Characteristics Of Precipitation Events In Dianchi Lake Watershed And Their Influence On Surface Water Temperature Of Dianchi Lake

As a provider of the earth's freshwater resources,precipitation plays an important role in the water cycle.Precipitation has the characteristics of uneven distribution and unstable changes in space and time.These characteristics often determine the occurrence of floods caused by super precipitation or the occurrence of long-term dry weather without precipitation,which has a bad impact on agricultural production.At the same time,with the progress of social development from rural to urban areas,the impervious surface expands rapidly,and the precipitation thermal runoff pollution generated by precipitation events under the action of impervious surface has an important impact on the recipient water area,which is directly manifested by the increase of water temperature on the surface of the lake.The surface water temperature of the lake is one of the important factors that reflect and determine whether the lake water ecosystem is good.In recent years,due to global climate change,the surface water temperature of lakes around the world has been on the rise,and this rising trend is more pronounced in urban lakes.Therefore,the thermal runoff pollution caused by precipitation events has an impact on the surface water temperature of lakes.The impact cannot be ignored.Therefore,this study takes Dianchi Lake Basin as the research area,collects and analyzes precipitation event data,land use data and Dianchi surface water temperature data in Dianchi Lake Basin.Based on these data,analyzes the characteristic changes of precipitation events in Dianchi Lake Basin and the influence of precipitation events on the surface water temperature of Dianchi Lake.The research results show that:(1)From 2001 to 2018,the annual precipitation and the number of precipitation days in the Dianchi Lake Basin showed an insignificant upward trend.The trend of annual maximum daily precipitation is downward.The frequency of heavy precipitation increased but the intensity decreased.The periodic oscillation of annual precipitation was strongest at the time scale of 8 years.Dianchi Lake basin is experiencing the cycle change of wet year,dry year and wet year.The trend change of seasonal precipitation events in four seasons is not significant,and there are seasonal differences in the cycle change.Among the monthly precipitation events,the characteristics of precipitation events in other months were not obvious except that the precipitation and maximum daily precipitation in March showed a significant upward trend,the number of precipitation days in May showed a significant downward trend,and the maximum daily precipitation in July showed a significant downward trend.The spatial distribution of precipitation events in the Dianchi Lake Basin is characterized by the obvious difference of dry season and rainy season,more in the south and less in the north,more in the east and less in the west,and the distribution of precipitation resources is seriously unbalanced.(2)The impervious surface of the Dianchi Lake Basin expands rapidly,and the impervious surface exchanges energy with the runoff generated by precipitation.The precipitation heat runoff has become another factor influencing the increase in the surface water temperature of the lake.Under the background of different impervious surface coverage rate and different growth rate of impervious surface area,the effects of precipitation events on lake surface water temperature are different.When the coverage rate of impervious surface is relatively low and the growth rate of impervious surface area is low,there is a negative correlation between annual precipitation and the average annual water temperature of the surface of Dianchi Lake(r=-0.483).When the coverage rate of impervious surface is high and the impervious surface area increases rapidly,the annual precipitation in the Dianchi Lake Basin is significantly positively correlated with the average annual water temperature of the Dianchi Lake surface(r=0.667).With the increase of the coverage rate of impervious surface,the correlation between the seasonal precipitation and the seasonal mean water temperature of Dianchi Lake surface changed from negative to positive in spring and autumn.In 8 out of 12 months,under the action of impervious surface,during 2010-2018,the correlation between monthly precipitation in Dianchi Lake Basin and monthly mean water temperature on Dianchi Lake surface changed from a negative correlation in 2001-2009 to a positive correlation,or the positive correlation coefficient was larger than the previous one.These experimental results show that in the previous time period,the precipitation event was mainly a natural factor,which had a cooling effect on the surface water temperature of the lake.This effect was mainly due to the large cloud cover,low solar radiation and low temperature when the precipitation occurred;in the latter time During the period,the precipitation and heat runoff pollution caused by the precipitation event became the human activity factor of the increase in the surface water temperature of the lake.With the increase of impervious surface coverage,the influence of precipitation and heat runoff on the temperature of lake surface temperature will increase.