Study On The Ecological Degradation Of Wetland In Nenjiang River Basin And Its Hydrological Driving Mechanism

The Nenjiang River Basin(NRB),located in the middle and west of Northeast China,is one of important areas with widespread wetland complex in China.The NRB is sensitive to climate change because it at the typical middle and high latitude region.In recent years,under the influence of climate factors and human activities,the hydrological processes of the basin and wetlands have changed greatly,which has led to considerable loss of wetland area.As a result,the function of wetlands has declined,threating ecological security in the NRB.Therefore,investigation on the hydrological driving mechanism of wetland ecological degradation from watershed scale has an important role and decision support for wetland ecological hydrological restoration and water resources regulation in the NRB.In this study,the temporal and spatial variation characteristics of hydrometeorological elements and runoff variation characteristics of main water supply rivers of typical wetlands were analyzed using the observed data in the past 50 years.The changes of the area and landscape structure of wetlands are evaluated.The status of wetland ecological degradation in NRB was clarified,and the ecological recession of watershed wetlands and international important wetlands is revealed.This paper focuses on the analysis of the degradation of riverine wetlands,explores the changes of the downstream hydrological situation caused by the operation of Nierji reservoir and its impact on riverine wetlands,and reveals the hydrological driving mechanism of ecological degradation of riverine wetland.The main results are as follows:(1)Evolution characteristics of hydrometeorological elements in the Nenjiang River BasinBased on the observed data from 39 national meteorological stations in the NRB and its surrounding areas,the law of climate evolution in the Nenjiang River Basin is analyzed.It was found that the drought occurred frequently in the Nenjiang River Basin,and the average temperature,annual precipitation,annual potential evapotranspiration and annual water deficit increased in the past 50 years.Based on the statistical analysis of the measured daily runoff data of the hydrological stations in the Nenjiang River Basin,it is found that the annual runoff of the Nenjiang River Basin and the annual runoff of main supply water sources of typical wetlands are decreased.(2)Analysis of wetland ecological degradation in the Nenjiang River BasinThe ecological degradation characteristics of the Nenjiang River Basin wetlands and typical wetlands were quantified from two aspects of wetland area and landscape structure by using wetland remote sensing interpretation data,Arc GIS and FRAGSTATS softwares.It is found that the wetland area of the Nenjiang River Basin declined 21% from 2000 to 2015;the area of Zhalong wetland,Xianghai wetland and Momoge wetland decreased by 0.2%,6.4% and 7.6% respectively.The wetland types of the Nenjiang River Basin are relatively scattered,the connectivity between wetlands is weak,the landscape structure is complex and the degree of fragmentation is serious.(3)Hydrological driving mechanism of wetland ecological degradation in the Nenjiang River BasinWith the increase of temperature,precipitation and potential evapotranspiration,the interaction between river and wetland is weakened due to the construction of water conservancy project,the runoff is reduced and the wetland area is reduced.Based on the principal component analysis of the wetland area and 32 hydrological indicators in Nenjiang River Basin,18 hydrological indicators affecting the ecological degradation of riverside wetland were obtained.The riverine wetland area decreased by 14% in 2000-2015.Among them,due to the moderate change of the downstream hydrological situation caused by the operation of Nierji reservoir,the wetland area decreased by 13.2%.The key factors that lead to the degradation of wetland area are the reduction of the maximum annual discharge of one day,the emergence time of the maximum annual discharge,the peak number of annual high discharge and the average continuous time.