The Responds Of Vegetation To The Groundwater Depth In The Grassland Open-pit Coal Mine Area

Hulunbuir grassland is the best-preserved high-latitude grassland in China,which plays an important role in maintaining and regulating the ecological environment security of arid-semiarid regions in north China.However,with the increase of the intensity and scale of glassland mining activities in recent years,the shallow groundwater of the grassland mining area is gradually drained,the environment in which groundwater occurs is changed dramatically.Groundwater is the key factor to maintain the normal growth of vegetation.The decrease of groundwater level makes the area of lakes and wetlands gradually shrink and degrades the grassland vegetation seriously.How to establish a complete response relationship between groundwater and vegetation in grassland mining areas,understand the impact of open-pit mining activities on regional hydrogeological conditions and ecological environment have become research hotspots for scholars at home and abroad.This paper takes Yimin open-pit coal mine in Hulun Buir grassland as an example,explores the influence of groundwater level drop caused by open pit coal mining drainage groundwater on regional vegetation: By analyzing the data of sample plot survey,the influence of groundwater level changes on grassland vegetation types,species richness,vegetation coverage,aboveground biomass and summed dominance ratio was explored in grassland mining areas;By using remote sensing methods,the data of Normalized Vegetation Index(NDVI)and groundwater depth of yimin mining area in 1985 and 2012 were extracted respectively,and the quantitative response relationship between regional groundwater depth and NDVI in recent 30 years was established.the final conclusion is drawn:1.In 1985,the groundwater depth in the research area was 0-16 m,and the groundwater depth in the research area was basically less than 5m;After the yimin open-pit mine was put into operation,the groundwater has been drained for decades,resulting in a sharp decrease of groundwater depth in the mining pit center,and the maximum depth of drainage reaches 112 m.However,the groundwater level around the Chedaminnuur lake and the northern part of the lake outside the drainage boundary,about 2000 m north of the mining pit,was not significantly affected.At present,the drainage volume of open-pit mine gradually approaches a fixed value(volume of runoff or rainfall recharge),and the drainage boundary is basically stable.2.In 1985,NDVI in the study area was 0.5-0.7,and in 2012,NDVI in the study area was 0.4-0.6,and the overall vegetation growth showed a downward trend.The relationship between NDVI and groundwater depth was established.It was found that when the ground water depth was 0-16 m,the tendency of NDVI to change with groundwater depth in 1985 is the same as 2012: NDVI with the groundwater depth at 0-1m was the highest;NDVI fluctuation was not obvious with the groundwater depth at 2-6m.When the groundwater depth exceeds 6m,NDVI shows a downward trend.3.The sample plot survey found that the soil was severely salinized at the first terrace of Chedarminore lake with the groundwater depth is 0-1m,saline meadows,mainly Suaeda glauca(Bunge)Bunge + Chenopodium album L.,were well-developed.The second terrace of Chedarminore lake,with the groundwater level buried in 1-2m,developed typical steppe in which the main types vegetation are Leymus chinensis(Trin.)Tzvel.+ Stipa baicalensis Roshev.+ Cleistogenes squarrosa(Trin.)Keng.;The area within the range of 2000-3000 m from the mining pit,with the groundwater depth was 2-6m,developed degenerated typical grassland in which the typed vegetation are Leymus chinensis(Trin.)Tzvel.+Stipa baicalensis Roshev.+Potentilla acaulis L.;The area within the range of 500-2000 m from the mining pit,with the groundwater depth was 6-40 m,developed degenerated meadow grassland in which the typed vegetation are Leymus chinensis(Trin.)Tzvel.+ Artemisia annua + Iris lactea Pall.var.chinensis(Fisch.)Koidz.;The area within the range of 300 m from the mining pit,with the groundwater depth was 40-60 m,developed degenerated meadow grassland in which the typed vegetation are Leymus chinensis(Trin.)Tzvel.+ Suaeda glauca(Bunge)Bunge +Artemisia annua.4.The results of the relationship of regional groundwater-vegetation obtained by the vegetation sample survey and remote sensing techniques show that the groundwater depth of the most suitable vegetation for the study area is 1-2m;the limit groundwater depth of the vegetation is 6m.5.Open-pit coal mining reduces the regional groundwater level and affects vegetation growth.The scope of the drainage funnel influence on the regional groundwater level and vegetation growth is about 3000 m in the periphery of the open-pit coal mine,and the the groundwater depth is 2-60 m.