## 3 Results
### The Suppression of Drought on NPP  
The NPP analysis calculated by the modified CASA model showed that NPP response to drought in the Yellow River basin indicated significant non-linear characteristics (Figure 3). High-intensity drought events (DI \(>\) 0.7) caused an average NPP reduction of 23.2 gC\(\cdot\)m\({}^{-2}\), representing 68% of total drought-induced productivity loss. The extreme case DEd (DI = 0.794) showed the most severe NPP deficit (\(\Delta\)NPP = \(-\)47.89 gC\(\cdot\)m\({}^{-2}\)), with its drought severity (DS = 928.31 km\({}^{2}\cdot\)h) strongly correlating with productivity loss (r = 0.91, \(p\) < 0.01). Low-intensity drought events (0.5 < DI < 0.6), including cases DEe and DEh, induced substantial NPP reductions despite their moderate intensity. Event DEe exhibited the most severe NPP deficit (\(-\)54.21 gC\(\cdot\)m\({}^{-2}\)), with this exceptional productivity loss primarily driven by the high ecological vulnerability characterizing its drought-affected regions (104.31\({}^{\circ}\)E, 36.07\({}^{\circ}\)N). This area is located in the semi-arid zone, covering the ecologically sensitive zone of the northeast edge of Qinghai-Tibet Plateau and the northern Loess Plateau, with low vegetation coverage and weak soil water holding capacity. The suppressive effect of water deficit on the photosynthetic carbon fixation process is significantly amplified. Spatial analysis revealed significant spatial heterogeneity in the intensity of NPP inhibition by drought. The \(\Delta\)NPP of the ecological fragile areas of the southern edge (event DEd) and the northern margin (event DEe) of the Yellow River basin reached \(-\)47.89 and \(-\)54.21 gC\(\cdot\)m\({}^{-2}\), respectively. However, the Hetao Irrigation District showed limited NPP loss (9.03 gC\(\cdot\)m\({}^{-2}\)) during event DEb, reflecting effective artificial irrigation mitigation. Similarly, despite severe drought conditions on the northeastern Tibetan Plateau during event DEf, observed \(\Delta\)NPP (\(-\)5.36 gC\(\cdot\)m\({}^{-2}\)) remained substantially below projected losses, potentially due to deep-rooting adaptations in alpine meadows. These cases demonstrate that ecosystem drought responses depend not only on climatic intensity but also on regional vegetation traits and human management practices. Drought-induced NPP suppression exhibited distinct temporal phasing, with the most pronounced productivity losses occurring during the initial 1-2 months of drought events. For instance, event DEe demonstrated a first-month \(\Delta\)NPP of \(-\)41.79 gC\(\cdot\)m\({}^{-2}\), representing 76.9% of its cumulative NPP deficit. As drought persisted, vegetation exhibited adaptive physiological responses including stomatal regulation and carbon reallocation, ultimately leading to positive \(\Delta\)NPP during the recovery phase of event DEe. This characteristic \"rapid decline-gradual recovery\" pattern demonstrates the Yellow River Basin vegetation's conditional drought resilience, though constrained by the underlying ecosystem's carrying capacity. The observed variations in recovery rates across ecological zones primarily reflect synergistic effects of vegetation functional traits, edaphic properties, and water acquisition strategies. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Event ID** & **Time Period** & **DD (Months)** & \
\begin{tabular}{c} **DA** \\ **(\(\times\)10\({}^{3}\) km\({}^{2}\))** \\ \end{tabular} &
\begin{tabular}{c} **DS** \\ **(\(\times\)10\({}^{3}\) km\({}^{2}\).h)** \\ \end{tabular} & **DL (km)** & **DI** \\ \hline DEe & July, 2010–November, 2010 & 5 & 449.98 & 246.08 & 1269.23 & 0.618 \\ DEf & April, 2008–July, 2008 & 4 & 496.91 & 618.94 & 833.98 & 0.601 \\ DEg & March, 2004–June, 2004 & 4 & 591.45 & 464.96 & 811.71 & 0.579 \\ DEh & February, 2013–May, 2013 & 4 & 599.37 & 714.31 & 378.54 & 0.558 \\ \hline \end{tabular}
\end{table}