## 1 Introduction  
Evaptranspiration (ET) and photosynthesis are coupled processes that mediate water, carbon, and energy fluxes between terrestrial ecosystems and atmosphere, which are important components of energy-water balances and global biogeochemical cycling [1, 2, 3]. In the context of climate change, observations of ecosystem water and carbon exchange, in order to deeply understand the trade-off between ET and the amount of terrestrial carbon assimilated, are currently one of the hottest issues in ecological hydrology research [4, 5]. Studies on the characteristics of water and carbon fluxes and their coupling relationships in agroecosystems are essential to improving carbon and ET models [6, 7], as well as predicting the effects of climate change on regional carbon and energy budgets [8]. Meanwhile, such study is also of great significance for maintaining and improving agricultural productivity and further understanding the material cycle of terrestrial ecosystems. The eddy covariance technique, which provides direct and dependable flux measurements, has been used to characterize the temporal and spatial variability of ecosystem carbon dioxide (CO\({}_{2}\)) fluxes (net ecosystem carbon exchange, NEE), ET, and climatic variables [9]. Based on this technique, measurements of high-frequency NEE allow us to investigate the dynamics of gross primary productivity (GPP), as well as estimations of ecosystem water use efficiency (WUE, the ratio of GPP to ET) [10; 11]. In recent decades, the Global Flux Observation Network (FLUXNET) has provided a large amount of data for the study of ecosystem water-carbon flux and its response to climate change [12], covering almost all major terrestrial ecosystems, including forests [13; 14; 15; 16], grasslands [17; 18; 19], and farmlands [20; 21; 22; 23]. Most previous studies have focused only on the water flux [20] or carbon budget [21; 22; 24] of single types of ecosystem separately, while ignoring the possible effects of parts outside the observational range on the ecosystem. The responses of GPP, ET, and their parameters related to environmental factors differed among ecosystems, resulting in uncertainties in WUE estimations [10; 25], which makes it urgent to find biophysical controls for inter-annual variations in GPP, NEE, ET, and WUE in the agroecosystems. [PERSON] et al. (2016) found that low soil moisture conditions in spring coincided with lower seasonal and annual carbon and ET, as well as lower WUE [26]. [PERSON] et al. (2018) studied the relationship between maize NEE and photosynthetically active radiation (PAR) by using the eddy covariance technique, and the results showed that NEE was almost linear with PAR under favorable climatic conditions [22]. Studies have shown that vapor pressure deficit (VPD) is an important environmental factor for WUE [27; 28]. The results of [PERSON] et al. (2009) indicated that, when LAI and solar radiation were large enough, WUE showed negative correlation with VPD in irrigated wheat and maize fields on the North China Plain [4]. Other factors also contributed to variation in WUE, and [PERSON] et. estimated the WUE of different ecosystem types at 32 FLUXNET stations in the northern hemisphere, showing that the WUE had obvious latitude zonality [29]. However, water-carbon couplings in cropland and other agroecosystems had their own particularities and the influencing factors were more complex. Few studies carry out comprehensive analysis and evaluation of water-carbon-energy processes based on years of continuous observation, making it difficult to fully reveal regional ecohydrological processes and their characteristics. The Changwu tableland is an important grain-producing area located in the warm temperate subhumid continental monsoon climate zone, south of the Loess Plateau, China. In recent decades, climate change and continuous improvement in agricultural productivity have significantly changed regional water-carbon processes.