Dynamics Of Shallow Groundwater Level In The Yellow River Delta And Its Relationship With Vegetations

Wetland is an important part of the ecosystem.At the same time,The Yellow River Delta is the most intact preserved warm temperate wetland ecosystem in China.At present,under the influence of both natural environment and human activities,a series of wetland ecological environment problems have been caused,such as soil salinization,wetland ecological function decline,wetland biodiversity loss.These problems are all related to vegetation.Soil moisture and salinity are the dominant factors of vegetation distribution,and groundwater plays an important or even decisive role in soil moisture and salinity in the Yellow River Delta.Therefore,it is important to study the relationship between vegetation and soil salinity and groundwater depth to effectively protect the health of wetland ecosystem.In this paper,the vegetation sample survey data and remote sensing image data are combined to analyze the relationship between vegetation distribution pattern and vegetation growth,soil salinity and groundwater depth in the Yellow River Delta by canonical correspondence analysis and correlation analysis.According to the actual hydrological situation of the Yellow River Delta,combined with previous studies,the groundwater flow model is constructed to simulate the dynamic change of groundwater level in the next five years.Based on the study of the dependence of different vegetation on soil salinity and groundwater depth,combined with the previous achievements,according to the predicted dynamic change trend of groundwater level in the next five years,and then revealed the impact of dynamic change of groundwater level on the growth,distribution and succession of vegetation in the Yellow River Delta.The results show that:(1)In this paper,Visual Modflow is used to build a two-dimensional conceptual model of groundwater flow in the Yellow River Delta.The strong implicit iteration method(SIP)is used to solve the problem.The parameters of the model are fitted and verified,and the groundwater balance is analyzed.The model can reflect the actual situation of the Yellow River Delta.Through the calculation of water balance,it can be seen that the main supply of groundwater infiltration in the study area is precipitation,and the main discharge is evaporation.Each supply and discharge item of groundwater is in a relatively stable state,and the supply amount is greater than the discharge amount.(2)The CCA sequence map reveals that soil salinity and groundwater depth are significantly correlated with the vegetation distribution pattern in the study area,and the vegetation distribution pattern changes with the change of soil salinity and groundwater depth.The influence of groundwater depth on vegetation distribution in the following aspects:with the increase of groundwater depth,the distribution of plant species decreased gradually,and Sargassum,Robinia pseudoacacia,Artemisia can survive in the area with larger depth.With the increase of soil salinity,the distribution of plant species decreased significantly;salt tolerant vegetation mainly includes Salicornia?Suaeda heteroptera?Tamarix chinensis.(3)The response of six vegetation communities to groundwater depth is different.There was no significant relationship between NDVI and groundwater depth in the communities of Suaeda salsa,Tamarix chinensis-Suaeda salsa.On the whole,the NDVI value of Tamarix chinensis-Phragmites australis+Suaeda heteroptera community increased with the increase of groundwater depth.The relationship between NDVI and groundwater depth in reeds community is complex.There was a linear relationship between NDVI and groundwater depth of Salix-Di community,and the NDVI value increased with the increase of groundwater depth,and R~2 reached a higher level of 0.8612.Groundwater depth has a significant effect on the NDVI of Robinia pseudoacacia community.The relationship between groundwater depth and Robinia pseudoacacia community can be fitted with a linear model,and R~2 reaches a high level of 0.7698.(4)Under the background of global warming,the groundwater level shows a rising trend,especially with the increase of years.With the rise of groundwater level,the groundwater depth becomes shallow,resulting in the increase of soil salinity,which inhibits the growth of vegetation.With the rise of groundwater level,the distribution area of wild Soybean,Robinia pseudoacacia and Chenopodium will decrease.With the increase of soil salt content,fresh water vegetation and salt intolerant vegetation are gradually replaced by salt tolerant vegetation.Salix-Di community,distributing in the Yellow River mouth area which is washed by fresh water,gradually becomes Phragmites australis community.The community of Limonium scattered in Tamarix chinensis shrub will be replaced by Tamarix chinensis gradually.In the low-lying area with high salt content in the tidal zone,the Salicornia can form pioneer plant community;the cattail community distributed in the inner side of the marsh type Phragmites australis community will gradually be replaced by the reeds community;the Suaeda salsa community will gradually be replaced by the Tamarix chinensis-suaeda salsa community;the Tamarix chinensis-suaeda salsa community will gradually be replaced by the Tamarix chinensis-Phragmites australis+Suaeda heteroptera community;the reeds community will gradually by Citronella community,which is suitable for growing in humid environment.