Effects Of Grassland Evolution On Surface Runoff And Its Generation In The Naqu Basin

The grassland is one of the most common vegetation types,accounting for 20% of the land surface over the world.It plays a key role in the ecology,food security,carbon balance and global climate change.Because of the global warming,population growth,and excessive land use in China,approximately 90% of the total grassland area has been degraded to a certain extent.The QinghaiTibet Plateau is one of the largest and most unique geographical unit on the Earth.Its mean elevation is more than 4000 m above the sea level and it is known as the “third pole” of the Earth.It plays a significant role in the ecological security of China.The grassland is the main vegetation type in the Qinghai-Tibet Plateau.Due to its vulnerability and the alpine condition in the region,it is extremely sensitive to the climate variation and human activities.The change of the grassland coverage affected the process of runoff yield.Although previous studies have shown that the grassland cover change was an important factor affecting water cycle and spatio-temporal variation of the water resources distribution.Few researches quantified the relationship between the grassland change characteristics and its runoff generation or process.Therefore,the study of the grassland changes affecting the watershed hydrological processes is a key issue for water resources planning,management and sustainable development.In this paper,we took the Naqu Basin as a typical region of the Qinghai-Tibet Plateau.Based on the identification of the grassland evolution,we analyzed the precipitation runoff changes thorough the indoor experiments and numerical simulations.We analyzed the variation of the surface runoff,the soil runoff and their main influencing factors under the different underlying surface conditions.Meanwhile,we also studied the the soil water transport process under different grassland coverage conditions.Through specific experimental sets,we obtained the relevant model parameters.We constructed the simulation model that was suitable for the process of the alpine grassland slope runoff generation.The mechanism of the grassland evolution affected the slope runoff generation process in the Naqu basin.Our main conclusions were as follows:(1)The Grassland Evolution characteristics in the Naqu BasinGrassland was the main vegetation in the Naqu basin,accounting for more than 90%.The whole grassland was deteriorating.The degradation degree was mian the moderate and mild classes.The spatial and temporal differences of the grassland evolution are obvious.The grassland coverage decreased in a whole level,but in various inter-annuals,it increased from 1980 to 2000 and decreased from 2000 to 2015.The spatial and temporal changes in rhe vegetation degradation were different over the watershed.However,the grassland coverage in the middle catchment was higher than the upstream and downstream catchement.(2)The response of the grassland coverage changes on the ground surface runoff concentration under the various control experimentsWith the increasing of the grassland coverage,the surface runoff and soil runoff also increased.When the grassland coverage increased 20%,the surface runoff and soil runoff increased by 19.8% and 13.4%,respectively.However,with the increasement of the grassland coverage,the accumulation of the surface runoff decreased and the accumulation of the soil runoff increased.When the grassland coverage increased 20%,the cumulative amount of the surface runoff decreased by 4.69% and the soil moisture increased by 6.66%.We also found that the rain intensity,the slope and the grassland coverage were the main factors affecting on the runoff generation starting and ending time,and the runoff cumulative amount.Comparation their the contribution values,the rainfall intensity > slope > grassland coverage.(3)Effects of the grassland coverage on soil water under the laboratory experimentsWith the increasing of the grassland coverage,the initial time of the soil water response to the precipitation decreased.Under the different slope gradients,the increment of the soil water under grassland was significantly higher than under the bare land.And with the increasing of the grassland coverage,the soil water increased.The soil layer advanced 10 cm,with the increasing of the slope,the initial time of the soil water response to the precipitation is shorter.In the 20 cm and 30 cm of the soil layer,the change rule of initial time of the soil water response to precipitation is not obvious.(4)Based on WEP-L model,the effect of the grassland degradation on surface runoff and its generationWe constructed the distributed hydrological model which was suitable for the alpine region.Thourgh setting the various grassland coverage scenarios,we analyzed the impact of the grassland degradation on the runoff generation and concentration.The results showed that the more serious of the grassland degradation,the generation amount of the annual runoff was greater.With the grassland coverage improved to 20%,the surface runoff decreased by 3.2%.