**3. Results**

#### *3.1. Past Changes in Land Cover*

We first identified past LCC. According to the data from Liu et al. (2014), we focused on four primary land cover types: forests, grasslands, croplands, and urban areas. As shown in Table 1, urban expansion played an important role over the last 30 years in the TNR, leading to a substantial decrease in the cropland and grassland areas of most basins. In the HRB, the urban area expanded from 2.5% to 9%, resulting in a decrease in cropland areas, and in the LRB, the urban area doubled, which caused a decline in the other land cover types. In the western three basins, the IRB, YRB, and HRB, due to afforestation projects, the forest area has slightly increased. The YRB is the basin where the forestland increased the most, from 12% to 13%, and the increases in the other two basins were under 0.4%. Meanwhile, in the other two basins, the SRB and LRB, the forest area decreased, especially in the SRB, where the reduction was greater than 3%. The reason for the reduction in forestlands in these two basins may be the expansion of croplands and urban areas. For the entire region overall, the proportion of forestlands, grasslands, croplands, and others land cover types all decreased because of the expansion of urban areas.


**Table 1.** Fractions of primary land cover types of the TNR in 1985 and 2015.

Due to afforestation programs and favorable climate conditions, the vegetation in the TNR has grown substantially. We calculated the vegetation index (i.e., the mean LAI from January to December) across the TNR, and the results are shown in Figure 3. Because of the growth of vegetation, the LAI peaked from July to August, and exhibited the lowest values from December to February. The average values between 1983–1986 and 2011–2015 were compared, and the results show that there has been a grea<sup>t</sup> improvement in the LAI over the entire region, especially in July and August when it increased by over 0.5.

**Figure 3.** Mean monthly leaf area index (LAI) of the TNR in 1985 and 2015.

#### *3.2. Future Climate Change*

To determine the temperature changes projected for the future under the RCP8.5 scenario, we averaged the datasets of five models, and averaged the maximum and minimum temperatures as the mean temperature. In order to correspond to the hydrological changes analyzed, we also selected the period from 2020–2099 as the experimental interval. The mean annual temperatures (MATs) across two different periods, 2020–2039 and 2080–2099, are shown in Figure 4. We found that the spatial pattern across the TNR may not change in future. The HRB will remain the hottest basin, while the SRB will remain the coldest, and the differences between the two periods were very similar in each area. The northeast of the SRB and the north and southwest of the IRB are the areas where temperatures will increase the most, by more than 3.6 ◦C, and in the south of the YRB, the change will be slightly less at ~3 ◦C. The MAT of the entire region will rise steadily from nearly 5.5 ◦C to over 10 ◦C (Figure 5).

**Figure 4.** Projected mean annual temperature (MAT) and mean annual precipitation (MAP) over two periods (2020–2039 and 2080–2099) and their differences (Δ).

**Figure 5.** Projected MAT and MAP across the TNR.

The spatial pattern of precipitation also may not exhibit obvious changes obviously; there will remain an obvious declining gradient from the southeast to northeast. As shown in Figure 3, the south of the LRB will remain the wettest area, where the annual precipitation will be over 1400 mm from 2080–2099, and the west of the IRB will be the driest, remaining under 50 mm. Additionally, except in some small zones in the west and north of the IRB, the precipitation will increase from 2020–2039 to 2080–2099 over the entire TNR. The spatial distribution of difference is consistent with that of the precipitation, with the southwest experiencing the most change (more than 180 mm), and the northeast experiencing the least, less than 10 mm or even less than 0 mm. The mean annual precipitation (MAP) of the entire region is shown in Figure 5. There will be a positive, linear trend overall, but the variability will also rise, which means that the discrepancy between the years will also increase. As for wind speed and downward shortwave radiation, no remarkable changes were found (the results were not shown.).

#### *3.3. Hydrological Responses to Future Climate Change*

We examined the changes in three hydrological variables, ET, R, and SM, from the VIC model, based on the input of the LC2015 data. As with the analysis of climate change, all values were averaged by the five simulations that were driven by data from the five GCMs, as described in Section 2.3.1. Additionally, we focused on the period from 2020–2099, since the period from 2006–2019 was set as the warm-up period for the VIC model.

The annual mean ET, R, and SM in two corresponding periods (i.e., 2020–2039 and 2080–2099) are shown in Figure 6. The distributions of ET and R are similar to that of precipitation. In the period from 2080–2099, ET decreased from over 1000 mm in the southeast to under 50 mm in the northwest, and R from more than 400 mm to less than 10 mm. Due to the complex spatial variabilities in climate and land surface conditions, the distribution of SM differed from those of ET and R. Across the three eastern basins (i.e., the SR, LR, and HRB), the SM in most areas will be greater than 700 mm, while over the majority of the other two basins, the SM will remain under 400 mm, and even under 200 mm in some areas. In each basin, it was obvious that the SM may rise in those areas where the soil was wetter, but decline in the drier areas.

