| Soil erosion,especially water erosion,is the one of the most important land degradation problems in the world and has become a global hazard.As a populous and agricultural country,complex historical and natural conditions make China one of the countries suffering from the most serious soil erosion around the world.Soil erosion is a multi-factors,multi-levels,multi-scales geographical problem,the occurrence and development of which are affected by various natural and social factors.Knowledge of soil erosion status and its dynamic changes at regional and national levels are closely linked with the investment and allocation of soil and water conservation efforts,development of regional society and economy,as well as the overall situation of the ecological environment.Quantitative current soil erosion estimation and objective spatial analysis of erosion characteristics across the country are of the utmost significance and provide scientific foundations in preventing and controlling soil erosion as well as protecting soil and soil resources.Although a wide variety of approaches have been employed to study soil erosion caused by water in different regions around the world.At present,studies on soil erosion at regional scale and national level are mostly focusing on qualitative and semi-quantitative methods.A case in point is the series of National Remote Sensing Survey of Soil Erosion in China.Quantitative researches mainly focus on sampling survey method or grid estimation methods.However,both categories have their own flaws and disadvantages.On the one hand,it is difficult for the sampling survey approach to achieve full-area coverage assessment,while the grid estimation methods usually simplify the erosion model and are characterized by a lack of considersation in both the microscopic process of erosion and soil conservation measures adopted.Based on the field sampling data collected from the Fourth National Soil Erosion Survey and the support of the National Land Use/Cover Database of China(NLUD-C)at the scale of1:100,000(developed by CAS),by employing the CSLE model and spatial interpolation and extrapolation methods,the dissertation quantified water erosion in the country using GIS,RS,and statistics techniques.A Quantitative estimation of the soil erosion modulus for the eight soil and water conservation regions across the country was made,the status of water erosion in different regions was also analyzed.Aditionally,by referring to the Standards for Classification and Gradation of Soil Erosion proposed the Ministry of Water Resources of China(MWRC),the quantitative estimates were divided into different erosion intensity levels compared with the 2010 updates based on the Second National Remote Sensing Survey of Soil Erosion,differences between the results of the two methods were revealed,causes behind were clarified and analyzed.The main findings of the dissertation are as follows:(1)Establishment of spatial databases for soil erosion affecting factors in the 8 soil and water conservation regions across the country.Based on multi-source data,including remote sensing imagery,rainfall data,DEM,soil data,land use data and field sampling data,the soil erosion factors in the CSLE model were retrieved and estimated in raster datasets.The results indicated that the spatial distribution of rainfall erosivity shows a decreasing trend from southeast to northwest,which is generally consistent with the distributi on trend of precipitation in China.The R values of the South Red Soil Region,the Southwest Karst Region,the Southwest Purple Soil Region,the North Soil-Rock Mountainous Region,the Northeast Black Soil Region and the Northwest Loess Plateau Region were 6783.71,4131.659,3908.89,2463.40 and 1470.38MJ?mm?hm-2?h-1?a-1 respectively.By Using the USLE equations and the EPIC model,a K-value map was generated and the then adjusted using measured runoff plots data in the national soil erosion survey.After the adjustment,the K value in the Northeast and North regions showed stable results with measurements,while the soil erodibility values in the other soil conservation regions showed a decrease in different degrees.The spatial distribution of LS factor values was consistent with the spatial distribution of slope gradients.MODIS NDVI data,land use maps and Primary Sampling Units(PSUs)data were used to estimate the vegetation coverage and factors of soil conservation measures.Areas dominated by cultivated land generally showed higher B values and lower T values,and the lower B values factor were mainly found in southern portions of the country,dominated by a permanent vegetation cover.(2)By combining the CSLE model calculation and the point(PSU)-to-regional methods based on spatial interpolation and extrapolation of PSUs data,soil erosion by water in China was quantified for the reference year of 2010.Erosion characteristics such as spatial distribution,soil erosion modulus,soil erosion ratios,erosion areas,intensity,soil loss amounts and erosion contribution ratios the six major water erosion regions were emphasized and revealed.The results showed that water erosion mostly occurs on the Secondary Ladder of Terrain in China and mainly distributed in semi-humid and semi-arid areas.The total area of water erosion in the eight soil conservation regions was 1.63×106 km2,accounting for 30.48%of the total land area of the entire water erosion area.The erosion-prone areas and soil erosion hotpots were identified.The most severe water erosion hot pots were found in the Loess Region and the Purple Soil Region.In terms of soil erosion modulus,the Loess Region,the Purple Soil Region and the Karst Region are the three regions suffered the most severe erosion,with average soil erosion rates of 1492.62,1461.66 and 1168.28 t/(km2·a),while the average soil erosion modulus for the Black Soil Region,the Norther Soil-Rock Region and the Red Soil Region were 618.01,529.048 and 335.64 t/(km2·a),respectively.