The Simulation And Analysis Of NPP Based On CASA Model In Hulunber Grassland

Net primary productivity is the most direct indicator of grassland growth characteristics and health status.Accurate estimation and quantitative analysis of NPP has always been an important scientific issue in grassland ecosystem research,and it is also of great significance for grassland carbon cycle research.In this paper,Hulunber grassland is selected as the research area,and the accuracy of grassland NPP quantitative inversion is taken as the core.Combined with remote sensing data,meteorological data and measured data,the photosynthetically active radiation?PAR?remote sensing product verification and screening is carried out,Based on the CASA model,the NPP of Hulunber grassland was simulated in2007-2016,and the temporal and spatial variation characteristics of NPP in different grassland types in Hulunber area were analyzed.The main conclusions are as follows:?1?The verification of grassland photosynthetically active radiation PAR remote sensing products and algorithms was carried out.Based on the data of 8 ground radiation stations in Inner Mongolia,the MCD18A2,BESS PAR products and the sunshine hour's algorithm on the monthly scale were cross-validated.The results show that BESS PAR products have the highest accuracy?R²=0.96,RSME=43.59,REE=0.01?,followed by MCD18A2 product data?R²=0.95,RSME=46.89,REE=0.07?,the sunshine illumination algorithm has the lowest simulation accuracy?R²=0.94,RSME=139.74,REE=0.55?.Based on the BESS PAR data,the temporal and spatial variation of PAR value in Hulunber grassland is analyzed.It is found that the PAR value of Hulunber grassland gradually increases from northeast to southwest,with low spring and winter season and high summer and autumn.?2?The accuracy verification of the NPP simulation results of the CASS model based on BESS PAR was carried out.Cross-validation of CASA-NPP inversion results,MODIS NPP products and ground measured data showed that CASA-NPP simulation accuracy?R²=0.67,REE=0.28?was significantly better than MODIS NPP products?R²=0.52,REE=0.78?.During the study period,the MODIS NPP product value was higher than the CASA-NPP inversion value of about 77.83gC·m^-2.The CASA model based on BESS PAR product can better reflect the overall situation of the Hulunber grassland NPP.?3?The temporal and spatial variation characteristics of NPP in Hulunber grassland are analyzed.The average annual NPP of Hulunber Grassland from 2007 to 2016 was 195.26 g C·m^-2.The NPP mean value of Hulunber grassland increased gradually from 2007 to 2013(slope=11.23 g C·m^-2),and it showed a significant downward trend from 2013 to 2016(slope=-30.59 g C·m^-2).In the past 10 years,the annual average value of NPP in the study area had gradually decreased from northeast to southwest.NPP of different grassland types showed different characteristics.The highest average NPP was mountain meadow,followed by lowland meadow.The warm meadow grassland was slightly higher than swamp,and the gentle grassland was the lowest.?4?The relationship between NPP and climate factors in Hulunber grassland was analyzed.The correlation coefficient?0.69?and partial correlation coefficient?0.65?between NPP and precipitation in Hulunber grassland are higher than the correlation coefficient?-0.22?and partial correlation coefficient?-0.02?between NPP and temperature.Precipitation is the main factor affecting NPP change in grassland.factor.Among different grassland types,warm meadow grasslands and warm grasslands are more sensitive to changes in climatic factors than lowland meadows and mountain meadows.The conclusions of the study provide a theoretical basis for the local herders to understand and master the ecological status of the Hulunber grassland and the local government to effectively manage the grassland and animal husbandry production,and also have certain guiding significance for the rational use of limited grassland resources.