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Net primary production (NPP) is one of the most important components in the carbon cycle of terrestrial ecosystems. Climatic and hydrologic elements are among the primary factors controlling the dynamics of NPP at global and regional scales. Thus, understanding the interactions between them is of great importance for optimal ecosystem management. This study aimed to investigate the spatiotemporal change in NPP and its responses to both climatic and hydrologic factors in a semiarid watershed-the Upper reach of the Wei River Basin (UWRB)-on the Loess Plateau, China. To this end, an integrated hydro-biogeochemical model (SWAT-DayCent) was applied to examine NPP during the period 1987-2016. The results show that the SWAT-DayCent performed well in simulating both the hydrologic and the biogeochemical components in this typical loess watershed. Though the basin average NPP increased slightly during the recent 30 years (1987-2016), the spatial distribution varied significantly in the region, with a relatively higher level in the southeastern and western parts and a lower level in the northern part. The strongly positive responses of major vegetations to precipitation indicate that precipitation was the dominant factor driving the ecosystem production, whereas warming may exert negative effects, especially in the southeastern part of the UWRB. Further, the strongly positive relationships between NPP and soil water/ET also suggest that the ecosystem production relied heavily on the water availability, indicating a tightly-coupled water-carbon cycle in this region. Overall, our findings are of great importance for identifying the key driving forces of the ecosystem production and the interaction between water and carbon cycles. The study may also aid policymakers in seeking better eco-environmental management when facing the climate change on the Loess Plateau.