The Hydrology Process And Ecological Functions Response Under The Vegetation Coverage Change Of Alpine-cold Meadow In The Headwater Region Of The Yellow River

The high-cold ecosystem in the headwater region of the Yellow River is the special one and easy to be influenced by the variation of the global environment and human activities, which has shown some degradation with different degrees. Vegetation degradation had made serious effects on the safe and the balance of whole high-cold ecosystem. This study is the key composition of the significant research project funded by National Natural Science foundation of China—"The response and the hydrological effect of high-cold ecosystem on global climate change in the headwater region of the Yangtze and the Yellow River". This dissertation mainly focus on the response of hydrological process to vegetation degradation and the influence of ecology function resulted by vegetation coverage change of alpine-cold meadow in the headwater region of the Yellow River. The typical areas with different vegetation coverage were chosen as the study area and alpine-meadow sample plots, where some experiments related to hydrological process were conducted, such as soil water infiltration, alpine-meadow interception, runoff and sediment yield, soil water distribution, coupling variation on soil water and temperature of seasonal frozen soil. At same time, some surveys were also made about plant community structure, ground and underground biomass, the variation of soil physical structure and chemical constituents on alpine-cold meadow with different degradation degree. Finally, the hydrology process and ecological function on the high cold region in the source of the Yellow River were implemented by VIC model. The main results and conclusions are as following:1. The field double-ring infiltration instrument was used to the soil infiltration measurement in loess hills area. The improved field double-ring infiltration instrument has been applied to test the soil water infiltration in the alpine-meadow. The results showed that this instrument can be better used to observe the one-dimensional vertical flow movement in the high-cold region. The soil infiltration process was fitted with different infiltration models, and the comprehensive evaluation was made at the transient phase (0-15min), the gradual change phase (15-60min) and the steady phase (longer than 60min). It showed that the Kostiakov infiltration model (f(t) = at^-b ) could better reflect the soil infiltration course than Philip model, Horton model and the general empirical formula in the high cold region.2. Soil water condition is the critical factor to determine the distribution and the growth of the alpine-cold meadow, and plays an important role in alpine-meadow research. The vegetation coverage and the terrain factor have a great effect on the soil water condition during growing season. With the increase of the absolute altitude, the soil water at different depth showed incensed at first, and then decreased. The relation between the soil water at different depth and the altitude satisfied the unary quadratic parabola. It showed a better agreement between above two at the soil depth of 60cm and (10-30cm) than that of others. The soil water with different vegetation types varied with the altitude: the reduction type and the fluctuation type, which included the shrub meadow, black soil types and the Kobresia meadow, degraded meadow respectively. The soil water with different degradation degree varied according to the following sequence as shrub meadow, Kobresia meadow, light degraded meadow, moderately degraded meadow, black soil type, and showed symmetric variation tendency with the light degraded meadow served as the symmetry axis. The variation tendency were fluctuating and reducing for both sides.3. The heat was the control factor which effected the soil water distribution and transfer of alpine-cold meadow. The regression model was established between the soil water and soil temperature under different vegetation coverage and soil depth by using the statistical methods in SPSS and DRS soft, and connected two parameters together. Theθ_v-T_s relationship showed a strong correlation with correlation coefficient R² range is (0.83-0.96) and (0.92-0.96) in freezing and thawing processes respectively. The interrelationship of soil temperature and water content in different periods of freezing and thawing provides a tool for discussing the effect of soil temperature and water content on soil frozen-thawing cycle. Better correlation between the simulated results and the observed data validated the models and the method.4. In analysis and computation of hydrologic data and water resources, only rainfall evaporation and rainfall infiltration in soil are often taken into account for rainfall losses, with rainfall interception by vegetation neglected, which is not imperfect in theory. The Kobresia meadow and the shrub meadow, which represent two different grass growth forms, were choosing to made contrast tests on interception. The results showed that the maximum interception of the Kobresia meadow and the shrub meadow can reach 1.8mm and 1.0mm respectively, for the existence of the difference of plant height, leave shape, leave area, underground biomass. When compared with the annual precipitation, the interception losses would reach 18.2% and 10.8%. The observing results showed that runoff and sediment yield process of alpine-cold meadow with different vegetation coverage was coincidence with power function (R = at^b ) distribution, while the black soil with coverage below 5% is more coincidence with exponential function (R = ae^bt) distribution. When coverage varying from high to low, the parameter a decreasing, and b increasing. The fitted regression equation between total vegetation coverage and runoff duration as following, C = 3.4881nt_p -11.772 (R²=0.9477). The equation shows that the vegetation is positive correlation to duration, i.e. duration is prolonging with the coverage increasing, which indicates that increasing vegetation coverage is an important measure to alleviate soil and water loss and reduce runoff on slope land.5. The surveys were made about plant community structure, ground and underground biomass of alpine-cold meadow with different degradation degree. The measured results on Aug., 2005 showed: the species diversity and evenness indexes had the largest values at the meadows characterized as being moderately degraded; the underground biomass at the depth of (0-15) cm accounted for 88.57% of total underground biomass in the light degraded meadow, while it accounted for 97.32% of total underground biomass in the heavy degraded meadow; the ratio of the underground and ground biomass was 4.89 and 3.47 for the light degraded meadow and heavy degraded meadow respectively.6. The vegetation coverage changes will lead the significant variation of soil environment. The research showed that the soil bulk density appeared the increasing trend resulted by vegetation degradation. The relationship between soil bulk density and vegetation coverage was most obvious at the soil depth of 0-10cm. The soil organic matter, total nitrogen (TN) and hydrolytic nitrogen (HN) losses on the whole profile (0-40cm) were 69%, 61% and 68%; the loss rates of available Nitrogen, available Phosphorus, and available Potassium were 51%, 38% and 21.5% respectively.