净生态系统生产力（NEP）是估算区域植被碳源、碳汇的重要指标。以甘肃白龙江流域为研究区，结合MODIS与气象数据对2000-2013年的流域净生态系统生产力时空变化进行了研究，并探讨了典型地形因子对NEP的影响。结果表明：（1）2000-2013年甘肃白龙江流域单位面积NEP平均为226.65 g C m-2 a-1，碳汇区主要分布在白龙江上游两岸、岷江西岸、白水江南岸、大团鱼河两岸的山地林区，碳源区主要分布在武都区、迭部县北缘的高寒草甸区等。（2）从不同植被类型上看，常绿阔叶林、常绿/落叶阔叶混交林单位面积NEP最高，高寒草地单位面积NEP最小，且耕地单位面积NEP增加最明显，常绿/落叶阔叶混交林单位面积NEP降低最明显。（3）2000-2013年研究区单位面积NEP总体上呈增加的趋势，增加明显的地区分布在流域的中部和西北部，4-9月为流域碳汇季节。（4）地形因子对甘肃白龙江流域NEP有明显影响，海拔4200 m以下多为碳汇区；陡坡区的碳汇能力的增长趋势低于缓坡区；阴坡的碳汇能力高于阳坡区。
Net ecosystem productivity (NEP) is an important factor for the regional estimation of carbon sources and carbon sinks,and the accurate estimation of these components of the carbon cycle is imperative to be able to predict climate change and establish reasonable environmental policies.In this study,we analyzed the temporal and spatial distribution of net ecosystem productivity on a unit area basis in the Bailongjiang Watershed,Gansu Province,China,from 2000-2013 via using MODIS and meteorological data and then explored the relationship between NEP and topographical factors.Four primary results were obtained:(1) The mean annual NEP was 226.65 g C m-2 a-1 from 2000 to 2013 in the watershed,and the carbon sink area accounted for 97.20% of the whole watershed area.The carbon sinks were mainly distributed in forest land along both sides of the upper Bailongjiang,the west bank of the Minjiang,the southern bank of the Baishuijiang,and both banks of the Datuanyu River.The carbon source areas were mainly distributed in the Wudu District and the northern border area of Diebu County.(2) Mean annual NEP was highest for evergreen broad-leaved forest and mixed evergreen and deciduous,broad-leaved forest followed by the mountain steppe.Mean annual NEP for cultivated land,coniferous forest,and shrubland was similar,and even smaller for the alpine steppe.While NEP tended to increase over time in cultivated land,the trend was a slight decline in mixed evergreen and deciduous,broad-leaved forest.(3) There was significant spatial heterogeneity in the trends of how NEP changed over time.Specifically,NEP tended to increase in the Bailongjiang Watershed,which accounted for 64.00% of the study area and were mainly distributed in the central and northwest regions of the watershed,particularly in the mid-Bailongjiang River,and NEP that tended to decrease was mainly distributed in the centre of Wudu District.Additionally,the period from April to September is when both NEP and the carbon sink areas increase.(4) Topographical factors played an important role in NEP in the Bailongjiang Watershed.The area below 4200 m was the primary carbon sink area,and the NEP growth rate and carbon sink capability on steep slopes (over 40 degrees) were higher than in areas with a gentler slope.Further,the capacity of carbon sinks on shady slopes was higher than for those on sunny slopes.