| There are many water systems in plain river network area.Water system plays an important role in increasing the storage and discharge capacity of river system and reducing flood risk.Over the past three decades,the Qinhuai River Basin has experienced rapid urbanization,with a large number of agricultural land transformed into urban land.The change of urban underlying surface has changed the water system structure and connectivity pattern of the river network in the basin,and led to the increased risk of flooding in the basin.Quantifying water system connectivity and exploring the impact of connectivity on hydrological processes such as flooded area and flood peak water level are the key to understand the impact of rapid urbanization and human activities on watershed ecosystem functions.Aiming at the aggravation of flood caused by the reduction of river network and blocked connection caused by urban land use change in the plain area of Qinhuai River Basin,firstly,the flooded area of the basin from 2000 to 2019 is extracted by using Landsat7 image data and improved water body index(MNDWI).On this basis,the temporal and spatial distribution characteristics and influencing factors of the flooded area of typical areas in the basin(including urban area and urban-rural junction area)are analyzed;Mike she model is used to simulate and analyze the impact of urban land use change on river water level change in urban area and urban-rural junction area;Finally,the circuit theory method in landscape ecology is introduced.Based on the land use data of different periods,the water system connectivity of Qinhuai River Basin is simulated.Combined with the flooded area and flood peak water level of typical areas of the basin,the possible impact of water system connectivity on the hydrological processes such as flooded area and flood peak water level is analyzed.The main conclusions are as follows:(1)Based on GEE remote sensing platform,the flooded water body is extracted from Landsat remote sensing images in the basin.The results show that during the study period(2001-2019),both sides of the Qinhuai River belong to the area with frequent flooding.The flooded area of urban area(area A)and urbanrural junction area(area B)shows a trend of decreasing first and then increasing.By calculating the flooded area density(flooded area / regional area)of each area,it is found that the flooded area of urban-rural junction area is more serious than that of urban area,and the difference is up to 0.23 km 2/ km 2.(2)The influence of rainfall,drainage pipeline and impervious surface on the flooded area in different urban and rural areas shows that in urban area(area A),rainfall has no significant effect on the change of flooded area;From 2001 to 2010,the growth rate of impervious surface is consistent with the change trend of flooded area,and the correlation coefficient r is 0.751,P is 0.020;Drainage pipelines in urban areas have a partial impact on the change of flooded area,but the impact has a certain lag.In the urban-rural junction area(area B),the change of flooded area was mainly affected by rainfall from 2001 to 2005(r = 0.961,P =0.005);From 2006 to 2015,the flooded area was significantly affected by the impervious surface(r = 0.781,P = 0.013).In addition,the construction density of urban drainage pipeline also has a certain impact on the flooded area in the region.(3)Through the calibration and verification of Mike she model parameters,the R2 between simulated and measured rainfall process values in four years of 2003,2007,2011,2013 is 0.70,0.83,0.62 and 0.63 respectively.The model can better simulate the change of river water level in the basin.Comparing the simulation results of urbanization and urbanization base year(2000),it is found that urban land use change is an important factor leading to the increase of river water level.For the same rainfall,the river water level is more sensitive in the urban-rural junction area with obvious land use change.(4)The application of circuit theory has certain practicability for the simulation of watershed connectivity.Since 2000,the degree of fragmentation of river system connectivity has increased significantly,and even a wide range of null value areas have appeared.The areas with dense connectivity and low value debris are consistent with the areas with frequent flooding monitored by satellite.Among them,the water system connectivity of urban area(area A)decreased by 2.7% from 2000 to 2005.At the same time,the flooded area increased to a certain extent,and the water system connectivity improved after 2005.The water system connectivity in the urban-rural junction area(area B)decreased by 3.4% from 2010 to 2018,while the flooded area increased by nearly 136.4%.The poor connectivity led to the obvious increase of the flooded area in the urban-rural junction area and the increase of flood peak water level. |
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