**4. Discussion**

#### *4.1. Extended Effects on Hydrological Regimes of Climate Change and LCC*

In this study, we detected the future hydrological regime based on current land cover condition (LC2015) at first. In previous studies about the predicted hydrological regime in TNR, especially under the most extreme scenario, some similar results were obtained. Over the entire region, annual ET may increase steadily under RCP8.5 scenario, possibly due to the increasing temperature and precipitation [23]. Other researchers found that annual R would increase in most areas of TNR, especially in eastern basins [38], which is consistent with our study. Additionally, other study also found the interannual changes in ET and R may be expanded under RCP8.5 scenario, which means they may be more fluctuating in future [23], resulting from the fluctuating P. Those studies all predicted

an uneven distribution in changes of R. In those humid areas, the increase in R may be much more than that in arid and semi-arid areas, and in some arid areas over Northwest China, the R may even experience a decrease [23,38]. The reason may be that in humid areas, the increase in P could compensate for the water loss caused by ET, while in those arid or semi-arid areas it could not. Many studies based on various scales and time series have summoned a similar result: "Wet ge<sup>t</sup> wetter, dry ge<sup>t</sup> drier" [39–41]. In our study, the SM in those areas densely covered by vegetation may experience a decrease in future, while in other areas the SM would rise in contrary. It can be attributed to the effects in promoting ET by vegetations, especially driven by increasing temperature. In other study, researchers predicted that over each basin in TNR, the SM would decline in future [23], which is inconsistent with our study, and those differences may be caused by the choose in soil depth.

Then we explored the dependence of the future hydrological regime on past LCC. With the progress of ecological restoration from 1986 to 2015, including afforestation, the forest area increased in North China (i.e., in the HRB) and in Northwest China (i.e., in the IRB), but decreased in Northeast China (i.e., in the HRB and LRB). The TNR also showed a significant increase in urban area of ~1.3% (52,701 km2) due to rapid urbanization. Results show that compared to the effects of climate change, those of LCC would be far less observable. Based on past climate condition, other study has found the similar conclusion that the LCC did not obviously alter the hydrological regime at a regional scale [24]. There was also study that found the afforestation did not help solve, and even aggravate the desertification in arid and semi-arid areas [15]. However, although the effects may be less observable than those of climate change, our study found that under the projected climate scenario they may also be intensified, and could be able to alter the hydrological regime in some areas. Growing temperature and precipitation may be responsible for the amplification in those effects. We also explored the effects in different seasons. Most improved R and SM may occur in summer, because P in this season may be much more than in other seasons and the abundance of water resources could intensify the effects of LCC.
