黑河源区高山草甸的冻土及水文过程初步研究

Preliminary Study of the Hydrological Processes in the Alpine Meadow and Permafrost Regions at the Headwaters of Heihe River

介绍了黑河源区野牛沟流域在试验点尺度和山坡尺度上所开展的冻土水文过程初步结果.冻土水文观测场建于最大冻结深度约为3.0 m的季节冻土区,近50 a来,该区降水量变化不大,器测蒸发量(20)和风速呈明显的降低趋势,而气温和地表温度则分别上升约1.0℃和1.7℃.研究区季节冻土冻结上限和下限深度均与地表温度呈二次多项式关系,这表明地表温度与冻结或融化区地温变化之间有一个滞后过程.在地表融化季节,季节冻土存在两层现象.当融化深度接近最大冻结深度时,存在向上和向下的双向融化现象,但自下而上融化速率较慢.2005年9月—2006年9月,具有较高代表性的3个山坡径流场均没有观测到产流量,结合蒸散发观测和野外调查,发现夏季高山草甸具有明显的地表径流拦蓄和水源涵养作用.COUP模型能够较好的连续演算试验场生长季节高山草甸-季节冻土-大气-维水热传输和耦合过程,但因其土壤完全冻结临界温度阀值设置偏高,影响了非生长季节的计算精度.

In consideration of global warming and deteriorating environments, the research on hydrological processes in permafrost and seasonal frozen soil regions in North China and the Tibetan Plateau, especially in the mountainous regions of an inland river basin, is very important and necessary. The research site is Yeniugou （99°32.5＇ E, 38°27.5＇ N, 3 320 m a.s.l.）, which is covered with alpine meadow, located in the middle reaches of the upper Heihe River, one of the long inland river in Northwest China. The averaged annual precipitation, yearly mean air temperature, averaged annual evaporation （Φ20） and the annual sunshine hours （1959--2004） is about 403.4 mm, 3.1 ℃, 1 275.9 mm and 2 701.7 h, respectively. In the recent 46 years, precipitation changed a little, evaporation （Φ20） and wind speed decreased with a R2 value of 0.38 and 0. 62, respectively, while air temperature and sunshine hours all increased with a R2 value of 0. 29 and 0.13, respectively. The averaged soil surface temperature is about 0.7 ℃ from 1980 to 2004, and the R^2 value of its increasing trend is about 0.51. The ground that is flat is seasonally frozen in the Yeniugou Site, with an averaged maximum frozen depth about 3.0 m. The relationship between the soil surface temperature and the up or down depth of the seasonal frozen soil is quadratic polynomial, which means that there is a hysteresis of the soil temperature from the soil surface to the thawing or frozen depths. In the thawing season, there are two frozen layers in Yeniugou Site. When the thawing depth was close to the maximum frozen depth, the thawing process occurred from both the up and down surfaces of the frozen soil. A meteorological station was built at the Yeniugou Site in 1959. In order to analyze the hydrological processes of the research site, an ENVIS system has been installed in 2004, in which nearly all the meteorological variables, soil temperature and water contents of 7 layers were measured. Three runoff plots were built, with an area of 83.4 m^2, 187.6 m^2 and 160.1 m^2, respectively, in August, 2005. The controlling depth is about 80 cm. Nine lysimeters with an inner diameter of 27 cm and a depth of 27 cm and six lysimeters with an inner diameter of 31.5 cm and a depth of 40 cm were installed in the runoff plots at the same time, to measure the evapotranspiration and to calculate the intending water balances. Of course, the soil properties of different soil layers were also measured. The Preliminary research results show that the alpine meadow holds up the surface runoff evidently in summer, and the surface runoff appears occasionally in these regions. The rain held up by the alpine meadow infiltrates soil slowly, resulting high water content in soil. Therefore, the alpine vegetation grows well and the underlying surface is wet, which forms a cold and wet microclimate in these regions. In this case, the ecological function of the alpine meadow is more than its hydrological effect. The COUP model could simulate the heat and water transfer processes in 1D-mode well in growth season, while＇ in the other season, not. The reason is that the threshold temperature value, below which the soil is assumed to be completely frozen, except for a residual unfrozen amount of the COUP model, is a little higher, which therefore affects the whole water and heat transfer processes in the frozen soil.