The Research On The Hydrological Cycle Based On The Synergistic Effect Of Vegetation And Frozen Soil In The Source Region Of Yangtze And Yellow River

The process of hydrology and energy under the global change and humanactivities are the control factors which retain the source region of Yangtze and Yellowriver as the "Chinese water tower". The LUCC and climate change seriously affect thebalance of hydrology and heat-water on the high-cold ecosystems of the Yangtze andYellow river. This thesis discuss the climate change and LUCC in the source region ofYangtze and Yellow river, and the influence of LUCC on streamflow in theeco-environment source region are studied by the SWAT model. Two typical smallwatershed are selected in the source region of Yangtze and Yellow river respectivelyfor the examination and observation on the process of hydrology, energy and ecologyin different stages. A SRM (snow melt runoff model) are used to discuss the responseof stream process to climate change. A frozen soil hydrology process model is built,and the heat-water process is discussed under different vegetation coverage based onmodel simulation and observation.1. The precipitation has slowly increasing trends in the whole source region ofYangtze and Yellow river, with 4.3mm every l0 years. However, the eight gaugingstation in the south region of Yellow river has a decreasing tendency, and the otherstations increase, the elevation dependency of precipitation changing is not significant.The temperature of 32 stations in and near the source region reflects significantwarming, with average warming amplitude of 0.318℃/10yr which is higher than theglobal Qinghai-Tibet Plateau. The elevation dependency of climatic warming is notsignificant.The grassland ecosystem is the most import ecosystem in the source region ofYangtze and Yellow river. The area of the high cover grassland decrease since 1950sespecially from 1986, and its ecosystem stability reduce; however, the area of the lowcover grassland increased, and its ecosystem stability increased. The degeneration oflake is mainly happened in the source region of Yangtze river, and the degeneration ofriver is mainly happened in the source region of Yellow river. The runoff gradually decreases since 1985s, and the Maduo is the most obvious.The decreased of the flood is smaller than the baseflow. The impact of the LUCC onstreamflow is about 19% over the whole year, but nearly 28% in the low water period.2. There are two obvious floods which are spring flood and summer flood in thepermafrost region. The spring flood is mainly consist by the precipitation, snowmelting and thawing water of frozen soil, while the summer flood is mainly consist bythe precipitation. The runoff coefficient in the stage of latter spring and autumn ishigher than the average and sometimes higher than 1 and the runoff coefficient inspring is much smaller.The SRM model is successfully used in the Fenghuoshan watershed. Whentemperature increase 2℃, the snowmelt season significantly shift towards earlier datesand the spatial distribution of annual runoff has also obviously been redistributedwhich present a trend of increase in spring. Increase or decrease in annual precipitationof 10% result in corresponding changes in annual runoff and peak flow.The frozen soil hydrology model simulate the process of hydrology and energy well,the model efficient coefficients of the simulated soil temperature at different depth areall over 0.85 and the streamflow is efficient coefficient is 0.731. The simulation underdifferent climate scenarios indicate that evapotranspiration is highest at the scenario oftemperature increase 1℃and the precipitation increase 10%, while it is the lowest atthe scenario of temperature unchanged and the precipitation decrease 10%. Thesimulation under different vegetation coverage reveal that with the decrease ofvegetation coverage, the evapotranspiration decrease and the runoff increase, and thenthe composition of runoff changed, with the proportion of surface runoff increased andsubrunoff decreased.3. The freeze-thaw process was sped up by the reduc-tion in vegetation cover, withthe date of onset of freez-ing for the seasonally frozen soil and of onset of thawing forthe permafrost soil being clearly earlier. With de-clining cover the integral of freezingdepth for the sea-sonally frozen soil increased, but decreased for the per-mafrost soil.The maximum invasion depth and duration of the negative isotherm for the frozenstage and of the positive isotherm for non-frozen stage increased with declining vegetation cover, however, positive isotherms (≥10℃) of seasonally frozen soil at thenon-frozen stage de-creased with declining vegetation cover, whereas the influence ofnegative isotherms (≤-5℃) for frozen soils is not obvious.There were two high soil moisture layers (0.40 and 1.20 m depths) and a lowmoisture layer (0.70 m) in the profile of the active layer of permafrost. Compara-tively,in the seasonally frozen soil the two high mois-ture layers occurred at depths of 0.10and 0.80 m, and a low moisture layer at a depth of 0.30 m. With a decline in vegetationcover, the permafrost soil moisture decreased in the top (0.2-0.60 m) of soil profile,but increased at greater depths. Comparatively, a decline in cover of the seasonallyfrozen soil resulted in a decrease in soil moisture over the entire soil profile.The simulation of the frozen soil hydrology model reveal that with the decrease ofvegetation coverage the latent heat decrease and the absolute value of ground heatincrease, which then accelerate the thawing process of the deepen frozen soil and theincrassation of the active layer.