###### Abstract
For providing effective, targeted solutions to drought [23; 24; 25] and formulating drought mitigation measures [26], the management of rubber plantations is inseparable from the mastery and understanding of vulnerability characteristics. Thus far, assessing vulnerability to drought on a regional scale has been a great challenge. However, remote sensing (RS) techniques are appropriate tools to analyze rubber ecosystems in Hainan Island, because they are statistically reliable, operationally robust, and economically cost-effective [25]. RS-based LUE (light use efficiency) models are powerful instruments for characterizing vegetation behavior under drought conditions on a regional scale [27]. Accordingly, the eddy covariance-light use efficiency (EC-LUE) [28] one of RS-based LUE models, is used to evaluate the drought vulnerability of rubber plantations distributed across Hainan Island. Terrestrial gross primary production (GPP), which is defined as the amount of carbon uptake through photosynthesis at the ecosystem scale [29], is the start point of the terrestrial biogeochemical cycle and, thus, serves as the energy that are required for almost all ecosystem processes. Therefore, GPP, which is closed to the water cycle, is an important biophysical parameter for ecosystem that plays a key role in evaluating the response to drought [30; 31]. Since the characteristics of drought are complex, in order to accurately grasp the impact of drought, many types of drought have been established [32], e.g., meteorological drought, hydrological drought, agricultural drought, ecological drought etc. To quantitatively reveal the impact of drought, various criteria and indexes have also been proposed [33; 34; 35]. Long-term monitoring [36], RS techniques [37; 38; 35] and a large number of models and tools [23] have been developed to assist researchers with in-depth drought research. In recent years, a new type of drought, flash drought, has become a hot topic in current research [39]. Compared with classic droughs, flash droughs have remarkably rapid onset [40], but the scientific community has not yet agreed on a definition [39], and the essential causes of flash droughs have not been directly proved. The earliest research on flash droughs began in North America, where it was found that a flash drought of any type is caused by a rapid loss of soil moisture, and most likely to occur during the growth period of crops [40; 41; 42]. A study focused on China indicated that the number of flash droughs had an increasing trend, and that they were more likely to occur in humid and sub-humid regions [43]. In addition, according to the latest research results [44; 45; 46], soil moisture in arid and semi-arid areas is the dominant water factor that limit plant growth, while in low latitudes like Hainan Island and humid areas, atmospheric water demand is the dominant factor limiting plant growth. Therefore, Standardized relative humidity index (SRHI), which is calculated by atmospheric relative humidity, has been selected as the index to evaluate drought level in this study. Rubber plantations are mostly managed manually, and irrigation and other interventions will be carried out to mitigate water stress when flash droughs have a significant influence; therefore, the present study only focuses on short-term flash drought (STFD) events that are more likely to affect the rubber ecosystem in Hainan Island, and aims to define such droughs according to the atmospheric water demand. In this study, vulnerability is defined as the rate of the loss of GPP compared to the optimal GPP, which is under the conditions of drought at a monthly scale. For STFD events, they are established by utilizing SRHI and historical climate data to gather multi-scenario input from various intensity gradients in different months.