Simulation And Projection Of Mountainous Runoff In The Shiyang River Basin Under Climate Change

Streamflow is an important component of water resources in inland river basins,especially when the regional water cycle process is profoundly changed by continuous climate change.For inland river basins,water resource is an important condition for the sustainable survival and development in arid zones,and mountainous areas in the upper reaches of the basin are the most important water formation areas,the changes of streamflow will directly affect the ecological environment and human production activities and life in the middle and lower reaches of the basin.Therefore,the simulation and projection of streamflow in the context of climate change can provide a basis for water resources management,allocation and coordination in the basin.To address the complexity of hydrological processes in alpine mountains and the spatial and temporal heterogeneity of streamflow responses among watersheds,this paper takes six tributaries in the upper reaches of the Shiyang river Basin,namely the Xidai River,the Dongda River,the Xiying River,the Jinta River,the Zamu River and the Huangyang River,the research analyzes the current characteristics of hydrological elements in the basin based on hydro-meteorological data from 1970 to 2014,and constructs a hydrological model applicable to each tributary.At the same time,Global Climate Models（GCMs）data were used to predict future climate change in the region,and future land use changes were predicted based on historical land use conditions,and climate scenarios and land use scenarios were combined to explore the potential hydrological response of the mountainous tributaries in the upper Shiyang River basin.The main findings are as follows:（1）The factors affecting the hydrological processes in the mountainous sub-basins of the upper Shiyang River basin are different,and snow changes have a significant impact on runoff changes in the mountainous areas,especially in the western part of the mountains.Besides,changes in surface production processes also have a strong influence on runoff.Meanwhile,the SWAT model has good applicability to all sub-basins in the mountainous area of Shiyang River basin.Among them,the simulation accuracy of Zamu River and Xiying River is the highest,both the R² and E_NS in calibration and validation periods are above 0.82,and the PBIAS is less than 5%;the simulation accuracy of Dongda River and Jinta River is higher,with R² and E_NSabout 0.76;the R² of Huangyang River and Xida River are 0.59 and 0.68,respectively,but the E_NS of Xida River is lower and the PBIAS is less than 10%.Therefore,the streamflow simulation of Xida River is better than Huangyang River.In addition,the runoff simulation results all reach high accuracy,indicating that the model has good adaptability in each tributary.（2）The corrected climate simulation by GCMs can better reproduce the historical changes of precipitation,maximum temperature and minimum temperature in the mountainous areas of the upper Shiyang River basin,which is helpful for analyzing the regional climate changes.The correlation between the corrected GCMs and observation from 1970 to 2014 is greater than 0.8,indicating that the application of the bias correction method can improve the simulation effect of GCMs on regional climate analysis.Meanwhile,the kappa coefficient of the 2020 land use scenario based on the MCE-CA-Markov simulation is greater than 0.9,indicating that the MCE-CA-Markov model also has a good simulation effect in this mountainous area.Therefore,the projected scenarios are comprehensive and reliability,which able to analyze the hydrological response in changing environments.（3）Precipitation,maximum and minimum temperatures in the mountainous areas of the upper Shiyang river have shown significant increases in the last 50 years,but the annual streamflow rate of the tributaries of the upper Shiyang river has spatial variability.The average precipitation rate is 1.42 mm/10a in the east and 5.54 mm/10a in the west.Among them,the maximum precipitation increase rate is 7.45 mm/10a for Huangyang River and the minimum precipitation increase rate is 0.87 mm/10a for Xida River.From the perspective of seasons,precipitation in each sub-basin is significantly influenced by summer,which contributes58.42%～60.96%to annual precipitation,followed by autumn and spring,and the minimum precipitation contribution is 1.26%～1.79%in winter.For temperature,the maximum temperature increases by 0.33℃/10a and the minimum temperature increases by 0.40℃/10a,and the increase rate of temperature show decreases from west to east.The greatest rate of increase in maximum and minimum temperatures was in Xida