(3)A spatial analysis of soil erosion rate by land use using the major 2nd level NLUD-C land use types was made.The results indicated that the dry land experienced the most severe erosion among all land use types,with an average soil erosion rate of2414.33 t/(km2·a).Significant differences in soil erosion rates of dry land from the 8soil conservation regions were found,the Purple Soil Region and the Karst Region were the two regions with the highest dryland erosion rates,with an averge value higher than4000 t/(km2·a),the average value of the dryland in the Loess Plateau Region was2782.25 t/(km2·a),the average soil erosion modulus of the dryland in the Black Soil Region was 1998.11 t/(km2·a),the average erosion modulus of the dry land in the Red Soil Region and the Soil-rock Mountain Region were 1182.69 t/(km2·a)and 927.46t/(km2·a),respectively.The total annual soil loss from dry land was about 2.056 billion tons.The soil loss contribution ratio of dry land was the largest among all land use types,reaching 58.76%of total soil loss with only 24.44%of the total land area of the water erosion area.Meanwhile,grasslands,forested lands,other woods and shrubs were also important sources of soil erosion,with annual soil loss of 399,398,290,and 157 million tons,respectively.(4)Comparison between the two estimates using two different methods(CSLE and qualitative method)China was made.The water erosion ratios of the country based on quantitative estimation and the qualitative remote sensing survey were 30.48%and33.56%,respectively.The results showed consistency in both the total area affected and spatial distribution of hot spots identified.Nevertheless,some differences in the internal composition of soil erosion were also found.On the one hand,the Loess Plateau Region is the most severely eroded region under the two estimations.The Red Soil Region,the Black Soil Region and the Soil-Rock Mountainous Region also showed close consistency between the two results.The difference in the erosion ratio in the Red Soil Region of South was only 0.67%.Hoever,the quantitative estimation of the Purple Soil Region was 8%less than that of the remote sensing survey method and it is also the region with a major difference in erosion ratio.Further analysis was conducted in Chongqing and Yunnan since significant differences were mainly found in these areas.The differences were analyzed from four perspectives:spatial distribution of erosion,soil erosion composition,grid-based comparison and land use backgrounds of the differences.The results showed that 74.81%of the areas in Chongqing related well between the two results.Areas with poor consistency were mainly found in the hilly and mountainous areas.The corresponding land use types of difference were dry land,mountainous paddy fields,industrial and mining lands,and other woods.To some extent,there are good consistency between woodland,shrub land,and grassland types in Chongqin.For Yunnan province in the Karst Region,spatial distribution of erosion and consistency of the intensity levels for the two results were poor,dry land and other woods are the corresponding land use types of difference.The average dryland erosion rate of the province reaches 4895.37 t/(km2·a),especially for the lower reaches of the Lancang River Basin in the southern portion,which even exceeds 7300 t/(km2·a).This is mainly caused by the steep slope farmland conditions,high rainfall erosivity and a lack of soil conservation measures.The results of field surveys demonstrated that soil loss of tea gardens and orchards(classed in other woods)are also major soil erosion problems.The main innovations of this dissertation are as follows:(1)Based on the CSLE and a combination of point-to-regional methods,the paper quantified soil erosion by water in China.By retrieving the factors in the CSLE model,meanwhile,by spatial interpolation and extrapolation of sampling data using land use maps,the estimation of soil erosion modulus was made at national level.(2)For the first time,comparison was made between quantitative estimation based on the Fourth National Soil Erosion Sampling Survey and the soil erosion status from the 2010 updates based on the Second National Remote Sensing Survey.The water erosion ratios assessed by the two methods were different by 3%.For most soil and water conservation regions,the results showed good agreement and the areas with significantly different were further analyzed.(3)In the process of estimating the soil erosion modulus at national level,the effects of soil conservation measures on the water erosion reduction were fully taken into account,which is of great significance for evaluating the effectiveness of soil and water management in different regions. |
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