## 3 Results
### Response of SIF to Compound Droughts
Therefore, compound droughts in the TRHR's grassland ecosystems pose a significantly higher threat to carbon uptake than singular droughts. Across the Yellow River source region, the pronounced interplay between high SVPDI and low SSMI in July triggered a discernible reduction in vegetation. This distinctive set of interactions underpins a more explicit manifestation of these conditions compared to other parts of the TRHR. To further analyze this phenomenon, we posed singular anomalies arising from atmospheric drought, soil drought, and compound drought, each contributing to the decline in SIF within the Yellow River source region. The delineation was based on atmospheric drought (SVPDI \(\geq\) 0.5), soil drought (SSMI \(\leq-\)0.5), and compound drought (both SVPDI \(\geq\) 0.5 and SSMI \(\leq-\)0.5) conditions as defining criteria. As shown in Figure 13a, atmospheric droughts caused SIF anomalies of \(-\)3.46%, \(-\)0.47%, and 1.2% in the CSIF,  
Figure 12: July SIF anomalies under atmospheric and soil droughts in the TRHR during 2001 to 2020. (**a**) CSIF, (**b**) GOSIF, and (**c**) RTSIF in response to atmospheric droughts. (**d**) CSIF, (**e**) GOSIF, and (**f**) RTSIF in response to soil droughts. (**g**) CSIF, (**h**) GOSIF, and (**i**) RTSIF in response to compound droughts. GOSIF, and RTSIF datasets, respectively. The decline in SIF under the soil drought constraint was slightly greater than that corresponding to atmospheric drought, with anomalies of \(-5.36\%\), \(-4.43\%\), and \(-3.46\%\) in the CSIF, GOSIF, and RTSIF datasets, respectively. Compound droughts caused larger decreases in SIF than singular drought events, with SIF anomalies of \(-11.29\%\), \(-9.09\%\), and \(-5.13\%\), respectively. The SIF values also markedly decreased as drought severity increased, the sole exception being atmospheric drought in the RTSIF (Figure 13b). When the criterion of compound drought was SVPDI \(\geq 1\) and SSVI \(\leq-1\), the SIF anomalies were \(-14.52\%\), \(-12.15\%\), and \(-7.02\%\) in the CSIF, GOSIF, and RTSIF datasets, respectively. The compound drought magnitude index in the Yellow River source region was the largest in July of 2017 (Supplementary Materials, Figure S4), so we used the July 2017 compound drought event as a typical case to analyze the decline in SIF. The average SIF  
Figure 13: Anomalies of July SIF triggered by atmospheric and soil droughts in the Yellow River source region during 2001 to 2020. (**a**) Threshold criterion for atmospheric drought is SVPDI \(\geq 0.5\); for soil drought, it is SSMI \(\leq-0.5\); and for compound drought, it satisfies both SVPDI \(\geq 0.5\) and SSMI \(\leq-0.5\). (**b**) Threshold criterion for atmospheric drought is SVPDI \(\geq 1\); for soil drought, it is SMI \(\leq-1\); and for compound drought, it satisfies both SVPDI \(\geq 1\) and SSVI \(\leq-1\). Light-red box indicates the 10 th–90 th range, and red line marks the mean SIF anomaly. anomalies of the Yellow River source region in that month were \(-\)13.14%, \(-\)10.32%, and \(-\)6.54% in the CSIF, GOSIF, and RTSIF datasets, respectively (Figure 14a-c). Almost the whole region presented negative SIF anomalies in the CSIF and GOSIF datasets, though these anomalies were especially pronounced in the western and southern parts of the region, where SIF decreased by up to 20%. We further analyzed the additional impact of compound droughs on SIF compared to individual drought events. In comparison to a singular atmospheric drought, the additional effect of a compound drought was significantly amplified, resulting in SIF anomalies reaching \(-\)9.83%, \(-\)9.67%, and \(-\)7.68%. These changes were especially accentuated in the southern parts of the region (Figure 14d-f). There was a more pronounced decline in SIF attributable to the compound drought in July 2017, surpassing that observed under atmospheric drought conditions; these effects further inhibited vegetation photosynthesis.