The Interaction Between Groundwater And Vegetationin Arid And Semi-arid Area

Ordos basin is the main energy chemical area with fragile ecology, which was mainly characterized with dry climate,lack of rainfall, intensive evaporation and scarcity of surface water resources. So the groundwater was the important factor that influenced the social and economic development and ecological environment. Therefore, it makes a significant sense for sustainable development of the groundwater exploitation and ecosystem protection by analyzing and discussing the mechanism of interaction between groundwater and vegetation in this area.Based on previous field investigation, the soil moisture/temperature monitor, soil water potential monitor, vegetation sap flow monitor and groundwater level monitor were adopted to conduct the in-situ test in the field with Mu Us Sand land as the research area. The relationship between groundwater and the plant was investigated at the field regional scale; With the Salix psammophila as studied object, the transpiration of plant and the mechanism of interaction between groundwater(soil water) and plant were researched at the proving ground scale. The main results as follow:(1): The groundwater depth had influence onsurface vegetation types and distribution: Which manifested with the groundwater depth varying from shallow to deep centered on lake basins, the surface vegetation types were changed from herbaceous plant to sandy shrub; Also, the moisture content, vadose zone salinity, groundwater salinity and hydrochemical typeswere consistent with the different eco-hydrogeological area. The main vegetations(Salix psammophila and Artemisia sphaerocephala) had different root and coverage distribution laws, which indicated that Artemisia sphaerocephala had few demand of groundwater, but strong demand of groundwater for Salix psammophila.(2) The sap velocity had a positive correlation with the branch diameters of the Salix psammophila. Sap flow rate increased gradually after7:00, and reached a peak between 10:00 and 15:00, and then gradually decreased from 16:00 and became the smallest value(near zero) after 21:00. Sap flow rate had aobvious seasonal variation: the maximum value was in July and minimum in November. Meteorological factors were the most significant factors that affected the sap flow rate of Salix psammophila in the following order: vapor pressure deficit > solar radiation > air temperature > relative humidity > wind speed. There was a significantly linear relationship between sap flow rate and reference evapotranspiration with a correlation coefficient of R2=0.77 in a daily time. The water table and the soil moisture content in 0-50 cm decreased with the increase of totally sap flow in shallow groundwater areas. However, the groundwater was little changed and soil moisture in 0-80 cm decreased with the increase of totally sap flow in deep groundwater areas.(3) The groundwater depth influenced the temporal and spatial distribution of moisture and temperature in the unsaturated zone. When the groundwater depth was deeper, the soil moisture waslower but soil temperature was higher. Results of the trials indicated that a 1m increased in the groundwater depth would increase the soil temperature by an average of 0.6°C but decrease the soil moisture by an average of 0.09cm3/cm3.(4) The vegetation influenced the distribution of moisture and temperature in the unsaturated zone under a certain condition of groundwater depth. The soil moisture content in the vegetation covering areas was less than the bare soil areas. Meanwhile the vegetation can influence the soil temperature and its thermal conductivity in the unsaturated zone indirectly: The soil temperature in the vegetation covering areas was a little higher than the bare soil areas and the thermal conductivity in the bare soil areas lagged behind the vegetation covering areas, and the delay time is about 0.5-2h.(5)Inthe shallow groundwater area, the evapotranspiration could lead to the continually descending of water table, meanwhile the water table stayedconstant in the deep groundwater area. The Calculation results showed that the evapotranspiration in the vegetation was greater than the bare soil areas and the evapotranspiration in the shallow groundwater areas was greater than the deep groundwater areas by the water balance method. The soil moisture and groundwater were absorbed by the vegetation in the shallow groundwater area but only the soil moisturewas consumed when the groundwater was deep.