Temporal And Spatial Changes Of Soil Erosion Characteristics And Its Response To Landscape Pattern In Small Watershed In Water-wind Erosion Crisscross Region, China

To study the mechanism of human factors to soil erosion in water-wind erosioncrisscross region in Loess Plateau,China, it is a significant element to analyze the temporaland spatial changes of soil erosion characteristics and its response to landscape pattern.Natural and man-made erosion environments have undergone great changes in this region.Therefore, it is necessary to study the variation of effective time and reasonable region forpreventing and controlling the water and wind erosion and it is practical significant forpromoting regional sustainable development. The typical water-wind erosion crisscross region,Liudaogou small watershed, was selected as study area. Our research was based on the resultsof history research, recent remote sensing images, DEM, rainfall and wind monitoring data,outcrop soil stratums distribution map and other basic data and Empirical soil erosionevaluating model was used to calculate the erosion modulus in GIS. Compare with the resultsfrom the study of Zhang Pingcang from1993to1996, I analyzed the changes of landuse/cover and soil erosion characteristics in recent20years. According to the "source andsink" landscape theory, the land use/cover pattern in this watershed was converted to the"source and sink" landscape pattern. Finally, Location-weighted landscape contrast index（LLI） was used to analyze the influencing mechanism of landscape pattern change withconfiguration of spatial elements, contribution weight and composition to soil erosionprocesses in Liudaogou small watershed. To facilitate the expression, the research of ZhangPingcang from1993to1996was described as previous study (pre-study) and this one from2009to2011as following study (fol-study). The main conclusions were as follows:（1）From1990to2010, due to the dramatically transference from slope farmland anddry terrace, especially after the implementation of the “Grain for green” policy in1999, thepercentage of the woodland and grassland increased from35.74%to62.79%and thepercentage of farmland decreased from33.87%（233.54hm~2）to7.61%（52.49hm~2）. The mainfarming area turned to concentrate in the land below the gully edge, whose proportion wentup from13.82%to74.04%out of the total area of farmland. Industrial area was extendedfrom0.28hm~2to14.89hm~2and the coal mining becomes more active.2）In the respect of underlying surface influencing erosion process, the vegetation coverage, especially the thatabove the gully edge, improved obviously. However, the increasingly more active coal mining,on one hand, disrupted the growth of vegetation and land use type in this region, on the otherhand, accelerated the erosion environment degeneration.（2）The annual average water erosion modulus was3545t/km~2.a in the previous studyand15002t/km~2.a in the latter study and annual average water erosion amount was24425t and103363t, respectively. Water erosion modulus and water erosion amount were significantlyreduced by11457t/km~2.a and78938t, respectively, as much as76.4%. In the previous study,the annual average wind erosion modulus3199t/km~2.a and annual average wind erosionamount was22041t. In the following study, the annual average wind erosion modulus985t/km~2.a and annual average wind erosion amount was6787t. In the past20years, winderosion modulus and amount was greatly reduced by2214t/km~2.a and15254t, as much as69.2%. The monitoring data of annual average erosion modulus were15040t/km~2.a inpre-study and3101t/km~2.a in fol-study and monitoring data of annual average water erosionamount were103626t in former and21428t in latter. The relative error were0.3%and12.5%,respectively.（3）Compared with the previous period, the erosion in Liudaogou small watershed wassignificantly reduced, especially the that in high-intensity level and middle-intensity level.High-intensity and middle-intensity water erosion area reduced significantly and the erosionmodulus of every intensity level decreased slightly. Wind erosion also clearly weakened,especially that in high-intensity and middle-intensity level. The middle-intensity wind erosionarea decreased dramatically and the wind erosion modulus in every intensity levels wentdowm steeply.In previous period, the high-intensity erosion occurred on the slope farmland, the deepvalley in middle and lower reaches and the head of gully. In following period, high-intensityerosion occurred on the head of gully and the two walls or slope foot of U-shaped deep valleyin the main channel. The erosion on slope decreased obviously, rarely high intensity watererosion occured. Wind erosion, gravity erosion and water erosion superimposed spatially ledto a high-intensity soil erosion on the foot of slope. Higher water erosion patch graduallytransferred to the lower reaches and the head of gully. The gully of this watershed wasdeveloping to stabilization, but the headward erosion was still relatively intensely. Caused byhuman impacts, such as arable land reclamation and coal mining, water erosion intensified inthe main channel.In both previouse period and following period, wind erosion occured mainly on the eastside trench wall of main channel and the steep slope of east coast. In recent20years, wind erosion on the slope weakened slightly that was due to the improvement of vegetation onslope. The wind erosion on east and west coasts were asymmetric. The multi-piece aeoliansand land covered on the west coast and wind erosion usually occurred on the east coast. Thespatial distribution of wind erosion amount was also very obvious differences, but itsconcentration was higher than the water erosion amount.Compared with the previous period, the sediment yield modulus of every soil stratumsobviously reduced. Their sediment modulus order changed from Malan loess (L1)> Eoliansand (S)> upper Lishi loess (L2)> lower Lishi loess (L3) to Malan loess (L1)> Eolian sand(S)> lower Lishi loess (L3)> upper Lishi loess (L2). From bottom soil stratum to top, theirsediment yield modulus ratio varied from1:1.5:5.1:4.0to1.0:0.6:1.9:1.6.（4）From1995to2010, the LLI （relative height, slope, projection distance, and flowlength） in Liudaogou watershed showed decreasing trend and distribution of landscapepattern became more reasonable. The risk of soil erosion reduced and soil erosion modulusover the same period showed the same trend. The decrease of cultivated land whosedistribution in relative height and slope reduced in these catchments and the increase ofgrassland were the main reason for improvement of watershed landscape pattern. The LLI（flow length） increment which reflected changes in the landscape pattern better weresignificantly negatively related to the soil erosion modulus increment. Other LLI （relativeheight, slope, and projection distance） increment was weak correlation with soil erosionmodulus increment and the trends were quite different to the measured values. This simulationmethod should be further improved in order to achieve accurate quantification.