## 4 Discussion
However, the northeastern basin areas and the Second Songnen River exhibited a lower ecosystem response rate to flash droughts. The ecological impact of water deficit- and heatwave-induced droughts in the Songnen Plain lasted for 13 and 17 days, respectively. During water deficit-induced droughts, the ecological impact timeframe in over half of the flash drought events within the Ersong watershed and the Nenjiang section was limited to merely 5-8 days--significantly shorter than in the northeastern Songhua River watershed. This extended impact duration suggests the vegetation's limited adaptability to drastic soil moisture changes. The observed increase in WUE and its underlying variant uWUE during drought conditions indicates enhanced ecosystem resilience to these flash climatic shifts. Notably, the Suihua and Qiqihar sections of the Songhua and Nenjiang Rivers, respectively, reported the highest WUE and uWUE values across both drought types, likely due to the superior WUE of forest ecosystems prevalent in these areas. The northeastern regions of the Songnen Plain boasted the highest forest coverage, in contrast to the significantly lower WUE observed in Changchun and Daan, and primarily because of the reduced forest area coverage. There are some uncertainties in this study. The soil moisture data derived from remotely sensed inversion may not fully account for the complex hydrological interplay between deep subterranean water reserves and surface moisture levels [51]. Future research should incorporate multi-layer soil moisture observations within an integrated hydrological model framework. Such an approach will provide a more nuanced understanding of soil water balance dynamics, offering deeper insights into the mechanisms driving flash drought evolution and their ecological consequences. In addition, it is evident that if the duration of the flash drought is too brief, it may not be sufficiently long to exert a detrimental effect on the ecosystem. Conversely, a mild flash drought could potentially stimulate vegetation growth [52]. Furthermore, vegetation's response to drought tends to be delayed; the longer the overall drought persists, the more significant its influence on vegetation becomes [53]. Consequently, solely focusing on the rapid development (or burst) phase of a flash drought limits our understanding of its ecological impact [54; 55]. It can be anticipated that a subsequent continuous drought following a flash drought could have profound effects on vegetation [56]. However, determining whether this impact should be attributed to a flash or continuous drought and from which perspective to assess the flash drought's impact are questions worth considering. In addition, it should be noted that the definition of flash droughts and the selection of indicators greatly influence the frequency of flash droughts [13]. Although we used the definition of flash droughts by [PERSON] et al. [2] found that can characterize the rapid onset and sudden propagation of flash drought events, we probably overestimated or underestimated drought patterns due to a lack of large-scale actual observations for verification. In addition, the selection of SM thresholds can impact the evaluation indicators of flash drought events [57; 58; 34; 41]. The number and duration of flash droughts may vary greatly under different thresholds, despite similar spatial patterns [13]. Furthermore, using different data sources (SM and meteorological variables) to define flash droughts can lead to some uncertainties in deriving their characteristics and potential drivers over a given region [59; 60; 61].