Study On The Spatial And Temporal Variation Of NPP And Its Influencing Factors In The Xiangride-Qaidam River Basin Based On CASA Model

As the basis of energy flow and nutrient cycling of organisms,NPP has an irreplaceable role in the world carbon neutrality and climate change.As the important oasis in the Qaidam Basin and an important base for food production and cash crop production in Qinghai Province,it is of practical significance to study the vegetation NPP in the Xiangride-Qaidam River Basin.In this paper,the spatial and temporal distribution characteristics of the net primary productivity of vegetation and its influencing factors in the Xiangride-Qaidam River basin were studied by using a combination of field survey and CASA model,the maximum value synthesis method,the one-dimensional linear regression trend analysis method and the geographic probe.By analyzing the changing characteristics and influencing factors of the net primary productivity of vegetation in the Xiangride-Qaidam River basin,we aim to provide a reference basis for local agricultural production and ecological protection.The main research results are as follows:（1）The CASA model can better estimate the value of vegetation net primary productivity in the Xiangride-Qaidam River basin,and the model has better accuracy and higher R2 after validation with the MOD17A3 product,therefore,the use of CASA model has better credibility for calculating the vegetation net primary productivity in the area.The highest NPP value in the growing season in the Xiangride-Qaidam River basin was 38.68 g C·m^-2-a^-1 in 2018,and the lowest was 28.83 g C·m^-2-a^-1 in 2000,and the overall NPP value in the last 22 years showed a fluctuating upward trend,with an annual average value of 34.91 g C·m^-2-a^-1 for NPP in May-October 2020-2021.The highest NPP value was 50.41 g C·m^-2-a^-1 in July and the lowest was 11.26 g C·m^-2-a^-1 in October,and the growing season NPP values were normally distributed.（2）The spatial distribution structure of NPP of vegetation in the Xiangride-Qaidam River basin is obvious.With the gradual increase of slope,the NPP value of vegetation gradually increases,and the lowest NPP value is 27.68 g C·m^-2-a^-1 at the flat slope and the highest NPP value is 39.88 g C·m^-2-a^-1 at the sharp slope.The highest NPP value appeared at 4000-4500 m,and the lowest value appeared at 2675-3000 m.The high values of vegetation NPP are mainly concentrated in the northeastern part of the study area,where the dominant vegetation is grassland and meadow,with high vegetation cover,high precipitation and high altitude,and less disturbance of vegetation by human factors,so the vegetation productivity is higher in this area.The northwestern part of the study area has lower vegetation cover,lower altitude,higher temperature,less precipitation,and is in an urban area where there is less arable land,and the vegetation growth is easily affected by human activities,so the net primary productivity of vegetation is low.（3）The changes in NPP were not driven by a single factor,but by the combination of each factor.The q-values after the two-factor interaction were all greater than those of the single factor,and the interaction was expressed as a two-factor enhanced and non-linear enhanced type.In the single-factor detection,the changes of NPP were mainly influenced by temperature（q-mean 0.48）,precipitation（q-mean 0.45）and elevation（q-mean 0.35）,indicating that meteorological factors were closely related to the changes of NPP.In the two-factor drive,the q-values of temperature and precipitation were higher after interacting with elevation and soil type in the physical geography category,which were elevation∩temperature（q-mean 0.61）,elevation∩precipitation（q-mean 0.6）,soil type∩precipitation（q-mean 0.59）,and soil type∩temperature（q-mean 0.58）,while elevation∩soil type（q-mean 0.4）had its q values were lower,suggesting that meteorological factors and physiographic classes jointly drive the distribution of vegetation NPP.