| The highly erodible Huangfuchuan watershed, belonging to the Coarse Sand Area, islocated in the northern of the Loess Plateau. Among the soil conservation practices, checkdam played a critical role for intercepting water and sediment, silting land and so on. Changesof soil erosion, sediment yield and sources can be well identified due to the historicalsedimentation records before the check dam. The results in the present study may providegood reference on soil erosion rates assessment in the area with limited observed dataset.Spatial and temporal distribution of check dams in the Huangfuchuan watershed wasobtained from the1:50000topographic map, remote sensing images and field trips.Yangjiagou Check Dam, located at downstream of Huangfuchuan was selected as the studyarea. The sediment deposition records of sediment couplets were analyzed and calculated withthe storage capacity curve which is built according to1:10000topographic map. The changesof the sediment yield intensity in the small watershed were analyzed by identifying therelationship between rainfall events and sediment couplets. The sediment sources wereassessed by using fingerprinting recognition method. The main conclusions are listed asfollow:1. Spatial and temporal variation and distribution of the check dams in the Huangfuchuanwatershed were obtained from topographic maps (1976), remote sensing images (1990and2007). There were392check dams in1976,445check dams in1990,504check damsin2007. Among these,233check dams were built between1976and1990, and172check dams were built between1990and2007. The distribution of the check dams in1990was similar to that of2007. Numerous backbone dams and small dams in damsystem were increased, compared with1976.2. Particle size distribution of the sediment profiles were analyzed in the check dam. Theirregularity coefficient and curvature coefficient of the most sampling layers lay between5～10and1～3. Sediment particle size mostly concentrated in the range of0.002～0.05mm. Median particle sizes of clay layer and coarse sand layers showed that thesampling layer of the profile had refinement trend from the bottom to the top. Particlesize analysis for three different profiles suggested that there were more coarse sand inprofile3corresponding to the tributary with widely distributed Pisa sandstones compared to the profile2. The finest sediment content was found in profile1, suggesting that thecheck dam has the function to silt crude sandy and row fine sandy.3. Storage capacity curve is determined, according to the siltation profile, and changes oferosion and sediment yield intensity in the dam controlled watershed were identified.According to the corresponding relationship between rainfall events and sedimentcouplets, the sediment deposition process was divided into1996～2003and2003～2011.In conjunction with the dam height-cumulative capacity curve, the average annualerosion modulus were2003.79and1292.22, respectively. In order to elimination theeffect of rainfall to erosion modulus, unit rainfall erosion modulus of the two periods inthe small watershed were6.158and3.204. Due to intensive human activities andincreasing soil and water conservation measures, soil erosion rates and sediment yieldintensity decreased in the past several years.4. The fingerprinting analysis method was applied to identify the sediment sources in thesmall watershed. Soil samples were collected on hillside field、grassland and bare SoftRock area, according to discriminant analysis to the elements of soil samples, the tracerproperties of Cu, Pb, P, La, W, Er and Cs formed the best fingerprint identificationcompositions. The results indicated that35.6%of the sediment was contributed by thehillside field,24.7%from grassland, and39.7%from Soft Rock area. However, this is notconsistent with the proportion of their area, of which the hillside field was only about5.5%, the bare soft rock is about9.5%, the grassland is about85%. |
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