Study On The Characteristics Of Soil Erosion, Sediment Yield In Small Watershed In Loess Plateau Hilly, Gully Area By Using Tracers

In this study, Wangmaogou watershed, one of the typical small catchments in Loess PlateauHilly and Gully Area was selected as the study object. With the theory of soil erosion, runoff—sediment relationship, sediment kinematical theory and mathematical statistical theory, ¹³⁷Csand ²¹⁰Pb_ex tracing technique, field observations and laboratory analysis, erosion status andsediment deposition under a certain storm of different physiognomy site and different land usepatterns were studied. Making use of deposition information in silt dam, the response process ofthe sediment in each sedimentary layer and erosive rainfall was inversed respectively, then theevolvement characteristics of erosion and sediment yield intensity in watershed was analyzed.Watershed soil erosion was quantitatively appraised from the point of causes of formation,process and development trends, as a result the variations of erosion and sediment yield inspatial and temporal scales were revealed. Utilizing ¹³⁷Cs tracer technique, the relative amountof sediment coming from inter-gully land and gully land was estimated. Researching on thesediment yield of single rainfall and rainfall characteristics in dam control basin, watershed soilerosion prediction model was established. The studies are of practical significance in evaluatingthe ecological environment restoration and comprehensive management effect, scientificallyadjusting macroeconomic policy, such as planning and designing programs, collocated keytechnique and optimizing the layout of father measurement, and promoting construction ofecological environment for further development.The main results are as follows:(1) The result that ¹³⁷Cs and ²¹⁰Pb_ex were mainly absorbed in fine soil particles were tested,the characteristics of no soil particle slectivity in the course eroded soil transmitting was openedout and the reliability of ¹³⁷Cs and ²¹⁰Pb_ex techniques was illuminated. With analying the ¹³⁷Cs,²¹⁰Pb_ex contens in different size soil particles and studying on the particles composes of grasssoil, terraces soil, sloping farmland soil and silt dam sediment, the researches showed that ¹³⁷Cs,²¹⁰Pb_ex were mainly adsorbed by clay particles of soil, with the soil particles growing fine, ¹³⁷Cs, ²¹⁰Pb_ex contents increased significantly; the particles composes of grass soil, terraces soil,sloping farmland soil and silt dam sediment had no significant difference, which indicated thaterosion sediment in transport process had no particle separation. This meant that the ¹³⁷Cs,²¹⁰Pb_ex tracer techniques were suitable for soil erosion research.(2) ¹³⁷Cs, ²¹⁰Pb_ex distribution characteristics and concentration variations in the soil profileof different land use patterns were analyzed; the relationships between ¹³⁷CS, ²¹⁰Pb_ex content insoil profile of slope-land with terrain factors such as slope length, slope gradient and aspect,and landuse types were investigated.Study results showed that ¹³⁷Cs,²¹⁰Pb_ex contents distributed uniform on the whole incultivated layer in disturbed soil (farming land) profile, and only a small number of ¹³⁷Cs,²¹⁰Pb_ex under cultivated layer. In undisturbed soil profile, ¹³⁷Cs had an "enrichment layer" in thesub-surface (2-4 cm), and decreased exponentially downward with the increasing ofsub-surface depth of the soil profile. ²¹⁰Pb_ex content exponentially decreased from the surfacedown, whose distribution depth was slightly bigger than that of the ¹³⁷Cs in undisturbed soilprofile. ¹³⁷Cs, ²¹⁰Pb_ex contents in slope soil profile increased volatility trends with the slopelength. Slope gradient was negatively correlated with ¹³⁷Cs, ²¹⁰Pb_ex contents, ¹³⁷Cs, ²¹⁰Pb_exconcentrations were significantly reduced with the increase in slope gradient within 15°～26°.The impact of sloping aspect on ¹³⁷Cs, ²¹⁰Pb_ex concentrations is very obvious, the orderfollowed by southern slope＞northern slope＞east slope＞west slope according to ¹³⁷Cs,²¹⁰Pb_ex contents of different slope aspect. The order was followed by grass＞woodland＞sloping farming land in accordance with ¹³⁷Cs, ²¹⁰Pb_ex contents of different land use patternsof soil.(3) The effects of terrain of slope land on soil erosion intensity were analyzed and the soilerosion intensity variations in different landuse patterns were illuminated.Researches showed that the soil erosion intensity increased fluctuatingly along the slope withthe slope length, which the upper and lower parts of the slope erosion larger, central smaller.Soil erosion intensity had a positive exponential correlation with slope gradient. Between 15°～26°, erosion intensity increased significantly with the slope gradient increasing. Slope gradientand soil erosion intensity of slope land was index relevant, meanwhile soil erosion fluctuatedalong the slope length, which meant that the impact of slope gradient on the soil erosion extentwas far greater than its length. In this basin, aspect impact on soil erosion intensity is veryobvious, which is that different aspect followed by western slope＞eastern slope＞northernslope＞the southern slope with respect to the erosion size. Different land use patterns accordingto the size of erosion were sloping farmland＞woodland＞tuff. If classified according to erosiondegree, sloping farmland belongs to extremely violent erosion; woodland and turf belong to moderate erosion.