Characteristics Of Soil Erosion And Its Response To Re-vegetation In The Upper Reaches Of Beiluohe River

Soil erosion has turned into one of the important environmental problem endangering the existence and development of human being.The Loess Plateau is limited in water resources and vulnerable to eco-environment,has become one of regions with the most serious water and soil loss in China and even the world.Since 1950s,many of massive soil and water conservation projects and forestry ecological engineering have been executed,especially the project of returning farmland to forest and grassland begin in 1990s has been achieving obvious effect in controlling loss of soil and water and improving eco-environment.Research on characteristics of soil erosion and its response to re-vegetation in the Loess Plateau can provide an important theoretical basis and reference to decision for forestry ecological construction and controlling loss of soil and water.This thesis was carried out in the upper reaches of Beiluohehe River located in the central part of the Loess Plateau and being one of the demonstration area of forestry ecological engineering.By using the method of statistic analysis,technique of digital watershed,theories of hydrology and landscape ecology,and based on the basic materials including observed data of hydrology and meteorology, remote sensing images of different resolution and digital elevation model(DEM),the time-space distribution characteristics of rainfall and land use and land cover change(LUCC) were analyzed.The object-geomorphic erosion risk in study area was evaluated after analyzing the geomorphic feature,and establishing the method of automated extraction of shoulder line of vally based on flow path from DEM data.According to studying the change trend,characteristic stage and driving factors for variations of runoff and sediment of basin,the regulation effects of runoff and sediment of re-vegetation were calculated.A series of topographic values were extracted from high resolution remote sensing image and critical topographic characteristics of shallow gully erosion on slope were obtained.Based on dividing district for landform,mending the algorithm of L factor and modified the valley-slop erosion model of single rainfall,the intensity and distribution of soil erosion on hill-slop and on valley-slop in different period were estimated by Revised Universal Soil Loss Equation(RUSLE) and valley-slop erosion model combined with Sediment Delivery Distributed Model(SEDD),respectively.By integrating the estimated results of soil erosion on hill-slop and valley-slop,simulating sediment discharge of basin,the responses of soil erosion and sediment yield of the different geomorphic units to re-vegetation were deeply studied.Besides,the relative contributing ratios of different environmental factors to space heterogeneity of soil erosions before and after re-vegetation were determined according to established erosion spatial distribution model.The main research results are as following: The annual rainfall of the upper reaches of Beiluohebe River is lower in the northwest region than in the southeast region.Over the past decade,rainfall increased a little,and the probability of occurrence of dry year and extreme rainfall event decreased.Average annual rainfall and rainfall erosivity after 1999 when massive re-vegetation were executed increase 5.07%and 8.64%than the previous period,respectively.This indicated that there is a climate background after re-vegetation to raise the erosion intensity and sediment yield.In the upper reaches of Beiluohehe River,the critical support area of channel network is between 0.5 and 1.0 km~2.The gully densities in different levels all are more than 0.91 km/km~2.The slop-up land account for 62.29%of total area of basin,and the rest is gully land.All of these indexes present the general geomorphic characteristics that ravines and guillies criss-cross.The region existing moderate and severe erosion risk accounts for 88.05%of the total area of basin.This indicated that there is the landform condition causing severe soil erosion in most parts of study area.The vegetation coverage of basin increases form 56.74%in 1986 to 76.76%in 2004.Among different land use types,forestland presents the most obvious rising trend,fastest changing speed. Besides,gravity centers of forestland and grassland with high coverage move to southeast of basin. Conversely,gravity centers of farmland,grassland with moderate coverage and grassland with low coverage move to northwest of basin.The patch number and their mosaic degree increase,but continuity of same type patches decrease.All of these lead to the landscape pattern showing the degree of fragmentation and diversity increase obviously.From 1980 to 2004,the variation of runoff and sediment of basin could be divided into three periods.Among them,the effecting period of re-vegetation is form 2001 to 2004.In this period,the runoff and sediment of basin reduce a total of 138.08 million m~3 and 142.51 million t due to re-vegetation.The average capacities of control water and soil loss of newly increased forestland are about 26 700 m~3/(km~2·a) and 27 500 t/(km~2·a),respectively.The shallow gully erosion on slope is mainly influenced by slope degree,slope length;slop aspects up-slop length.The upper limited critical slope steepness of shallow gully erosion is between 26 and 27°,and the lower limited critical slope steepness is between 15 and 20°.In addition,the critical slope length ranges from 50 to 80 m.Taking 1980 to 1994 as the reference period and 2001 to 2004 as effecting period,the average annual intensity of soil erosion of total basin reduce from 12 652.06 t/(km~2·a) to 6 036.72 t/(km~2·a),and the average annual sediment yield reduce from 38.97 million t/a to 17.96 million t/a.Among different geomorphic units,the average annual intensity of soil erosion on hill-slop reduce from 5 770.46 t/(km~2·a) to 1 437.93 t/(km~2·a),and sediment yield reduce from 13.09 million t/a to 3.22 million t/a.At the same time,the average annual intensity of soil erosion on valley-slop reduce from 28 093.92 t/(km~2·a) to 16 196.91 t/(km~2·a),and sediment yield reduce from 25.88 million t/a to 14.73 million t/a.On the hill-slop, the average capacities of reducing erosion and sediment of newly increased forestland are about 11 752.10 t/(km~2·a) and 11 400 t/a,respectively.The proportion of sediment yield on valley-slop in total sediment yield of basin increase form 66.41%to 82.04%.The valley-slop has been as the most main source of erosion and sediment yield..Before re-vegetation,the relative contributing ratios of terrain,vegetation and rainfall to space heterogeneity of soil erosion are 17.30%,20.79%and 61.91%,respectively.After re-vegetation,the relative contributing ratios of relevant environmental factors change to 15.73%,57.67%and 26.60%。The relative contributing ratios of vegetation grows 36.88%and become the most effective factor from the least one due to re-vegetation.