Vegetation Ecological Risk Assessment Under The Impact Of Groundwater Withdraw In Wulannao Area, Ordos Basin

Ordos Basin is rich in coal, oil, natural gas and other mineral resources, has become an important energy base , and water resources demand growing. Ordos region is located in arid and semi-arid areas, lack of surface water resources. The exploitation of groundwater resources to make up for shortage of water resources has become a practical way. Wulannao water resource region located in west of Yijinhuoluo County, Erdos City, Inner Mongolia Autonomous Region, was an important water sources for the Shenhua energy base .Previous studies have shown that the exploitation of underground water resources will change the natural state of groundwater. The natural state of groundwater and groundwater hydrology process change will have an inevitably affect on vegetation and the ecological environment, and bring on ecological risk. The risk is mainly reflected in the groundwater-table change. When it's a bit more than the suitable water-level of native vegetation will cause the degradation of vegetation or even replacement .In the Wulannao region Ordos, based on a great deal of the vegetation sampling and hydrogeological field survey results, analyzed the relationship between the distribution of vegetation with water table depth. According to previous studies, plants and environmental factors are usually in accord with the non-linear quadratic curve model, that is, single-peak model, one of the most representative model is Gaussian. Ecological meaning of Gaussian model is that abundance of a plant increase when the environmental factors increase; when the environmental factors increase to a particular value, the abundance of plant species to achieve maximum, environmental factors at this time become the most appropriate value. Thereafter, when environmental factors continue to increase, the species abundance decline, until disappear.Therefore, selected the Gaussian regression analysis methods base on the mathematics of vegetation ecology ,and use that to analyze the data of the water table and vegetation in Wulannao , then get the suitable water table of vegetation. According to the suitable water table of vegetation to part Wlannao into different sensitivity zones . Use the numerical model of groundwater to simulate the condition of extraction, combined with the results of groundwater prediction to estimate the ecological risk caused by the exploitation.Through analysis data from the vegetation Survey can know that, in Wulannao there is mainly two kind of plants, low humidity vegetation and desert grassland vegetation, the former distributed in the lake basin and surrounding beaches, the latter located in aeolian wavy terrain and ridge. Through the vegetation coverage, abundance and highness statistics and calculate the dominance of vegetation. The results can know the Artemisia, Caragana, Artemisia frigida, Stipa desert, Leymus, Carex, etc, are the main dominance species of the area .Part of vegetation in the region did not show obvious correlation between water depth and abundance. The survival of such vegetation does not need to rely on groundwater water or only partially dependent on groundwater. And this side of the sample is major for herbaceous vegetation in arid grasslands, the number characteristics of tall shrubs can not be accurately calculated. The suitable waterlevel of shrubs is mainly reference YangZeyuan's study in the TuWei River Basin, most shrubs good survival of the water level is in 1.5-3m, stressed water level in 3-5m, alert waterlevel is in 5-8m depth range, growing non-performing, more than 8m, poor growth, there is blasted branches.On account of dominance species are the principal part of plant communities, this time mainly use the Gaussian model to regression analysis dominance species of vegetation. Judging from the results of the analysis, when the depth to groundwater level is less than 1m , evaporation of water consumption is very easy to form salinity, and disadvantageous for vegetation except some alkali-resistant vegetation ; water table depth at 1-3m in the beach is suitable for the vegetation likes water, such as Leymus, splendens, Carex ,Leymus ,Alkali buttercup , etc, have the best growth, high vegetation coverage; water table depth at 3-5m, low humidity vegetation can grow, vegetation coverage and abundance decreased significantly, and this depth range is most appropriate the drought-resistant vegetation, such as Cynanchum komarovii, Artemisia Frigida, Ordosica, these vegetation have the highest coverage and abundance and became the main body of plant communities; when water depth deeper than 5m, the vegetation coverage declines, and most of the vegetation types in this area does not need to rely on groundwater water or only partially dependent on groundwater, such as Stipa desert, Skybiue Broomrape Herb, Euphorbia esula, etc. Based on the conclusion of the relationship between the vegetation and water table, when the groundwater level changes, the abundance of vegetation will change ,either . According to the change of vegetation, part Wulannao into vegetation sensitivity zones . Lake beaches has shallow depth of groundwater level in most areas, are the first sensitive area. Aeolian Sand terrain zone and the plateau for the second sensitive zone. Wavy plateau was part into a non- sensitive zone ,because the depth to water-table was deeper.Based on the analysis of the hydro-geological conditions of the study area to built the numerical model of groundwater. Use the numerical model to predict different exploitation project. Through the selection, the most reasonable project is that in the detailed investigation of the district, north-south disposal wells in tow lines, distance between wells is 1.5km, the distance between well line is 3km, exploitation 10000m3/d. Calculating comparison proves that the program is guaranteed. Forecast the future water-level of this project in case of groundwater extraction. On the basis of the relationship between total vegetation abundance and water-level , establish vegetation indicators of ecological risk ,and make an assessment of ecological risk caused by groundwater exploitation ,then part Wulannao into vegetation ecological risk zones.From the numerical simulation results, the exploitation of water to the affected area is limited, mainly concentrated in the detailed investigation of the area, so this time mainly assessed ecological risk of vegetation this of area .From zoning map of ecological risk which caused by groundwater exploitation, the high-risk zones of vegetation ecology are mainly located around the water resource. Ten years after the exploitation of groundwater, the water level in the central area of exploitation of confined aquifer dropped 9-16m, unconfined water level dropped 2-4m, depth to water table at 4-5m, more than the suitable water-level of low humidity vegetation .And the existing vegetation and low humidity will be degraded, vegetation type and quantity will drop obviously, so it's the area of groundwater extraction for high-risk.Medium-risk zones are mainly in the outside of central area of the exploitation in the lake beach, this region affected by the exploitation of groundwater weaker than high-risk zone. In this part of the region the low humidity vegetation can grow, and there won't be major change in vegetation ecology.Low-risk zones are mainly distributed in the northern and northeastern of the detailed investigation area, the impact of groundwater extraction is no or less in the aeolian terrain of the area. Vegetation here is normal growth. Security-zones are main distributing in the ridge of bedrock, the groundwater level deeper than other areas. And most vegetation here does not need to rely on groundwater water or only partially dependent on groundwater, the area affected by the exploitation of groundwater impact is negligible.Described above, the total area of vegetation ecological security zones and low-risk zones are more than 70% of area of detailed investigation. High-risk zones and medium risk zones are mainly in the area concentrated in and around water sources. High-risk zones are only in the center of the groundwater funnel .Through risk analysis and evaluation, groundwater exploitation in Wulannao has little ecological impacts on vegetation except the central area,. Artificial cultivation and prohibition of grazing can effectively prevent deterioration of the ecological environment.