On The Impact Of Urbanization On The Precipitation And Runoff Based On The Feedback Between Land Surface And Atmosphere

Urbanization is the process during which the natural surfaces are changed into urban areas characterized by concrete,impervious surface.It change the land use and cover and biogeophysicial features,such as albedo,surface roughness,leaf area index,which influenced the surface water and heat budget.On one hand,altered water and heat budget will influence the distribution of heat and atmosphere pressure,which will impact on 2m air temperature and precipitation,etc.at local,even regional climate and weather.On the other hand,increasing imperviousness will impact the runoff yield and routing process,i.e.,less intercepted water by vegetation,infiltration and evaportraspiration water,but increasing runoff and routing speed.In all,the water circulation are intervened a lot by urbanization.As global urbanization process speeds up,urban storm flood and deterioration of urban enviroment have been dominating under which effect of urbanization on environment has been a hot spot which draw much attention of many scholar with the background of geography,hydrology,meterology and environmental science.Yangtze River Delta(YRD)is one of highest urbanized and densely populated region,and also the one of the largest river delta and metropolitan area.Fast urbanizaiton occurred here since 1980s,and there was increasing trend of extreme precipitaton and flood disaster with tremendous economic loss.Past urban hydrology research only focused on the runoff yield and routing,while it ignored the effect of alter heat budget on the local air temperature,precipitation and evaportranspiration.In fact,the influenced temperature and precipitation can directly impact the runoff yield and routing process on the surface.Above all,based on the heat and water budget and feedback bewteen land surface and atmosphere,this research investigate the impact of urbanization on the surface,precipitation and runoff by integrating techniques of geography,hydrology,meterology.The main content and conclusions are describled as follows:(1)On Land use and cover and river characteristicThree periods of Landsat TM images were selected to describe the land use change,and it showed that urbanization in recent 30 years was classified into 1990-2000 featured by normal speed urbanization and 2000-2010 by high speed,and the percentage of urban area increased from 4.9%in 1990 to 9.2%in 2000 and 19.94%in 2010.River network were extracted based on the topographical maps and Google images with resolution of 2 meters.Result showed river density and water area ratio have declined in the background of urbanization,and that they showed larger decreasing magnitude in highly developed area and it has positive relationship with imperviousness(r=0.97).(2)On characteristic of precipitation variationBased on daily precipitation data of meterological station within Yangtze River Delta,the trend of annual and seasonal precipitation extremes were investigated in the period 1957-2013 as well as wet periods(WPs).Result showed that 1)the increasing trends of extreme precipitation can be detected in the past 57 years in YRD,but the increasing tendency were not statictically sifnificant;Heavy events exhibited overwhelming upward tendency in summer throughout YRD;Mean wet periods had become short most of YRD indicating that precipitation was gradually concentrated on short precipitatin periods that the occurrence and fractional contribution of short periods were overwhelming.Based on the rain gagues with dense spatial distribution,the potential effect of urbanization on the precipitation was discussed by analyzing the differences of rainfalls between urban and suburban zones.Result showed that there was effect to enhance precipitation that more annual and extreme precipitation was found in urban area than suburban,and this effect have been strong during 1963-2006 but weak from 2007 to 2012,which showed the complex effect mechanism in different stages of urbanization.The driving forces showed that there was increasing tendency of extreme rainfalls under the background of global warming,but urbanization caused the precipitation differences between urban and suburban area,which provides observational proof for the numerical simulations later in this thesis.(3)Setting up rainfall and runoff simulations based on the feedback between land surface and atmosphere.WRF(Weather Research and Forecasting Model)were applied to the numerical simulation in our research.It is found that the performance of model can be largely improved by coupled with single-layer urban capony model,real-time sea surface temerature(SST),high resolution of land use and cover in the one-way triply nested configuration.The corelation coefficient of simulated air temperature and precipitation with observational values was 0.89,0.72 in control experiment,and these figures were 0.94 and 0.80 in validation experiment,indicating that the incorporated model have good performance in simulating the air temperature and precipitation chracteristic in YRD(4)On the numerical simulations on the effect of urbanization on precipitation and rainfalls.Three scenarios representing the characteristic of land type and cover in 1990,2000 and 2010 were designed to explore the impact of urbanzation on precipitation and runoff,trying to figure out the physical mechanism in it.The simulations showed that more summer precipitation and rainy days were observed in urban areas,which was consistent with the observational results.The effect of urbanization was in the form of,1)the scale of the city,precipitation change in the area around the city of Shanghai was obvious,while it was weak in other cities.2)Spatial heterogeneity that increasing precipitation was seen in urban zone and downwind region,but decreasing rainfalls in cores of cities.3)different periods,increasing precipitation was seen in the upwind region with the land parameters in 1990 replaced by 2000(1990-2000),while decreasing precipitation was observed in this area in 2000-2010,and increasing rainfalls in downwind region.Alteration of land surface affected the partition between sensible heat and latent heat,which led to urban heat island,and enhanced convective activites have increased convective precipitation to some extent,but the different effect of urbanization on other type of rainfalls(frontal,typhoon rain)made it complex on influencing total precipitation in summer.In the whole YRD,the increasing magtitude of runoff and yield coefficients was significant as result of alteration of land surface during the urbanization,meanwhile,regional runoff 15km surrounding the cities increased slightly.From the basin with different urbanization degree,increasing amount of runoff peak in highly urbanized basin(Huangpujiang basin),while it was least in almost natural basin(Xitiaoxi basin).The change amounts in natural,low level,moderate level and high level of urbanization was-4.66mm,47.75mm,58.25 and 93.22mm,respectively induced by changing the land surface from 1990 to 2010.