| The variation of runoff and sediment load in the Yellow River has been always attracted the attention of many researchers. In recent years, the runoff and sediment load in the Yellow River have sharply decreased due to the influence of climate change and human activities. It is of great importance to quantitatively evaluate the changes of runoff progresses and sediment load for river basin management and water resources management. The Hekou-Longmen region is the major soil erosion control area of eco-construction of soil and water conservation in the Yellow river. However, a decreasing trend has occoured and the relationship between precipitation and runoff and sediment load has changed since 1980 s in the Hekou-Longmen region. The study area is Hekou-Longmen region and its typical tributary—Huangfuchuan. Several mathematical statistics methods such as Mann-Kendall trend analysis, Pettitt analysis, hydrological method and elastic coefficient method were applied to evaluate the variation of precipitation, runoff, sediment load and their responses to climate change and human activities from 1954 to 2012 in the study area. The effects of climate change, land use and land cover, as well as check dams on runoff and sediment load decline were evaluated by SWAT model. A small watershed in Huangfuchuan basin were selected to investigate the sediment yield and to validate the performance of the SWAT model. The main results are as follows:(1) The variation of runoff and sediment load and their response to climate change and human activities in the Hekouzhen-Longmen region were investigated in this study. In the past 56 years, the average annual runoff and sediment load have remarkably decreased with average reduction rates of 1.101×108m3 and 0.195×108t, respectively. Both the annual runoff and sediment load showed the same changing points in 1979,while the precipitation did not change significantly, which showed changes in relations to precipitation-runoff and precipitation-sediment load. The human activities were the main cause of such variation, and the contribution of human activities to the variation of the runoff and sediment load accounted for 79.2 % and 88.1% respectively.(2) Precipitation, runoff and sediment load and their driving factors in the Huangfuchuan watershed from 1954 to 2012 had extremely uneven distribution pattern. The concentration period was steady with a range of 195.85°~234.54°, which mainly occurred in July and August. The average coefficient of variation was 1.26. The monthly precipitation showed seasonal variation with high values in July and August and low values in other months. The monthly runoff and sediment load at Huangfu station showed a strong consistency with the precipitation with the concentrating on July and August. The runoff and sediment load have significantly decreased with the change point for runoff in 1984 and the change point for sediment load in 1989. Human activities were the main cause for the decreases of the runoff and sediment load in Huangfuchuan watershed. The sensibility of the runoff on precipitation calculated by elasticity analysis was 1.3, which indicated that the contribution of precipitation to the decrease of the runoff accounted for 16.9%. The contribution of precipitation on runoff was 23.7% by means of mathematical statistics, which was roughly consistent with the result by elasticity analysis. These methods can serve as references for the quantization of the influence of precipitation on the runoff.(3) Based on the change point year of runoff and sediment load in Huangfuchuan watershed, the SWAT model was calibrated and validated in the period from 1976 to 1984. The results revealed that both Nash-Sutcliffe coefficient(NS) and determination coefficient(R2) for runoff and sediment load exceeded 0.6, which indicated that the model results met the demand of precision, and could be used to evaluate the response of the variation of runoff and sediment load to the changes of climate, land use and check dam project.(4) There were dramatic variations in land use and check dam projects in different periods in the Huangfuchuan watershed. By comparing the statistics of land use in 1978, 1990 and 2006, from 1978 to 1990, there was an increasing trend in arable land, residential land and forest land with the highest increasing rate in arable land, while from 1990 to 2006, there was a decreasing trend in arable land and sandy land; on the contrary, there was an increasing trend in grass land. The construction of check dams could be divided into 3 stages. Before 1973, the check dams increased slowly with a total number of 45 dams. The number of check dams have increased from 45 to 303 with an average increase of 9.5 per year in the second stage from 1973 to 2003. From 2004 to 2009, there was 200 more with an annual average increase of 28.6 with a total reservoir capacity of 5.7×108m3.(5) The influence of land use/cover and check dam projects on the variation of runoff and sediment load was investigated. In recent years(1999-2012), human activities contributed for about 74.6% and 89.2% to the reduction of runoff and sediment load of Huangfuhcuan watershed, respectively, it is in consistent with the contribution calculated by hydrological method. The the increasing check dam projects were the main reason that made the change of runoff and sediment load, which took up 65.7% and 69.4% of all the human activities respectively. The simulation results of with check dams and wihout them were different, for the results of without check dams had higher runoff and sediment load than with check dams in runoff and sediment load for 0.32×108m3 and 0.122×108t. The variation of land use and land cover in different periods exerted great influence on water yield and sediment yield. Compared with the land use condition in 1978, the water yield caused by land use in 1990 and 2006 have decreased by 15.9mm and 20.8mm respectively, resulting in the decreases of 4, 300 t/km2 and 6, 200 t/km2 in specific sediment yield.(6) This research has made a comparison between the simulations of SWAT model and field investigation. A small check dam at Huang Jiagou in the Huangfuchuan watershed was selected to calculate the sediment yield. By collecting the sample profiles and analyzing its particle size analysis, the erosion specific sediment yield could be calculated with the data on rainfall and topography. The results suggested that from 1974 to 2015, the annual erosion specific sediment yield in Huang Jiagou watershed was 15, 770 t/(km2·a), which belonged to the severe soil erosion area. In the sediment soil particle composition in the profile of check dams, there are the highest sand content and the least clay content. The increasing trend in the profile median particle diameter changed with the depth showed that the impoundment of the sediment in check dams was fine in recent years. From 2001 to 2012, the average simulation specific sediment yield in Huangfuchuan watershed was 18, 540 t/(km2·a), while the yield calculated through the field profile sampling in Huang Jiagou watershed is 15, 510 t/(km2·a). With the deviation of 19.5%, it is obvious that the simulated value is higher than the measured one. |
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