The three hydrological variables exhibited significant temporal changes. Comparing the results from the two periods (2020–2039 and 2080–2099), we found that ET will increase over the vast majority of the TNR because of the increases in precipitation and temperature. This increase may approach over 150 mm in the southeast, especially in those areas with forest cover. As for SM, the changes appeared closely related to the land cover type. In the entire region, rising temperatures will aggravate the water loss by ET, especially in the areas with forest cover in the SRB, LRB, and HRB, as well as in the areas with grassland cover in the IRB and YRB, leading to a decrease in SM, although precipitation will also greatly increase SM. In other areas, the increase in precipitation will be able to compensate for the loss by ET, so the SM will increase. The center of the SRB and northwest of the IRB are the areas where SM will increase the most; in the southwest of the YRB and as the south of the SRB, the SM will decrease

the most, by over 20 mm. The changes in R will be directly influenced by the changes in precipitation, ET, and SM. In most areas, the R will increase, while decrease only in some small zones.

**Figure 6.** Mean annual evapotranspiration (ET), runoff (R), and soil moisture (SM) during two periods, 2020–2039 (**left**) and 2080–2099 (**middle**), and their differences (**Δ**).

The annual mean ET, R, and SM for the entire TNR are shown in Figure 7. As with the changes in precipitation, the ET and R will both experience a positive and fluctuating trend. Because of rising temperatures, the trend of ET is more pronounced than that of R, and the fluctuations in R are greater than those of ET. The SM will increase in the first 20 years and then remain relatively stable at 510 mm from 2030 onward.

**Figure 7.** Projected mean annual ET, R, and SM across the TNR.

#### *3.4. Hydrological Effects of Past Land Cover Changes (LCC)*

Based on the two simulation experiments performed in this study, we can quantify the contribution of past LCC on the future hydrological regime. The differences in ET, R, and SM between the two experiments for the period from 2080–2099 are shown in Figure 8. It is obvious that across nearly all the TNR, LCC will result in a reduction of ET, as the areas with vegetation cover decreased, as illustrated in Section 3.1, though the LAI increased to some degree. In particular, the center and the southeast of the HRB is the area where the ET decreased most distinctly, by over 40 mm. Only in a few grids, such as in the center and the east of the SRB and the center of the YRB, where the ET increase within a range of 40–60 mm. As for the changes in R, the distribution is somewhat complex. Contrary to that of ET, in a majority of the eastern part of the TNR, R will increase, which is mainly because of the reduction of ET. The center and southeast of the HRB will be the place where the difference is the greatest, by over 40 mm. Some grids in the east of the SRB are the areas where R will decrease the most, by more than 60 mm in 2080–2099. As for the west part of TNR, R will increase in nearly half of the grids and decrease in the others grids, while the range may not be greater than 20 mm. As SM is very sensitive to the ET process, its distribution is similar to that of R across the majority of TNR, but the maximum increase will be in the center and southeast of the HRB, where urban expansion is the most intense, and will reach over 80 mm. SM will decrease in some areas in the SRB by over 80 mm. In contrast, in some areas across the northeast of the IRB, SM will decrease by more than 120 mm. Overall, the effects of LCC on R and SM may be positive in the SRB, LRB, HRB, and YRB, but negative in the IRB.

**Figure 8.** Shifts in annual ET, R, and SM for 2080–2099 driven by past land cover changes (LCC).

The annual mean changes in ET, R, and SM are shown in Figure 9. Obviously, the changes in land cover will cause a decrease in ET, as well as increases in R and SM. Similar to the change in annual precipitation, there is a positive trend in these effects over time, which means that under the RCP8.5 scenario, LCC will more strongly influence the hydrological cycle in the future than at present.

**Figure 9.** Projected average annual changes in ET (**left**), R (**middle**), and SM (**right**) between the two results, based on different land cover datasets across the TNR.

The shifts in the hydrological regime due to past LCC also exhibit obvious seasonal variability; Figure 10 shows the shifts in ET among the four seasons across every basin. Across most of the basins, the effects in spring, summer, and autumn will be negative, while they will be positive during winter. For example, across the YRB from 2080–2099, the shift will be nearly 0.1 mm in winter, and −1.9 mm, −3.1 mm, and −0.4 mm in spring, summer, and autumn, respectively. Moreover, the HRB will be the most strongly affected, with a change of over 10 mm in the summer; the magnitude of change observed in the HRB is followed by that for the LRB (~9 mm in the summer), and in the IRB, where effects are the weakest, there will be a change of no more than 0.5 mm. The effects of LCC on R and SM across the TNR are shown in Figure 11. In every season, the shifts in R and SM due to LCC will be positive, and stronger from 2080–2099 than in other periods. Especially in the summer, R and SM may significantly by ~2 mm. A similar positive pattern will likely appear in each of the five basins despite different amounts of change.

**Figure 10.** Projected changes in ET between the two results (based on different land cover datasets) of each season and different periods, over the five basins in the TNR.

**Figure 11.** Projected changes in R and SM between the two results, based on the different vegetation parameters of each season.
