Study Shallow Groundwater Table Dynamics And Its Relationships Withvegetation Based On Visual Modflow In The Yellow River Delta

At present, due to climate change and human activities lead to large areas of wetlanddegradation and salinization, the wetland ecosystem stability and health of wetlandecosystems poses a serious threat. Yellow River Delta wetland also appeared natural wetlandarea is reduced, soil salinization degree aggravated ecological functions of wetlandsdegradation, decreased diversity of wetlands and other issues. All these issues is closelyrelated to the wetland vegetation, as the Yellow River Delta special geographical location andnatural conditions, groundwater makes the region has very close hydraulic relationshipbetween groundwater and vegetation, groundwater through the "saturated zone-vadose zone-vegetation" between vertical water contact produce extremely important to the ecologicaleffects from the point and the surface. At present, study the dynamic characteristics ofgroundwater in the region and its relationship with vegetation, in order to operate ecologicalin the Yellow River and provide scientific basis and management idea in the Yellow RiverDelta wetland ecosystem restoration projects.First of all, in order to study the relationship among the typical vegetation (Robiniapseudoacacia, Suaeda heteroptera, Tamarix chinensis, Phragmites australis, Cotton), soilsalinity and groundwater depth in the Yellow River delta, field survey and statistical analysiswere conducted. Secondly, the numerical simulation method was used to establish in theYellow River delta wetland that the paper based on the hydrogeology survey work of YellowRiver delta wetland area established the conceptual model of shallow groundwater movementin the Yellow River delta wetland system. Using the Visual Modflow to establish the shallowlayer groundwater movement model, identification, validation, and solving. Finally, thegroundwater numerical simulation model based on after verification and identification,prediction of groundwater dynamic change under different scenarios, to study their effects onvegetation growth and distribution. Following results are obtained:(1)The groundwater depth has significant influence on soil salinity and the averageinfluence coefficient of groundwater depth on soil salinity is0.327.Other results were below: the salinization is severe when the groundwater depth is between0.5m and1.5m; thevegetation growth is poorer in the study area with the NDVI less than0.4in78%of studyarea; the groundwater depth and soil salinity have significant influence on vegetationdistribution; the soil salinity has significant influence on the NDVI of vegetation (Robiniapseudoacacia, Suaeda heteroptera, Tamarix chinensis, Phragmites australis, Cotton); thegroundwater depth has significant influence on the NDVI of Robinia pseudoacacia andSuaeda heteroptera and has insignificant influence on the NDVI of Tamarix chinensis,Phragmites australis and Cotton.(2) According to the dynamic change rules of groundwater and water balance analysis inthe study area shows that the precipitation and evaporation Cause the groundwater dynamicchanges. Overall direction of subsurface flow runoff is from land to sea, and like the directionof surface elevation changes. Subsurface flow runoff based on the current channel of theYellow River and the Yellow River communist-held watershed, divided into north, northeast,southeast three mainstream direction of discharge to the sea. Near the Yellow Rivergroundwater level is high, the groundwater contour is densely, the Yellow River rechargegroundwater. Water level slightly higher than the surrounding and formed with the ridge ofcommunist-held groundwater watershed in the highlands of communist-held Yellow River.Flat area of groundwater, groundwater contour is scanty, groundwater depth change gently,hydraulic gradient is small and consistent with the direction of the surface elevation drop.Groundwater contour in waterfront is scanty, groundwater depth change gently, hydraulicgradient is small, groundwater discharge to the sea, flow slowly.(3)Under normal conditions, the groundwater level will be upward trend at the next fewyears a, the groundwater level of low lowland, coastal lowlands rise about1to2m. Under thiscondition, Vegetation succession as follows: depression, low lowlands, coastal lowland due toimpounded surface water, suaeda heteroptera can not adapt to the impounded surface waterenvironment and gradually replaced by a reed community, in the long term is graduallyreplaced by imperata cylindrica community, until the succession for temperate broad-leavedforest. In the case of increased rainfall, the groundwater level overall rose in the study area,With the increase of time, the larger the range of groundwater level increase is; In this case,Due to rainfall leaching effect, soil salinity decreased in the study area, the salt tolerance ofreed by fresh water reeds, Suaeda heteroptera, Tamarix chinensis, gradually replaced byimperata cylindrica community, until the succession for temperate broad-leaved forest. In thecase of evaporation increased, the groundwater level overall decline in the study area. Withthe increase of time, the obvious of groundwater level decline; After evaporation increasemay lead to vegetation succession process is: due to increased evaporation make the buried depth of groundwater become deeper and more serious soil salinization, soil salt contentincreases, the reeds and imperata cylindrica community gradually by salt-tolerant Suaedaheteroptera and Tamarix chinensis meadow.