipitation and land use change on runoff and sediment,it is found that land use change has more significant effect on runoff and sediment production.(4)The change of land use pattern has an important influence on the process of runoff and sediment production.According to empirical statistics,on the annual scale,the contribution rate of precipitation and land use change to runoff reduction was 249.07% and-149.07% respectively.The main source of runoff is precipitation,the average precipitation from 1998 to 2016 decreased by 3.91% compared with that from 1980 to 1997,while the average runoff depth decreased by 5.66%.The precipitation change contributes greatly to runoff reduction,while the change of land use pattern in the basin plays a role in increasing runoff.On a seasonal scale,precipitation changes in spring and summer play a dominant role in runoff reduction,with contribution rates of 64.73% and 78.37%,respectively,while land use change is the main driving force for runoff reduction in autumn and winter,with contribution rates of 105.33% and 180.81%,respectively.This is owing to runoff in the rainy season is more responsive to rainfall in the current season,while in the dry season,land use change has a more prominent impact on runoff.At the same time,the slope of the double cumulative curve of runoff depth and sediment volume from 2013 to 2016 is higher than that from 2009 to 2012,that is,under the same precipitation conditions,the sediment production capacity will be stronger in the later period,and the change of land use pattern in this period will increase the sediment yield in the basin.(5)The change of land use pattern has an important effect on soil and water loss in the basin.Typical correspondence analysis(CCA)was used to analyze the relationship between landscape pattern and water and soil loss in the watershed.The results showed that:At the type level,the distribution size,morphology and connectivity of each land use type are closely related to water and soil loss.The clumpiness index(CLUMPY)of cultivated land,the fractal index distribution(FRAC＿CV)of forest,the connectance(CONNECT)of orchard land and construction land had a great influence on runoff yield,the landscape shape index(LSI)of cultivated land,the clumpiness index(CLUMPY)of forest land,and the average patch area(AREA＿MN)and landscape shape index(LSI)of orchard land and construction land had more significant effects on sediment transport.Meanwhile,the average patch area(AREA＿MN)and patch connectivity(CONNECT)of the watershed were positively correlated with runoff.The contiguity index(CONTIG＿CV),patch density(PD)and landscape division index(DIVISION)were positively correlated with sediment production and transport.It shows that the landscape pattern evolution in the watershed has a significant impact on water and soil loss,mainly in the spatial distribution size,morphology,connectivity and other aspects.Therefore,to control water and soil loss,land use pattern can be optimized by enhancing the control effect of dominant landscape,improving patch uniformity,enriching landscape types,reducing the physical connection between patches,and strengthening the aggregation degree of landscape patches.(6)According to the evaluation of the ecological environment quality of the Xunwu watershed by the remote sensing ecological environment index(RSEI),the mean values of the RSEI in 1995,2005 and 2015 were 0.554,0.544 and 0.550 respectively,which showed a decreasing trend and then an increasing trend,indicating that the ecological environment was improved after the deterioration.The occupation of forest land caused by the rapid expansion of orchard land caused by the development of Xunwu fruit industry is the main cause of the deterioration of ecological environment,and the main reasons for the improvement of the ecological environment are returning fruit(cultivate)to forests,protecting ecological forests and afforestation.