Study On The Eco-environmental Characteristics And Effects Of Irrigated Agriculture In The Qinghai-Tibet Plateau

The Qinghai-Tibet plateau,as an important ecological barrier in China and in Asia,has the characteristics of climate change sensitivity and ecological environment vulnerability.Ecological environmental protection has always been the bottom line of the development for the Qinghai-Tibet Plateau.Irrigated agriculture is one of the important ways of surface resources utilization in the Qinghai-Tibet Plateau.Compared with the development for traditional low-altitude irrigated agriculture,the management mode of plateau irrigated agriculture is significantly different,where the development direction of irrigated agriculture is constrained by the fragile ecological environment.However,there is little research on the relationship between the development of plateau irrigated agriculture and ecological environment,which leads to the lack of systematic understanding of the ecological environment effect of plateau irrigated agriculture and restricts the sustainable development of plateau irrigated agriculture.The Lhasa River basin is a typical irrigated agricultural area on the Qinghai-Tibet plateau,located in the core area of southern Tibet.In this study,the material flow analysis,field sampling,geostatistics and regional model simulation were used to systematically explore the environmental effects of irrigated agriculture in the Lhasa River basin,including the characteristics of farmland nitrogen and phosphorus flow and its water environment effect,the quantitative analysis of the impact of irrigated agriculture on local vegetation dynamic change,the impacts of irrigated agriculture underlying surface changes on local climate,as well as the threshold of cultivated land ecological security index and the spatio-temporal pattern of cultivated land ecological security.The eco-environmental effects of irrigated agriculture in this study mean the watershed water environment,vegetation dynamics and regional climate.It is of great significance for guiding the sustainable development of plateau irrigated agriculture.The main research contents and conclusions are as follows.（1）Based on the material flow analysis and field water quality sampling,the nitrogen and phosphorus nutrient flow models of plateau farmland system considering the non-anthropogenic pollution source were constructed.The characteristics of nitrogen and phosphorus flow in watershed farmland system were clarified.Then the contributions of nitrogen and phosphorus loss in farmland system to the water ecological occupation in irrigation area and watershed were quantified.Finally,the driving mechanisms of nitrogen and phosphorus loss in the farmland system were revealed.The results showed that from 2006 to 2018,the total nitrogen input from the farmland system in the Lhasa River basin continued to decrease from 12.9 million kg to 7.9 million kg,while the total phosphorus input first increased from 5.4 million kg to 11.2 million kg and then decreased to 7.8 million kg.The non-anthropogenic pollution sources contributed 9.6%and 1.5%of the nitrogen and phosphorus input from the farmland system respectively,which confirmed the necessity of considering non-anthropogenic nitrogen input in the calculation of agricultural non-point source pollution load on the plateau.About 15%of nitrogen input in the farmland was lost via leaching and runoff,while only 0.58%of phosphorus input entered into the water environment.The nitrogen and phosphorus nutrients from irrigated agriculture were not the largest contributor to the water environment in the whole basin,and its contribution rate decreased.The effects of irrigated agriculture on water environment were limited to the river nitrogen pollution only at the irrigated area scale.The irrigated agriculture had no adverse impacts on the water environment at the basin scale.The driving mechanism of nitrogen and phosphorus loss in plateau farmland lied in the nutrient utilization efficiency and crop planting structure.（2）Based on statistical methods such as multiple correlation analysis and multi-source data regression,this study explored the temporal and spatial variation of vegetation dynamics in the Lhasa River basin,identified the attributes of the human disturbance and climate change to the vegetation dynamics,put forward a method to extract the impacts of irrigated agriculture on vegetation dynamics separately,and quantified the impacts of plateau irrigated agriculture on vegetation dynamics.Finally,the characteristics of plateau irrigated agriculture on vegetation dynamic change were revealed.The results showed that the vegetation growth in the Lhasa River basin was generally improved,but the spatial heterogeneity was strong.The vegetation growth in the lower reaches and northwest of Lhasa River basin tend to deteriorate,but the vegetation growth in the high-altitude areas in the upper reaches of Lhasa River basin tend to improve.About 73%of the vegetation growth in the basin was dominated by human factors,mainly distributed in the low-altitude valley areas in the lower reaches of the basin.The vegetation growth in the northwest and southeast of the basin was mainly affected by the warm and humid climate.The influence of irrigated agriculture on the vegetation dynamics had a logarithmic distance attenuation,and there were spatial differences in the disturbance of different landscape substrates of grassland and farmland.The reservoir construction lead to the vegetation degradation with grassland as the landscape matrix at the rate of 0.25 gC m^-2 yr^-1 and 0.0012 yr^-1,and the occupation of cultivated land directly lead to the vegetation degradation with farmland as the landscape matrix at the rate of 0.8 gC m^-2 yr^-1 and 0.004 yr^-1.The construction of irrigation area was conducive to the accumulation of vegetation with farmland as the landscape matrix at the rate of 1.20 gC m^-2 yr^-1.（3）Comparing the differences between changing trend of air temperature in high and low proportion of irrigation areas,the impacts of irrigated agriculture on air temperature in the Lhasa River basin was analyzed.The data processing module of local climate model WRF was improved to realize the dynamic reading of underlying surface data.The coupled WRF-CLM model was adopted to reveal the impacts of the underlying surface change for irrigated agriculture on the local surface temperature from the perspective of radiation budget and energy balance by setting up two groups of different underlying surface simulation experiments（1980 and 2015）.The results showed that the irrigated agriculture had the effect of slowing down the increase in regional air temperature,and the cooling effect on the maximum temperature(T_max)（0.017℃/10a）was greater than that on the minimum temperature(T_min)（0.011℃/10a）.The change of underlying surface for the irrigated agriculture has the greatest impact on local surface temperature（T_a）in summer.The decrease of agricultural land lead to the significant increase of T_a（0.14℃）in spring and T_a（0.23℃）in summer.The reservoir construction lead to the significant decrease of T_a（0.14℃）in winter and a slight decrease of T_a（0.10℃）in summer.The increasing floodplain forest land lead to a slight decrease of T_a（0.10℃）in summer.The influence mechanism of changing underlying surface on local T_a mainly lied in evapotranspiration and surface albedo.There were differences in the impact mechanism of underlying surface change on local T_a.The impact of decreased agricultural land and increased floodplain forest land on local T_a mainly existed in the change of net radiation,which was more allocated to latent heat flux,and the evapotranspiration played a leading role.The impact of reservoir construction on local T_awas attributed to the change of net radiation,which was more allocated to sensible heat flux,and the surface albedo played a leading role.（4）A multi-level cultivated land ecological security index system considering the relationship between cultivated land system and adjacent environment was constructed,and the index threshold of multi-source factors for every level was scientifically defined.Then the spatial and temporal variation characteristics of cultivated land ecological security and the key constraints factors were identified.The results showed that based on the cultivated land ecological security index system consisting of 13 factors considering the relationship between cultivated land,water and vegetation,the irrigated agricultural development had an adverse impact on the ecological security of cultivated land.The cultivated land ecological security went through the process of decline first and then improvement.The development of cultivated land ecological security in various counties in the basin was not coordinated.The impact factors had obvious regional similarity and spatial heterogeneity.The total power of agricultural machinery,the nitrogen fertilizer and the phosphorus fertilizer were the main factors.On the basis of cutting nitrogen at the source,this study put forward the ecological protection strategy of focusing on the construction of riparian ecological buffer zone and forming a double-layer structure of riparian buffer zone surrounding farmland,contributing to promoting the harmonious development of plateau irrigation agriculture and ecological protection.