Groundwater Flow Patterns And Numerical Simulation Of Typical System In An Arid Inland Basin,Northwest China

Manas River Basin is a typical mountain-oasis-desert arid inland river basin in Northwest China.As one of the biggest irrigation area in Xinjiang,mulch drip irrigation technology has been applied in the Manas River Basin.Now the Manas River Basin has become the key base of ecomonic development in Xinjiang.Rapid population and agricultural growth have been achieved through the heavy utilization groundwater during the past fifty years.Large-scale groundwater exploitation has caused many eco-environmental problems,such as soil salinization,land subsidence,and groundwater quality degradation.Understanding the various groundwater flow pathways from recharge to discharge is critical for sustainable utilization of groundwater resources and maintaining ecosystems.Based on the geological and hydrogeological conditions,the groundwater flow patterns under natural condition and human activities have been analyzed by using hydrogeochemistry,environmental isotopes,Bromide tracer tests and numerical simulation in the Manas River Basin.The major conclusions of this research are as follows.Three groundwater flow patterns?local,intermediate and regional flow patterns?are identified in the Manas River Basin under natural condition.In the piedmont plains,local groundwater flow systems contain modern water residence times younger than 50years.Groundwater velocities in this region are greater than 0.5 m/d.The groundwater is characterized by the type of HCO₃·SO₄-Ca with TDS lower than 400 mg/L.In the oasis plains,precipitation and evaporation are considered as recharge source and groundwater discharge,indicating that local flow systems operate.The groundwater residence times are between 50 and 180 a.The intermediate and regional flow systems contain groundwater residence times between 10 and 30 ka in the deep aquifers across the whole basin,with groundwater type from HCO₃·SO₄-Ca in the recharge area to Cl-Na in the discharge area.Among these two systems,groundwater velocities are lower than 0.05 m/d.Soil flow systems operate with irrigation water providing a highly evaporated recharge source under mulch drip irrigation in the oasis plains.Intermediate and regional flow systems have been changed into mixing flow patterns under human activities.Intense groundwater pumping has caused the groundwater to leak from shallow and deep aquifers to intermediate aquifers.This results in the mixing patterns of exploitation,irrigation and infiltration.The groundwater contains modern water and old water with groundwater residences between 5 and 10 ka.Groundwater recharge and evapotranspiration within soil flow systems have been determined by Bromide tracer tests and unsaturated model.Eight Bromide tracer tests have been carried out to calculate the evaporation rates in the Manas River Basin.The results show that the evaporation rates range from 0.02 to 0.71 mm/d.Compared to the evaporation rates of Bromide tracer tests and unsaturated model,the soil hydraulic parameters are reliable for the model construction under mulch drip irrigation.The results of thirty unsaturated flow models show that the evapotranspiration vary from500 to 750 mm/a,and the evapotranspiration during the period of cotton growth are between 460 and 650 mm/a.Groundwater recharge in the basin is between 5 and 38mm/a.The results of the coupling of unsaturated and saturated flow models show that the groundwater exploitation increased by 10%or 20%in the piedmont plains would result in the variation of water table decreased by 0.2 m/a.The groundwater exploitation in the Mosuowan Irrigation reduced by 20%would result in the variation of water table decreased by 0.3 m.In contrary,the groundwater exploitation increased by 10%would result in the variation of water table increased by 0.16 m/a.The impact of groundwater pumping on groundwater flow is significant in the oasis plains and oasis-desert transition zone.If the groundwater overexploitation or further expansion happened,ancient groundwater would be exploited,causing the water table to continuous decline.This would impact the development of agriculture and ecological environment in arid inland river basins.This research can help provide scientific bases for the protection of ecological environment and the utilization of groundwater resources in the arid inland river basin,Northwest China,and other similar semi-arid to arid,inland basins around the world.