*3.3. Effect of Climate Change on Production Flow Variation during Different Periods in the SRB* 3.3.1. Production Flow Variation during Different Periods

The non-freeze-thaw was the main annual production flow reduction period. Production flow reduction during the non-freeze-thaw period accounted for 80.7% of the annual total under the influence of climate change (Tables 7 and 8). The production flow reduction during the thawing period accounted for 20.4% of the annual total. The production flow during the freezing period slightly increased and accounted for −1.1% of the annual total.


**Table 7.** Changes in meteorological factors.

**Table 8.** Production flow variation during different periods.


3.3.2. Production Flow Component Variation during Freezing Period

The freezing period temperature/precipitation data series for the base period were replaced with those for the change period to study the influence of climate change on runoff component variation during the freezing period. The temperature and precipitation increased by 1.1 ◦C and 5.5 mm, respectively (Table 7). Hence, there was a 1-mm increase in the total production flow during the freezing period (Table 9). Relative to the BS scenario, the rate of change in the total production flow during the freezing period was 13.9% in the BSTP scenario.

**Table 9.** Influences of temperature and precipitation on the runoff component variation during the freezing period.


The increase in the base flow accounted for most of the increase in the total production flow. The rate of change in the base flow during the freezing period was 8.5%. Nevertheless, the increase in base flow explained 60.0% of the increase in total production flow during the freezing period. The increase in surface flow accounted for 40.0% of the increase in total production flow during the freezing period. However, the rate of change in the surface flow was 99.3% during the freezing period. There were minimal changes in the soil flow, and increases in it explained 0% of the increase in total production flow during the freezing period.

#### 3.3.3. Production Flow Component Variation during the Thawing Period

The thawing period temperature/precipitation data series for the base period were replaced with those for the change period to study the influence of climate change on runoff component variation during the thawing period. Increases of 1.57 ◦C and 10.3 mm precipitation caused a 3.2-mm decrease in runoff during the thawing period (Table 10). Relative to the BS scenario, the rate of change in the total production flow during the thawing period was −8.4% in the BSTP scenario.

**Table 10.** Influences of temperature and precipitation on runoff component variation during the thawing period.


The rate of change in the surface flow was −11.4%, but the reduction in the surface flow accounted for 112.5% of the total reduction in the production flow during the thawing period. The reduction in the surface flow explained most of the reduction in the total production flow during the thawing period. The soil and base flow increased during the thawing period. The increases in soil and base flow accounted for −3.1% and −9.4% of the change in the total production flow during the thawing period, respectively.

#### 3.3.4. Variations in Production Flow Components during the Non-Freeze-Thaw Period

The temperature/precipitation data series for the non-freeze-thaw period were replaced with those for the change period to study the influence of climate change on runoff component variation during the non-freeze-thaw period. The temperature increased by 1.4 ◦C while the precipitation decreased by 41.7 mm. Thus, there was a decrease of 17.7 mm in the total production flow during the non-freeze-thaw period (Table 11). Relative to the

BS scenario, the rate of change in the total production flow during the non-freeze-thaw period was −15.3% in the BSTP scenario.

**Table 11.** Influences of temperature and precipitation on the variation in runoff component during the non-freeze-thaw period.


The rate of change in the surface flow was −11.6%, but the reduction in surface flow accounted for 65.0% of the reduction in the total production flow during the nonfreeze-thaw period. The rate of change in the base flow was −40.5%, and the reduction in the base flow accounted for 33.9%. The changes in the soil flow were minimal, and the reduction in the soil flow accounted for 1.1% of the total production flow during the non-freeze-thaw period.

#### *3.4. Effects of Climate Change on Groundwater Recharge*

The foregoing analysis demonstrated that under climate change, the surface flow caused most of the reduction in production flow in the SRB. In contrast, the reduction in base flow accounted for a relatively small proportion of the reduction in production flow in the SRB. The base flow increased during the freezing and thawing periods. Relative to the BS scenario, the rates of change in the groundwater recharge during the freezing and thawing periods increased by 9.2% and 4.1%, respectively, in the BSTP scenario (Table 12).

**Table 12.** Influences of temperature and precipitation on groundwater recharge variation.


The water use was 27.5 billion m<sup>3</sup> , the groundwater exploitation was 9.4 billion m<sup>3</sup> , and the groundwater exploitation accounted for 34.3 % of water use and 29.1% of groundwater resources in the SRB between 1980 and 2018. The utilization rate of groundwater resources is far below the red line for development and utilization of 40%, the internationally recognized alarm line, which shows potential for development [43]. The groundwater exploitation was much less in the SRB compared with the Yellow River Basin and Haihe River Basin in northern China. The attenuation of the production flow aggravates water resource shortages. Appropriate attention should be given to groundwater utilization in areas with relatively less groundwater exploitation.

### **4. Conclusions**

The WEP-N model was used to simulate the SRB's hydrological cycle, and its overall performance was acceptable. The flow simulation was accurate, NSE > 0.75 and RE < 5 % for three hydrological stations and close to the actual measurements.

Climate change and water use were the main factors influencing the SRB's reduction in the annual production flow. Compared with the BS scenario, the rate of change in the annual production flow was −28.2% under the BSWM scenario. According to a multifactor attribution analysis, the rates of the contribution of climate change and water use to the reduction in annual production flow were 77.0% and 23.0%, respectively. Thus, climate change was the dominant factor attenuating runoff. The decrease in annual surface flow caused a 62.1% reduction in the annual production flow in the SRB. By contrast, the decrease in annual base flow accounted for only 35.7% of the reduction in the annual production flow in the SRB.

The change in annual production flow occurred mainly during the non-freeze-thaw period. The reductions in production flow during the non-freeze-thaw and thawing periods accounted for 80.7% and 20.4% of the annual reduction in the production flow, respectively. The production flow slightly increased during the freezing period. The change in the production flow occurred mainly during the non-freeze-thaw period. The increases in surface, soil, and base flow accounted for 60.0%, 0%, and 40.0% of the total increase in the production flow under the influences of increasing temperature and precipitation during the freezing period. The variations in surface, soil, and base flow accounted for 112.5%, −3.1%, and −9.4% of the total reduction in production flow during the thawing period. The reductions in surface, soil, and base flow accounted for 65.0%, 1.1%, and 33.9% of the total reduction in production flow during the non-freeze-thaw period. The foregoing analysis showed that surface flow caused the reduction in production flow in the SRB, where reduction in base flow accounted for a relatively small proportion under climate change. The base flow increased during the freezing and thawing periods.

Relative to the BS scenario, the rates of change in groundwater recharge during the freezing and thawing periods increased by 9.2% and 4.1%, respectively, in the BSTP scenario. The attenuation of the production flow aggravated the water resource shortage. Attention should be directed towards certain areas of SRB with less groundwater exploitation and similar areas in northern Eurasia and northern North America.

**Author Contributions:** S.L. performed the model programming and simulations. Z.Z., J.L. (Jiajia Liu), P.W., C.L. and J.L. (Jia Li) contributed to the model programming. S.L. and Z.Z. performed the writing. X.X., Y.J. and H.W. also contributed to the writing of the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (51679257, 51779270) and the National Key Research and Development Program of China (2016YFC0402405).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The datasets and model codes relevant to the current study are available from the corresponding author upon reasonable request.

**Acknowledgments:** The authors are grateful to the editors and reviewers; the comments and suggestions have contributed significantly to the improvement of the manuscript.

**Conflicts of Interest:** The authors have no relevant financial or non-financial interests to disclose.

#### **References**

