###### Abstract  
Understanding the relationship between drought and the water use efficiency (WUE) in terrestrial ecosystems can help reduce drought risk. It remains unclear what the correlation between the cropland water use efficiency (CWUE) and drought during drought events. We aim to identify the spatiotemporal relationship between drought and the CWUE and to ensure the service capacity of cultivated land ecosystems. In this study, the cubist algorithm was used to establish a monthly integrated surface drought index (mISDI) dataset for the Huang-Huai-Hai Plain (HHHP), and the run theory was used to identify drought events. We assessed the spatio-temporal variations of drought in the HHHP during 2000-2020 and its influence on the CWUE. The research results were as follows: from the overall perspective of the HHHP, the mISDI showed a downward trend. Drought had an enhanced effect on the CWUE of the HHHP, and the enhancement of the CWUE in the eastern hilly area was more significant. The CWUE response to drought had a three-month lag period and a significant positive correlation, and it was shown that the cultivated land ecosystems in this area had strong drought resistance ability. This study provides a new framework for understanding the response of the CWUE to drought and formulating reasonable vegetation management strategies for the HHHP. water use efficiency; drought; remote sensing; cropland; ecosystem; drought events; the Huang-Huai-Hai Plain; time-lag effects 2022 An increasing number of scholars have explored the WUE. There are many definitions of the WUE [6; 7], the more common of which are the net primary productivity (NPP)/evapotranspiration (ET) and gross primary production (GPP)/ET. The time scale of the WUE can also be divided as instantaneous, daily, seasonal, and annual [8]. Spatially, the variation characteristics of the WUE can be studied using multiple spatial scales, such as latitude (LAT), altitude [9], ecosystem type [10], and biome [11]. Several previous studies have successfully demonstrated the obvious spatial heterogeneity of the WUE [7; 9; 10; 11]. Some researchers investigated the WUEs of different ecosystems and found that changes in different species are affected by the structure of plants. Under the same external conditions, the WUE increased in areas with closed shrubs and woody sparse trees but decreased in all other grassland types [12]. Moreover, related research on the WUE in mixed plantations provided a reference for the rational use of water resources in forest ecosystems [13]. However, cultivated land is different from other natural ecosystems, because it is managed by humans and is closely related to human development. Chinese agriculture uses only 7% of the world's cultivated land to feed approximately 22% of the global population [14]. As the main grain-producing area in China, the Huang-Huaiai-Hai Plain (HHHP) has contributed to 45% of the total grain production in China over the past 30 years. Studies have shown that droughts in the winter and spring in the HHHP impact the yields of winter wheat and summer maize [15; 16]. Climate change and ecosystems are closely related to drought [17]. Therefore, to better predict the response of cropland ecosystems to climate change, it is necessary to explore the relationship between the cropland water use efficiency (CWUE) and drought. Droughts are a traditional topic of research that has gained increasing attention in recent years [18]. Accurately reflecting drought characteristics and identifying drought events remain difficult tasks. With advances in drought research and improved accuracy in drought-characteristics measurement methods, the use of multi-source data combined with data mining technology to monitor droughts has become a new approach for drought research [19; 20; 21; 22]. The optimized drought index provides a possibility for an in-depth study of droughts and an ecosystem's carbon-water cycle.