| Water evaporation as the first stage of the hydrological cycle is one of the main causes for water isotope fractionation.In recent 20 years,owning to the rapid development of the isotope analytical techniques and high precision of soil pore water extraction methods,the soil water movement features and groundwater recharge are widely estimated by using the stable isotopes.However,there are still some deficiencies concerning the water isotope fractionation and groundwater recharge though comprehensive reviews have been obtained:?1?The evaporation fractionation factors affected by the salinity and meteorological factors in the North China Plain?NCP?are still ambiguous.?2?The mixing and isotope fractionation effects in the low water content soil do not draw sufficient attention,and the reason for the isotope fractionation is less studied.?3?The key factors causing diffuse flow and preferential flow in the unsaturated zone are inconclusive,especially in the central area of the NCP with different vegetation and soil texture land.More works have been conducted in this thesis in the Hengshui and Luancheng experiment stations in the NCP.The fractionation factors were obtained under the local meteorological conditions in the NCP.Soil water mixing and fractionation effects were studied by simulating the precipitation infiltration in the low water content soil columns.Subsurface-water flow pathways in different land-use and soil texture in the Hengshui experiment station were investigated using oxygen(18O)and hydrogen?2H?isotopes spatial and temporal variations in water samples of precipitation,soils,and groundwater.The groundwater recharge rates were calculated by comparing the depths of spike soil water ?18O values and the precipitation infiltration time.The precipitation infiltration characteristics were therefore analyzed.The main conclusions are as follows:?1?The experimental results showed an exponential relationship between ?2H and ?18O in residual water and f?the volume ratio between the residual water and the initial water?.The slopes between ?2H and ?18O in the residual surface water were 4.78 and 4.69 for the fresh water and saline water,respectively.The ratios between the ?2H and ?18O variations of the residual surface water and the evaporation were 2.68 ‰/cm and 0.56 ‰/cm,for the fresh water;and were 2.78 ‰/cm and 0.61 ‰/cm for the saline water during the experiment period.?2?The experimental results showed that soil water ?2H and ?18O values were much depleted than that in precipitation and decreased exponentially with increasing soil depth.The fractionation trend for the oven dry soil column experiment is straightly opposite to the kinetic Rayleigh model result?h=0%?in the ?2H-?18O diagram.The soil water ?2H and ?18O values are located nonlinear connection between the initial soil water and precipitation in the air dry soil column experiment,which indicate that both the mixing effect and fractionation effect occur during the precipitation infiltration.?3?The precipitation ?2H and ?18O values seasonal variations were not obviously observed.The precipitation amount was effect was significant in the rainy season.Soil water ?18O values revealed that diffuse flow was observed in the grass land,and preferential flow was observed in both the poplar land and maize land at a small scale?<5 km?.There was no necessary correlation between the inftiltration flow patterns and soil water content and precipitation intensity.?4?The annual mean natural precipitation recharge rate during the year of 1995 and 2006 in the Hengshui experiment station was 76.5 mm.The results also indicated that the fast precipitation infiltration in the rainy season domain the total recharge rate.In the Luancheng experiment station,the annual mean precipitation and irritation recharge rate during the year of 2001 and 2011 was 152.6 mm. |
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