## 4 Discussion
On the seasonal scale, the increases in ET and GPP mainly occur in spring and summer and the increases are relatively small particularly in winter. Similar results are also found in the northeastern part of China [55]. The major reason for the increase in ET and GPP in spring and summer is obvious rising temperature and increasing precipitation in the two seasons, which may be due to the availability of more accumulated temperature and water during the growing season. In autumn, significant solar dimming should lead to lower ET and GPP uptends, while the humidity decreases significantly and lower increases in precipitation and temperature cause little ET and GPP increase in winter. Besides, the interactive effects of the climate variables, eCO\({}_{2}\) and LUCC are significant in spring and winter, and have positive effects on the ET (16.2%) and GPP (27.8%) uptrend in spring and negative effects on the ET (\(-\)24.4%) and GPP (\(-\)38.0%) uptrend in winter. Meanwhile, the interactive effects are also significant in autumn in ET increase (42.9%). This finding is consistent with the conclusion that interactive effects are important in water-limited catchments, whereby the ecosystem structure and function are mainly constrained by water [39; 56]. Additionally, this may be related to the strategy mechanism of the ecosystem in adapting to changing circumstances. The ET and GPP increase in the water-source region of the Xiong'an New Area, and increase more during the growing season from April to September, which can cause adverse impacts more easily on agriculture and the ecosystem [57; 58]. As we know, water resources were relatively scarce in semi-arid area, the increased GPP was helpful for regional-vegetation recovery and more water consumption may have led to runoff reduction under global climate change in this region, subsequently resulting in a water shortage for social-economic usage and river environmental requirements. Besides, the increased ET mainly caused by increased transpiration will lead to more soil water-consumption of the soil layer and intensify the drought, resulting in high ecological sensitivity and serious vulnerability in water-limited zones [6]. The attributions of ET and GPP changes reveal that the multitude of factors dominating water and carbon fluxes are different in the region over different time scales. Precipitation is responsible for both ET and GPP changes, which proved the determination of the vegetation by water in the semi-arid region. Rising temperature and eCO\({}_{2}\) also contribute greatly to both ET and GPP changes. Increasing precipitation, rising temperature and eCO\({}_{2}\) lead to greater GPP and ET for vegetation. Meanwhile, more precipitation may lead to greater water availability and fewer droughts, but a warmer temperature could enhance evaporation from the soil, producing more droughts [59]. The complex mechanisms of ET and GPP changes may bring more uncertainties of the drought in future and pose challenges to drought management. The attributions of ET and GPP changes are quantified in this study by considering climate variables, eCO\({}_{2}\) and LUCC, at both annual and seasonal timescales using the water-carbon coupling DTVGM ecohydrological model. We consider the water-carbon interactions by better capturing the mechanisms of ET and GPP changes, and it could especially reflect the interactive effects of hydrological processes on GPP by the climate forces and the vegetation dynamics on ET by the elevated atmospheric carbon dioxide, which cannot be well understood by the ecological model or rainfall-runoff relationship based on hydrological model. Besides, as global climate change may increase drought frequency and intensity in the future [60], more effective models to evaluate ET and GPP are needed considering anthropogenic influences such as irrigation [61].