Study On Tracing Source Of Sediment In The Loess Hilly-gully Region Based On Mid-infrared Spectroscopy

Soil erosion seriously harms the ecological environment and restricts social and economic development.The identification of sediment source is a hot and difficult point in soil erosion research.Accurate identification of sediment source plays an important role in correctly simulating soil and water loss,making policies according to local conditions and arranging water conservation measures reasonably.Soil erosion is serious in the hilly-gully region of the Loess Plateau.It is of great significance to quantitatively identify the source of the sediment in this area to reveal the occurrence mechanism and development process of soil erosion.In this study,seven small catchments in the hilly-gully region of the Loess Plateau with severe erosion were selected,and the characterization of loess properties based on the mid-infrared diffuse reflectance spectroscopy was studied.Firstly,mid-infrared diffuse reflectance spectroscopy characteristics of loess was analyzed,and then the calcium carbonate（CaCO₃）and soil organic carbon（SOC）which could reflect the remarkable characteristics of loess were taken as an example,the contents of CaCO₃and SOC were quantitatively predicted by establishing a modal based on mid-infrared diffuse reflectance spectroscopy.On this basis,the study on the source of the sediment in small catchments of the Loess Plateau hilly-gully region by mid-infrared diffuse reflectance spectroscopy was carried out,combining with the method of using silt sediment to calculate soil erosion.Contributions from the slope and gully to sediment in nine typical small catchments were quantitatively identified.The results were compared with those of the traditional geochemical tracing method.Then,by means of the mid-infrared diffuse reflectance spectroscopy tracing method,source identification of riverbed sediment and river suspended sediment in the hilly-gully region was carried out.The main conclusions are as follows:（1）Loess has the characteristics of high CaCO₃ and low SOC,and its mid-infrared diffuse reflection spectrum shows obvious characteristic peaks of related groups of CaCO₃,organic matter and clay minerals.Quantitative models of CaCO₃and SOC based on mid-infrared spectrum were established.The prediction performance of the models was affected by spectral preprocessing methods,modeling regression methods and the calibrating set samples.The results showed that partial least squares regression（PLSR）method was better than principal component regression（PCR）method and support vector machine（SVM）method in this study.In addition,the relative deviation of prediction for CaCO₃ and SOC was affected by the contents of CaCO₃ and SOC.During the establishment of a quantitative prediction model based on mid-infrared spectrum,not only the spectral characteristics of the target components were used,but also the spectral characteristics of other components of the loess played an important role.The contribution of each spectral characteristic to the model was different with different prediction objectives.（2）Combined mid-infrared spectroscopy with stoichiometric method,the tracing technology of sediment source based on mid-infrared spectroscopy in hilly-gully loess region was constructed.On the basis of correctly identifying the sediment sources of slope and gully soil,the sediment source quantitative model based on mid-infrared spectroscopy was constructed.And the results showed that the slope soil was the main source,the average contribution of gully soil was about 35%.Comparing the spectroscopy tracing method with the traditional geochemical tracing method,the gully contribution calculated by the traditional geochemical tracing method was lower than that obtained by the spectroscopy tracing method in most sediment samples;however,the weighted average contribution of the total depth gully calculated by the two methods was 27%and 21%respectively,and the results were basically the same.The spectroscopy tracing method has obvious advantages due to simple sample preparation,less sample requirement,no damage to the sample,no pollution to the environment,fast measurement and low economic cost.（3）Using the constructed sediment source tracing technology based on the mid-infrared spectroscopy and the PLSR method,a river sediment source tracing model based on the mid-infrared spectroscopy was established,and the source of the riverbed sediment during flood period and base-flow period and the source of the suspended sediment during the flood event were predicted quantitatively.The results showed that slope erosion was the main source of sediment collected whether during flood period or during base-flow period,and the contribution of slope erosion was 65%and 71%respectively.The source variation of sediment in different tributaries was different.During flood period and base-flow period,the riverbed sediment of upstream tributary was mainly from slope erosion;however,the riverbed sediment of downstream tributary was mainly from slope erosion during flood period,the gully contribution only accounted for 14%,while gully contribution increased significantly during the flood period,reaching 60%.During the flood event,the main source of the suspended sediment of the catchment outlet was from gully erosion,the contribution of gully erosion changed between 39%and 97%,the average ratio of gully to slope was roughly 12:5,the proportion of clay and silt particles of the suspended sediment was significantly higher than that of the source samples,the median particle diameter was smaller than that of the source samples.The correlation analysis showed that the contribution of gully erosion was positively correlated with SOC content,CaCO₃content,clay volume percentage and silt volume percentage,and negatively correlated with sand volume percentage and median particle diameter.The relationship between suspended sediment concentration and gully contribution ratio of the suspended sediment and the relationship between suspended sediment concentration and median particle diameter were showed parabolic fitting.