青海湖流域理物理侵蚀速率对气候变化的响应

Response of physical erosion rates to climate changes within the Lake Qinghai catchment

气候要素是否影响地表物理侵蚀速率目前还存在诸多争议。以往对此方面的研究均集中在高降雨区域,而对于低降雨条件下气候因子的控制机制研究目前还很有限。青海湖流域对气候变化十分敏感。本文通过对青海湖流域最大两条河流布哈河和沙柳河每日的温度、降雨、径流和河流悬浮物浓度进行为期一年的持续监测,探讨干旱/半干旱区气候要素对地表物理侵蚀速率的影响。研究结果显示:在气候要素中,降雨以及其产生的径流是青海湖流域物理侵蚀速率最主要的控制因素,而温度的影响较弱。最重要的是,降雨强度决定着最终的侵蚀速率。布哈河和沙柳河流域一次瞬时高强度降雨能产生全年侵蚀通量的30%以上。此外,相同流量条件下,季风前期的侵蚀速率要高于季风中、后期,反映了季风前期低温条件下的冰冻作用,以及春季降尘产生的大量细颗粒物质,增加了雨季来临时侵蚀物质的输出。这些认识对于理解长时间尺度上青海湖沉积物钻孔中沉积速率和古气候要素的关系具有重要意义。

Background, aim, and scope Whether climate controls erosion rates is still controversial. Some found significant correlations between erosion rates and climate （e.g. precipitation, discharge）, and others didn＇t. In addition, Previous studies were mainly focused on high precipitation and tectonic active regions, such as the Taiwan, Alps, Andes and Himalayas, whereas the study in low precipitation area remains scarce. Lake Qinghai catchment, located in the NE Tibetan Plateau, is quite sensitive to climate changes. The detailed hydrological and meteorological data in the Lake Qinghai catchment provides a good oppommity to in depth understand the effects of climate factors on erosion rate in arid/semi-arid regions. Materials and methods The daily air temperature, water discharge, precipitation and suspended particulate material （SPM） were monitored over a whole year period in the Buha River hydrological station in 2007 and the Shaliu River station in 2009. The water discharge was monitored twice a day, and when in flood periods, the monitoring frequency was inereased to 4-5 times one day. The daily SPM concentration was measured only from the period of May to October, since the SPM was extremely low during dry seasons. The physical erosion rates （PER） were calculated by daily SPM concentration multiplied water discharge. Results The results showed that the courses of air temperature, water discharge, precipitation and suspended fiver sediment in the Buha and Shaliu Rivers exhibited great seasonal variations with their high values during the monsoon seasons and low values during the dry seasons. In the Bulaa River catchment, the total precipitation was 389.8 mm in 2007, and the mean daily air temperature and water discharge during a year ranged from -20.5℃ to 16.5℃ and from 2 m3·s-1 in February to 220 m3·s-1 in August, respectively. In the Shaliu River catchment, the annual precipitation was 431 mm in 2009, with the mean daily air temperature and water discharge ranging from -25.5℃ to 12℃ and 0.1 m3·m-1 in February to 107 m3·s-1 in July, respectively. It is observed that the daily SPM concentrations vary from 〈0.001 kg·m-3 in the dry season to 2.280 kg·m-3 in the monsoon in the Buha River, and from 〈0.003 kg·m-3 to 2.350 kg·m-3 in the Shaliu River. Seasonal PER exhibits significant changes in the Buha and Shaliu Rivers. The SPM fluxes vary from 〈1 tons·day-1 in the dry season to 23155 tons·day-1 in the monsoon in the Buha River, and from 〈1 tons·day-1 to 21078 tons·day-1 in the Shaliu River. Discussion The relationships between temperature and erosion rates were weak （R2 is 0.22 and 0.17 in the Shaliu and Buha River, respectively）, whereas the water discharge showed good correlation with erosion rates （R2 is 0.77 and 0.68 in the Shaliu and Buha River, respectively）. In addition, some samples with abnonnaUy high physical erosion rates were corresponded to heavy rain events. In the Shaliu River, a 26 mm rainfall within 5 hours on 15th, July generated 21078 tons of SPM, accounting for 35.2% of the total year flux in 2009, whereas totally 92 mm rainfall in the entire August only contributed 6937 tons. Similarly, a highest daily rainfall （20 mm） in the Buha River brought in a highest SPM flux on 18th July, and produced 22.5% of the total year flux in 2007. Meanwhile, it was also observed that at a given discharge, the suspended sediment flux was generally higher from May to June than from July to October in both river catchments, indicating that freezing processes before monsoon and input of eolian dust in spring season provided large volumes of fine materials which was finally flushed out at the onset of monsoonal rainfall resulting in the high suspended sediment fluxes during the pre-monsoon period. Conclusions In the Lake Qinghai catchment, water discharge and precipitation controls erosion rates, but temperature shows only a weak control. The most important is that precipitation intensity rather than total rainfall volume or discharge finally determines the erosion rates. One storm rain event can produce more than 30% of the total yearly suspended river sediment flux. Higher erosion rates during pre-monsoon period than mid- and post-monsoon are ascribed to freezing erosion and eolion dust input. Recommend__m__＇_ons and perspectives We firstly detailed discussed whether temperature, discharge and precipitation control physical erosion rate in the two largest rivers in the Lake Qinghai catchment, which are critical to explore their relationships at add/semi-arid climate condition. Also, this study is significant for understanding the relationship between deposition rate of Lake Qinghai sediment and paleoclimate changes over long-time scale.