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.