## 4 Discussion  
### The Spatiotemporal Variation Characteristics of GPP in Arid Regions  
A comprehensive analysis of GPP station data from three different types of ecosystems (farmland, desert vegetation, and grassland) shows that the intra-annual variation in the daily average and daily cumulative average GPP of various ecosystems in arid areas presents an inverted \"U\" shape. GPP starts to increase slowly from March (when vegetation begins to grow), peaks in June and July, begins to decrease slowly in August, and levels off in mid-to-late October (the end of the growing season). During other periods, GPP fluctuates insignificantly and remains low. There are differences in GPP attribute values among different ecosystems, with the order being farmland > grassland > desert vegetation. This is because the impact of drought duration and severity on GPP is remarkably significant [58]. Among these values, in farmland areas, the minimum daily average GPP (0.00002 gC/m\({}^{2}\)/day) and the minimum daily cumulative GPP (0.0003 gC/m\({}^{2}\)/day) occur in January. The inflection points are in March (when vegetation is planted and GPP begins to increase), July (the peak, with a maximum daily average GPP of 0.94 gC/m\({}^{2}\)/day), and late October (when vegetation dies and GPP levels off). This is because crops like corn and cotton in farmland areas have relatively short growth cycles. Their large leaf area and artificial irrigation lead to intense photosynthesis during the growing season, while photosynthesis sharply declines during other periods, resulting in a regular seasonal variation in GPP. This finding is consistent with [PERSON]'s [59] study on the comprehensive characteristics of GPP for various crops on a global scale. In grassland areas, the minimum daily average GPP (0.00003 gC/m\({}^{2}\)/day) and the minimum daily cumulative GPP (0.0005 gC/m\({}^{2}\)/day) occur in December. The snow melts in mid to late March, and vegetation starts to grow, so GPP begins to increase. July is the peak period, with a maximum daily average GPP of 0.31 gC/m\({}^{2}\)/day. In early November, snow accumulates and vegetation dies, and GPP levels off. This is because the grasslands in arid regions start to green at around the 110 th to the 150 th day of the year, begin to grow around the 140 th to the 160 th day, and start to wither at around the 270 th to the 300 th day. Without irrigation or other human influences, the growth cycle is stable. This finding aligns with [PERSON]'s [60] revelation of the spatial-temporal variation characteristics of Xinjiang's grasslands over 20 years. In desert vegetation areas, the minimum daily average GPP (0.00001 gC/m\({}^{2}\)/day) and the minimum daily cumulative GPP (0.0001 gC/m\({}^{2}\)/day) occur in January. In late March, the snow melts and short-lived vegetation starts to grow, so GPP begins to increase. July is the peak, with a maximum daily average GPP of 0.11 gC/m\({}^{2}\)/day. In late October, snow accumulates and short-lived vegetation withers, and GPP levels off. This is because desert vegetation areas represent a unique underlying surface in arid regions. Short-lived vegetation is scarce, its leaves are small, the adequate photosynthesis time is short, and the mechanism is complex, resulting in carbon sinks through non-photosynthetic processes most of the time. This research result is aligned with the comprehensive changes in greenhouse gases in the Taklamakan Desert revealed by [PERSON] et al. [61] and in the Gurbantunggut Desert indicated by [PERSON] et al. [62]. Research into several frequently used SIF satellite products applied to GPP estimation within arid regions reveals that the data from RTSIF satellites yield favorable estimation outcomes in three respects [24]. First, in terms of spatial characteristics, they have significant attributes and are highly consistent with the characteristics of arid areas.