Fingerprinting The Source Of Erosion Sediment In Forest Watershed Disturbed By Production And Construction Activities

The increasing intensity of human activities,and particularly the frequent production and construction activities in recent years,have caused new soil erosion in the mountainous areas and rapidly increased river sediment in the neighboring areas.It has caused great challenge to the ecological security and water resources utilization in related areas,and urgently needs to be treated.Information on catchment sediment source is a key prerequisite for the design and implementation of management strategies.However,establishing catchment suspended sediment sources is fraught with difficulty due to the general complexity from varied terrain,the lack of basic research and monitoring data as well as disturbance of production and construction in the mountainous areas,and thus has made it difficult to implement targeted and efficient catchment governance and protection.In this study,the Dadingshan small watershed disturbed by production and construction activities was selected as study area,locating at the mountainous area of western Guangdong Province.In the Dadingshan small watershed,we evaluated the feasibility of different composite fingerprinting methods to identify sediment sources,and identified the information on catchment sediment sources and analyzed the temporal and spatial changes of soil erosion by using field survey,composite fingerprinting method and model simulation.The main conclusions are as follows:（1）In the Dadingshan watershed,the soil was mainly coarse particles and the average particle fraction<2 mm for sediment samples was 68.5%,representing the dominant particles being transported during the erosion process of the catchment.Therefore,the component of particle fraction<2 mm was used to analyze the sediment source in the catchment.In addition,the cluster analysis showed the optimal source classification scheme was plantation subsoil,plantation topsoil and the wind farm.（2）The optimal composite fingerprint shows that three optimal composite fingerprint groups consisting of geochemical elements collection were screened,and their discrimination ability were 97.6%,97.2%and 100%,respectively,indicating that geochemical elements were robust fingerprint factors within the Dadingshan catchment.According to the average results of three fingerprint factor groups,plantation subsoil（58%）was the main source of sediment downstream in the Dadingshan small watershed,followed by plantation topsoil（30%）and wind farm（12%）.The results of the three composite fingerprint groups were similar,and the average goodness of fit of the model was greater than 80%,indicating that the optimal composite fingerprinting method could be used to identify the sediment source in catchment disturbed by production and construction.But it is notable that the correction factor of the model may lead to unreasonable source analytical results.In the Dadingshan small watershed,particle size correction and variation weighting of the in-source fingerprint factor caused the misleading results calculated by the source analytical model,which indicated that the model correction factor might not be applicable to all erosion environments,and the basic conditions of the correction factor should be carefully considered.（3）The multiple composite fingerprinting method was used to obtain 32 sets of non-contradictory composite fingerprints（that is,the analytical solution of positive definite can be obtained）,and their discrimination ability was 53.7%-100%with the average value of 74.5%.According to the average results of 32 non-contradictory composite fingerprints,the downstream deposited sediment mainly came from the plantation subsoil（55.9%）,followed by plantation topsoil（31.4%）.The least sediment sources were wind farms（12.8%）.The use of the average method made the 95%confidence interval of the three source calculations narrower and improved the accuracy of quantitative discriminant results.（4）Both multiple composite fingerprinting method and optimal composite fingerprinting method could be used to identify sediment sources in the watersheds disturbed by production and construction activities in mountainous areas.The multiple composite fingerprinting method used multiple fingerprint factor pairs to calculate the sediment source,but the source discrimination ability of single fingerprint pair was different,and the average discrimination ability was lower than that of optimal composite fingerprint method.And the error of a single fingerprint pair within multiple composite fingerprinting method was large.The standard deviation of mean contribution of 32 composite fingerprint factor pairs were still larger than that of optimal composite fingerprinting method.Therefore,the optimal composite fingerprinting methods performed better in this environment.（5）The high contribution of plantations subsoil（58%）was mainly attributed to severe erosion of unpaved roads and landslides on steep slopes in logging areas,transporting excessive sediment downstream.Sediment contribution from disturbed area of plantation（88%）was much greater than that from disturbed area of wind farm（12%）.Though soil erosion in disturbed areas of wind farm projects was severe,its contribution to the downstream sediment was least due to the limit of disturbance area and transferred distance.（6）The erosion modulus was approximately 1948.1 t·km^-2·a^-1（belonging to light erosion）in 2019 in the Dadingshan watershed.However,the spatial variation of soil erosion was very large with variation coefficient of 5.12.Slight erosion accounted for 80.6%of the total watershed area.And the moderate erosion and above were mainly distributed in the Eucalyptus young forest area with the less than 30%of the vegetation coverage,which contributed nearly 75.7%of total soil erosion.The destruction of natural vegetation resulted by plantation exploitation was the primary cause of soil erosion of afforestation areas.The soil erosion significantly increased with the increase of slope gradient in the young forest area,which was aggravated by extreme rainfall.However,soil erosion gradually decreased with the increase of the age of Eucalypt plantation.Therefore,the hot spot of soil erosion was young forest areas of Eucalypt plantation with a slope greater than 25°,and the key period for soil erosion control was the first 2-3 years after Eucalyptus planting.