| Hilly-gullied Loess Plateau region where suffers serious soil erosion and shortage of water resources is the main source of coarse sediment in the Yellow river basin. How much eroded sediment has been removed to the river is a critical problem correlation to the safe of the river channel and is important to the Yellow river management and exploitation. Sediment delivery ratio is a good connection between amount of soil erosion and the sediment discharge. It is a key factor to define the change of erosion and sediment production in a basin. The object of the present study is to calculate the sediment delivery ratio in basins with different spacial scales on the basis of analysis the sediment discharge in the middle reach of the Yellow River and to supply scientific evidence for Soil and Water Conservation Planning and river regulation. In order to get this object the data from runoff plots tests and erosion modulus of elemental small watershed were used to calculate the sediment delivery ratio in basins with different space scales. The main results of the study as follows:(1) Generally speaking, sediment discharge of middle reach of the Yellow Rive tends to decrease. During the period of grain for green (from 1998 to 2007), the sediment discharge decreased the most, while sediment discharge decreased little from 1970 to 1989. The sediment decreased wider margin mainly located in the following regions: Toudaoguai to Wubao in the main stream of Yellow River, lower reaches of Weihe, Wudinghe, Weifenhe, Lanyihe, Hunhe, Pianguanhe, Huangpuchuan, Qingshuihe, Xianchuanhe. The sediment decreased relatively narrow margin mainly located in the following regions, such as Jinghe, Beiluohe, Qingjianhe. But there were same control regions of same of special hydrologic station with sediment discharge increased, such as Fugu to Longmeng in the main stream of Yellow river, Bajiazui to Yaoxinzhuan, Luoyuping (Banqiao and Qingyang), Lintong (Taoyuan, Yanyang, Qinduzhen, Gaoqiao, Maduwang). During the Grain for green period the sediment transport modulus was still more than 2500 t/km~2a in same watersheds, e.g. Huangpuchuan, Gushanchuan, Jialuhe, Quchanhe, yanhe and qingjianhe, so more conservation and management measures should be applied in the up regions. In this period, the contribution rate of precipitation and human activity to the sediment discharge was 37.6% and 62.4% respectively. (2) In the small watershed with area less than 1 km~2, the sediment delivery ratio was nearly to 1 under the no conservation conditions and ranged from 0.41 to 0.50 under the conservation conditions. In the watershed with area ranging from 1 to 100 km~2, the sediment delivery ratio changed from 0.83 to 0.90 under the conditions without conservation and ranged from 0.37 to 0.62 under conditions with conservation. In the watershed with area less than 100 km~2, the sediment delivery ratio got from the calculation of elemental small watershed data was very close to the test of runoff plots. Thus, these results indicated that it was possible to get the delivery ratio from the calculation of elemental small watershed data when there were no runoff plots.(3) In the hydrologic station controlled regions with single soil erosion type and with area ranging from 100 to 1000 km~2, the sediment delivery ratio changed from 0.78 to 0.96 and the results got from calculation of elemental small watershed data (area < 1km~2). If use data from different elemental small watershed with area 1~10 km~2 and area 10~100 km~2, the relative error was 2.3%and 4.6% correlated with the results got from the watershed with area < 1km~2. It suggested that watershed with area 1~10 km~2 and area 10~100 km~2 can be used as elemental small watershed to calculate sediment delivery ratio when there was no elemental small watershed with area < 1km~2.(4) In the tributary with area ranging from 1000 to 6000 km~2, the sediment delivery ratio ranged from 0.48 to 0.97 got from calculation of elemental small watershed (area <1 km~2) when know the proportion of area with conservation and without conservation. The error was relatively big when use watershed with area 1~10 km~2, 10~100km~2 and 100~1000km~2 as elemental small watershed.(5) Under conditions with conservation or without conservation, analyzed effects of following factors on sediment delivery ratio, such as runoff generation precipitation, watershed area, density of gully, gradient of channel, length of main channel and degree of conservation on the. The results showed that under conditions without conservation, the sediment delivery ratio was significantly related to the gradient of channel (related coefficient was 0.95), and without clear correlation with other factors; while under conditions with conservation, there was no clear correlation between sediment delivery ratio and the influence factors. It suggested that the change of sediment delivery ratio was a result of synthesized influence of many factors. |
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