(4) The characteristics of soil erosion intensity transformation in grassland coverage,uncultivated farming land and fanning land in hill top and valley slope were clarified inWangmaogou Small Watershed.Researches found that the ¹³⁷Cs concentration loss in grass coverage land in hill top andvalley slope is lower than that in uncultivated farming land, and is significantly lower than thatin cultivated land. According to soil erosion modulus size, the order is followed by cultivatedland >uncultivated farming land＞grass coverage land. If hill top and valley slope werecultivated to farming land, the erosions would increase sharply.(5) Applying ²¹⁰pb_ex tracer method, the average erosion modulus of slope fanning land wascalculated, and its results were contrasted with that of ¹³⁷Cs tracing method.Based on ²¹⁰pb_ex content in sloping land soil profiles, the average soil erosion moduluswere calculated on slopes farmland in Wangmaogou small watershed in Loess Plateau Hillyand Gully Area, results of which were compared with that of Zhang Xinbao ¹³⁷Cs model. The²¹⁰pb_ex estimating results were the average erosion rate of the past 110 years, however, ¹³⁷Csmethod results were the average of the recent 50 years. The former is bigger than the latter,which indicated that there had been an increasing trend of soil erosion since the ¹³⁷Cs fell(1954). This might be caused by augmentation of the population, cultivation of forest andgrassland, the increasing of plow land, intensify of human activities after the Liberation, and soon.(6)The distribution laws of grain composition and ¹³⁷Cs contents of sludge in each damsedimentary cycle layers in the 3rd and 4th check dam were opened out.By analyzing test, the following results were drawn: the grain diameter of silt insedimentary cycle layers in the 3rd and 4th check dam were mainly distributed among the rangefrom 0.005 mm to 0.1 mm, sludge soil particles were distributed unevenly, and along the damto the deep profile there were no obvious changes. The ¹³⁷Cs contents of sludge in each damsedimentary cycle layers in the 3rd and 4th check dam profiles had very similar distributionwhich demonstrated that the 137Cs technique is feasible to note specific environmental historicalevents.(7) The relationships between the single rainfall characteristic with sedimentary quantity in3rd and 4th check dam controlled basin were differentiated and analyzed;accordingly, thesediment yield forecasting model under single rainfall was established in respective silt damcontrol basin.By the cross-section measurement of the 3rd and the 4th darn area, in Wangmaogou watershed,combining the rainfall cycle deposition layer thickness of the 3rd, the 4th silt dam since 1959, the sediment yield of each sedimentary cycle in the 3rd and the 4th check dam was calculatedaccording to the damaged section after 1987 and dam land area buried in the lower dam withslope gradient modified of both sides of the valley slope. Through studying the 3rd and the 4thsilt dam construction, operation history, mud amount of respective sedimentary cycles, inconjunction with analysis of rainfall data, using noting time function of ¹³⁷Cs technology, inconformity to the rainfall in response to sediment principle which is bigger rainfall produced alarger number of sand, the sediment record of the 3rd and the 4th were deciphered, thesediment yields of single rainfall in 3rd and 4th check dam control basin were identified from1959 to 1987, respectively. Through the multiple regression analysis of rainfall characteristicsresearch, such as rainfall, the biggest 30 min intensity of rainfall, rainfall erosivity andsedimentation cycles corresponding rainfall deposition of mud (that is soil erosion amount ofthe dam control basin), the results indicated that sediment yield had the best correlativity withrainfall erosivity within the dam control basin. Therefore, rainfall erosion and sediment yieldforecasting model was established for the 3rd and 4th dam control basin. Through testing, themodel had high precision, which could simulate and forecast rainfall erosion and sediment yieldin a small basin.(8) Relative contribution of sediment yield for inter-gully land and gully land in silt damcontrolled basins in Wangmaogou watershed was illuminated, respectively.The average ¹³⁷Cs contents in sediment of sedimentary layer and hilly slope farming land soilin Guandigou No.3, No.4 silt dam controlled basins were calculated, the sediment coming fromvalley didn't have ¹³⁷Cs, so relative contribution of sediment yield for inter-gully land and gullyland was worked out by proportion method, respectively. The studies showed that theinter-gully land was the main source of sediments in Guandigou No.3 and NOo4 check damcontrolled basins, and 70% of No.3 and 66% of No.4 of sediment comes from inter-gullyrespectively. It indicated that silt dam played very important role in reducing soil erosion andsediment yield.(9) On basis of the corresponding of rainfall events and silt dam sedimentary cycle, theevolvement trend of sediment aggradation speed, erosion and sediment yield intensity indifferent operational period of the silt dam in Guandigou No.3 and No.4 check dam controlledbasins were opened out.According to the corresponding of rainfall events and silt dam sedimentary cycle, thedeposition rate, erosion and sediment yield intensity in the silt dam in different operationalperiod in Guandigou No.3 and No.4 check dam controlled basins were analyzed. The resultsshowed that during the early stage of the construction of silt dam, dam sedimentation rate waslarge, which is 1.98～2.50 times of the average rate of 29 years. Later the trend was gradually reduced, but for different drainage areas there were certain differences, and during differentperiod of the silt dam operation, from the initial period, development period to stability,watershed erosion and sediment yield intensity was significantly reduced.