River,with 0.34℃/10a and0.43℃/10a,respectively,and the slowest rate of increase was in Huangyang River,with0.30℃/10a and 0.36℃/10a,respectively.In this context,the mountain streamflow in the upper Shiyang river tends to decrease slightly,with a rate of-0.01 m³/s per 10a,and the average depth of Shiyang river is 207.49 mm.However,the variation of each tributary in the mountainous area of the upper Shiyang River has spatial variability.Among them,Xida River and Xiyang River basins showed an increasing trend,with a larger increase in the Xiyang River and a change rate of 0.16 m³/s per 10a.Meanwhile,Dongda River,Jinta River,Zamu River and Huangyang River showed a non-significant decreasing trend,among which the Dongda River showed a larger decrease with a change rate of-0.25 m³/s per 10a.Seasonally,runoff from June to September accounts for about 60%to 75%of the total annual runoff.Among them,August is the peak period of annual runoff with an average flow of 15.06 m³/s;February has the smallest runoff with an average flow of 1.43 m³/s.（4）Compared with the historical period,the future precipitation,maximum and minimum temperatures in the mountainous tributaries of the upper Shiyang river basin show an increasing trend under different emission scenarios,especially under the SSP585,where the climate change increase is particularly obvious.Under SSP585,the annual precipitation increases with a rate of 8.30 mm/10a in the upper Shiyang river basin,with the largest increase in annual precipitation at 13.09 mm/10a in west and the smallest increase at 6.83 mm/10a in east.Compared to the historical period,the contribution of spring and autumn precipitation to the increase of annual precipitation,while the contribution of summer precipitation decreased with the increasing emission concentration.For the total mountainous areas,the contribution of summer precipitation ranged from 29.17%to 36.02%under the SSP126 to SSP585 scenarios;and the contribution of spring precipitation ranged from 23.00%to 37.50%.In addition,the annual maximum temperature（minimum temperature）increases with a rate of 1.01℃/10a（0.99℃/10a）.The sensitivity of warming showed differences among scenarios,and the climate response in eastern was more sensitive and significant in the mountainous areas of the upper Shiyang River basin.（5）The variation in streamflow was not significant under different land use scenarios,but there were significant differences under different climate scenarios.Compared with the historical period,the annual streamflow in the mountainous areas of the upper Shiyang river basin increased by 49.84%under the SSP126 scenario and 56.12%under the SSP585,but the streamflow of each tributary showed a significant increase followed by a decrease under the SSP585.Overall,the annual streamflow in the mountainous areas of the upper Shiyang river basin showed a weak increasing trend with a rate of change of 0.04 m³/s per 10a under the SSP126 and a significant decreasing trend（P<0.05）with a rate of-0.28 m³/s per 10a under SSP585.Seasonally,the increase in streamflow in the mountainous areas of the upper Shiyang river was mainly concentrated in spring,followed by winter and finally summer and autumn.Finally,summer and autumn.In spring,the increase in the western part of the mountainous area in the upper Shiyang river basin was significantly higher than that in the eastern part,with109.71%and 134.33%increase in streamflow in the western part of the mountainous area in the upper Shiyang river basin and 57.90%and 86.90%increase in streamflow in the eastern part under SSP126 and SSP585,respectively.（6）For the mountainous areas in the upper Shiyang River basin,future runoff will be recharged significantly by precipitation,but a significant increase in temperature can also lead to a decrease in runoff.The correlation test shows that precipitation is positively correlated with streamflow in all tributaries and scenarios,while the effect of temperature on runoff varies with the scenario.Under SSP585,maximum temperature is significantly negatively correlated with streamflow,and its correlation effect is much larger than that of precipitation,indicating that there is a close relationship between maximum temperature and streamflow reduction,and a significant increase in maximum temperature is likely to weaken or exceed the positive effect of increased precipitation on streamflow recharge.In addition,the correlation coefficient between maximum temperature and streamflow was higher in this scenario than that of minimum temperature,indicating that under the scenario with high emission scenario,maximum temperature is an important climatic element affecting streamflow changes in the sub-basin,followed by minimum temperature.