| Due to the construction of the Three Gorges Dam,the Three Gorges Reservoir has become an important water storage space in the Yangtze River watershed,which directly changes the temporal and spatial distribution of water resources in the Yangtze River.With the construction of the Three Gorges Dam,land cover and landscape pattern have undergone very dramatic changes,such as increased water area,urban backward relocation and large-scale construction of supporting infrastructure;After the formation of the reservoir area,continue to practice the ecological route,the implementation of soil and water conservation,afforestation and returning farmland to forest and grassland projects have directly or indirectly changed the regional water cycle process.Located in the heart of the Three Gorges Reservoir,the Daning River is a typical 'mountain-forest'small watershed.Also affected by the construction of the Three Gorges Dam,the water level of the lower reaches of the Daning River rises and falls periodically with the impoundment of the Three Gorges Reservoir,and its hydrological factors change.Clear the temporal and spatial variation characteristics of hydrological elements in natural state of Daning River watershed;identifying the sensitivity of the runoff change of the Daning River to meteorological and non-meteorological factors;To explore the influence of land use change on runoff temporal and spatial change.It is of great significance for scientific planning,utilization,control of water resources in the watershed and adjustment of future land use direction.And lay the foundation for the study of the influence of periodic impoundment of the Three Gorges Reservoir on the runoff change of the Daning River.Based on the daily observation data of five meteorological stations and one hydrological station in the Daning River watershed from 2003 to 2017 and the land use data of 2005 and 2015,this paper analyzes the change characteristics of meteorological and hydrological factors from the time and space scales.The simulation value of runoff in the Daning River watershed was obtained by constructing SWAT model,and the sensitivity and attribution analysis of runoff change in the Daning River watershed was carried out based on the Budyko assumption of water and heat balance equation.The control variable method is used to further clarify the impact of land use change on runoff change in the Daning River watershed.The main conclusions are as follows:The temporal variation characteristics of meteorological and hydrological factors in the Daning River watershed were analyzed by trend analysis and mutation test.The results showed that: 1)The interannual and seasonal changes of precipitation and runoff in the Daning River watershed were similar.The annual value(annual precipitation and annual runoff)fluctuated around the annual average value,and the overall fluctuation was not significant.Spring and winter showed a downward trend,the change rate is close;Autumn and summer change in the opposite direction,that is,rising in autumn and falling in summer,the absolute value of the change rate is close.In addition,the change trend of runoff and precipitation is more obvious than that of runoff.2)The interannual variation of temperature showed a significant upward trend in general.Taking 2012 as the dividing point,it showed an insignificant downward trend from 2003 to 2012,and a more obvious upward trend from 2012 to 2017.By setting 12 months in 2003 as the warm-up period,2004 – 2011 as the rate period,and 2012 – 2017 as the validation period,the SWAT model of the Daning River watershed was constructed to simulate the monthly runoff of the Daning River watershed.The determination coefficient R2,Nash efficiency NSE and percentage deviation PBIAS of the model evaluation index reached good,excellent and excellent,respectively,that is,the model can better reflect the hydrological characteristics of the Daning River watershed.According to the SWAT model simulation results,the temporal and spatial variation characteristics of runoff in the Daning River watershed are studied.The results are as follows: 1)The annual variation of runoff in the Daning River watershed shows a downward trend overall,but there is little difference between the sub-basins;The interannual spatial variation of production flow is generally distributed in groups,with an average distribution in the upstream and a relatively obvious difference in the middle and lower reaches.In terms of the annual change rate of production flow,the annual change rate of high-yield flow area is small,while that of low-yield flow area is relatively large,namely,high-yield and low-change,low-yield and high-change.2)In terms of seasonal variation of abortion flow in the Daning River watershed,the spatial distribution of annual runoff yield in spring(March-May),summer(June-August),autumn(SeptemberNovember)and winter(December-February)in the four seasons is similar to that in the Daning River watershed,that is,the high-yield flow area is distributed in the left bank branch of the Daning River,and the low-yield flow area is mostly located in the right bank branch.Spring,autumn,winter similar degree is high,from upstream to downstream runoff gradually increased;In summer,the upstream tributary of the Daning River and the right bank tributary have more runoff.Seasonal change rate showed a downward trend in spring,summer and winter runoff,but the downward trend was not obvious in spring and winter;In summer and autumn,the change rates showed obvious downward and upward trends,and the absolute values of the change rates in summer and autumn were close.In the spatial distribution of seasonal change rate,the regions with obvious downward trend in summer and obvious increase in autumn are more consistent in space,that is,the high-yield flow area has high change,and the low-yield flow area has low change.In spring and winter,the change in high-yield flow area is low,while the change in low-yield flow area is high,which is consistent with the annual change rate.Based on the sensitivity of Budyko hypothesis and Choundhury-Yang equation to runoff variation in the Daning River watershed;The contribution rate of meteorological and non-meteorological factors to runoff is studied.The results show that the elasticity coefficient of each branch of the Daning River watershed changes little,which means that there are some differences between the Daning River watershed,but the difference is not obvious.The contribution rate of meteorological and non-meteorological factors to runoff in each sub-watershed of the Daning River watershed is basically consistent.Meteorological factors are more than 80%,and non-meteorological factors contribute less than 20%.There were small differences in each tributary,and meteorological factor gradually decreased from upstream to downstream,while non-meteorological factors gradually increased.Based on the analysis of land use quantity,spatial change and distribution of land use types under different topographic factors in the Daning River watershed,the SWAT model parameters and other data are ensured to be unchanged by using the control variable method.The runoff changes in the Daning River watershed are simulated by using the land use scenarios in 2005 and 2015,respectively.By comparing the temporal and spatial changes of the two runoff yields,the results show that: 1)It is precisely because of the wide distribution and increasing trend of forest land area in the Daning River watershed that the cultivated land and grassland show a decreasing trend,which promotes the canopy interception and the increase of total evaporation in the watershed,thereby reducing the runoff yield;In addition,due to the large elevation fluctuation and steep slope in the Daning River watershed,the trend of runoff reduction is further intensified;Due to the large proportion of cultivated land in the tributaries of the left bank of the Daning River watershed,and it is also the main increase area of construction land,the regional runoff changes greatly.2)In spring,autumn,and winter,the area with positive value of discharge change is larger than the area with negative value;while in summer,the opposite is true.The negative value of the inter-annual change of the flow of production can indicate that the change of the flow of production in summer is deeper and more sensitive to changes in land use. |
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