Plant Water Consumption And Its Response To Groundwater Table Drop In Arid Area

Groundwater is considered as an important water supply source and an important factor affecting the social and economic development and ecological environment in the arid and semi-arid regions which are widely distributed in NW China,where the climate is dry,precipitation is low,evaporation is strong,and surface water resources are scarce.Because of the irrational exploitation of groundwater,vegetation in arid areas is damaged.Therefore,the interaction between groundwater and vegetation is of great significance to regulate and control water resources and to develop groundwater sustainable in arid areas.Vegetation water consumption and the impact of water table decline on vegetation are the main contents of groundwater-plant interaction.Aiming to address the above-mentioned problems,the interactions were studies at both site and regional scale.Firstly,using the observation data from groundwater and ecological field observation base of Yulin,transpiration of Salix matsudana and Populus simonii was calculated and the relation between transpiration and meteorological factors was explored.In order to regionalize the results from the site observation to other climatic regions with different water table depth and soil conditions,48 meteorological stations were selected in NW China.By using the Hydrus 1D,transpiration was calculated for the 48 sites with different water table and soil types.According to the relation,a forecasting model was developed and verified.Hydrus 1D is a widely used model to study the relationship between groundwater and plants,however in many cases the initial conditions can not be specified easily.It is found that the different initial conditions have different effects on the simulation results.Systematic study was performed on the influence of initial conditions on modelling results in this study.The following conclusions were obtained:(1)Using sap flow to calculate vegetation water consumption,the variation of sap flow in orientation and depth must be considered.Otherwise,the error can be up to 104%.(2)The correlation of plant water use to metrological factors indicates that the most correlated factor is the net radiation and the second correlated factor is air temperature.The time lag analysis show that there is no time lag between transpiration and net radiation,while there is one hour time lag between transpiration and vapor pressure deficit.(3)The average daily water consumption of Salix matsudana and Populus simonii is2.9L and 2.4L,respectively.(4)An exponential decay was observed between the normalized transpiration(Tan)and the depth to water table and a positive linear relationship was found between the Tan and mean annual precipitation(MAP).Based on these two relationships,a predictive model was developed to forecast the response of phreatophytes to groundwater drawdown using MAP only and the validation in the other independent 20 sites indicates that the predicted response is comparable to the modelling results.(5)With deep water table and dry soil,the memory of the initial conditions is longer.The results also prove that rainfall will reduce the influence of initial conditions on the simulation results,obviously in sand and clay.Field water capacity is not suitable to be used as the initial condition for loam and silt and spin-up is an effective method when observation data is unavailable.The paper comprehensively uses various technical methods to carry on the related research such as observation and measurement in situ,model,mathematical statistics,and remote sensing.Study on the relationship between groundwater and ecology was carried out at the site scale and regional scale.The relationship between water consumption of vegetation and meteorological factors was identified,and the impact of depth to water table and precipitation on groundwater-depend vegetation was assessed.Finally a prediction model was developed.Furthermore,some technical problems in the instrument installationand model setting are solved.It provides a technical support for rational exploitation of groundwater and ecological environment protection.