Evapotranspiration And Runoff Response To Urbanization In The Qinhuai River Basin

The Yangtze River Delta has experienced dramatic urbanization during the last several decades.Converting water stress-free paddy fields to‘dry'urban uses contributes directly to the rapid decline of ecosystem services function of rice paddies.Thus,the watershed hydrological processes are bound to be affected.Evapotranspiration(ET)is a major variable involved in the understanding of the impacts of land use and land cover change on the ecohydrological systems.Various researches focus on the ecohydrologic effects of urbanization.However,our knowledge of the disturbance mechanisms and key processes of ET and other ecohydrological processes is incomplete.Therefore,we conducted our case study in the Qinhuai river basin(QRB),a typical rapidly urbanized watershed in the Yangtze River Delta region.MIKE SHE(MIKE Système Hydrologique Européen/European Hydrological System)was applied to assess the potential dynamic of hydrological elements with different land use types at different spatial-temporal scale.The flood hydrological effects of urbanization were simulated through scenario analysis method which revealed the dominant hydrological disturbance processes controlling water balances and their variations with the converting of paddy fields to urban uses.The main conclusions are as follows:(1)The performance of MIKE SHE model was well for simulating the hydrological processes in the QRB.Two types of hydrological fluxes were used for model calibration and validation including monthly observed discharge data(1990-1999)from the Wudingmen gauging station at an outlet of the Qinhuai River and daily observed actual evapotranspiration(ET_a)rates data(2016-2017)from eddy covariance technique in the rice field located at Lishui Experimental Station.The coefficient of determination(R~2)and Nash-Sutcliffe's model efficiency coefficient(E)between simulated and observed discharge were 0.89 and 0.79,respectively for the calibration period.The R~2 and E were 0.74 and 0.64,respectively for the validation period.The model was able to capture the ET_a seasonal patterns with an optimal R~2of 0.72.(2)The reduced transpiration as a result of urban expansion was the primary factor driving the change of watershed-level.Although the land use conversion increased ponded water evaporation,there was overall a significant reduction in watershed level ET_a(p=0.01),since the total decrease rates for T,E_i,and E_swere about 2.5 times as much as the total increase of E_p.The total ET_a in rice-wheat rotation fields was highest among different land use types in this study area.Thus,conversion of rice-wheat rotation field to urban had more impacts on the change in ET_a and its partitions than the conversion of other land-cover change scenarios,especially in the rice paddy growing season.At present,most of remote sensing technology and hydrological models cannot estimate the change of evapotranspiration and its components.Therefore,the importance of vegetation conversion should be especially emphasized in predicting the water cycle under varying environment(e.g.urbanization).(3)Rapid rise in urban areas by converting paddy fields inevitably led to huge alterations in the watershed hydrology.With the same rainfall dynamics,the peak flow value and the total runoff under LU2011 scenario were larger than that under LU2000.In addition,low flow was more sensitive to land use change compared to high flow.Compared with LU2000scenario,flood discharge for 2-year and 50-year return period was increased by 33.6%and 10.2%in LU2011 scenario,respectively.Moreover,antecedent moisture condition is closely related to ET_a,which had a significant influence on runoff processes and extreme flood.Replacing rice paddy fields with high ET_a with urban land uses with low ET_a may result in extreme floods not only under extreme precipitation events but also under regular precipitation condition.(4)During the urbanization processes,entire watershed water cycle was altered and flood risks may be exacerbated.The impacts of urbanization were modeled by applying the model to four land use scenarios(LU2000,LU2004,LU2007 and LU2011).Degradation in rice-wheat field led to increased soil moisture content and groundwater storage.Expansion in urban land use reduced the rate of infiltration into the soil,resulting in enhanced streamflow,base flow and groundwater level.