QUANTIFICATION OF THE HETEROGENEITYIN WATER TRANSPORT THROUGH THEUNSATURATED ZONE OF SANDY SOILS USINGENVIRONMENTAL ISOTOPES

摘要

A new method, which combines mathematical modelling with environmentaland hydrological data, was investigated to estimate the heterogeneity of theunsaturated soils by separation of preferential and matrix flows, quantify both fluxesand determine their transit times. Finally, the transit time distribution functions wereused to construct vulnerability diagrams of different soils without plants. The modelcomplexity was simplified using a lumped parameter approach that combines an inputand output function of environmental tracer contents with hydraulic measurements.Assuming a two parallel flow-paths model the environmental deuterium ( 2 H) with itsseasonal variation in precipitation and in lysimeters outflow was taken to estimate themean transit times as well as the amount of preferential and matrix flow and enabled toquantify the heterogeneity of seven sandy lysimeters (L = 2 m, A = 0.125 m2) installedat the area of the GSF, Germany. The calculations were performed using weekly 2 Hcontents in precipitation and discharge during an eight year period. The fraction ofpreferential flow directly appearing in the outflow within one week, varied between17% and 30%. The amount was practically independent from the texture and flowrates. The crucial parameter influencing the fraction of preferential flow was foundto be the saturated hydraulic conductivity (K s ). The vulnerability diagrams yieldeddifferent patterns for all soil materials depending on the mean water content and thesaturated hydraulic conductivity. Coarser material with low mean water content andhigh K s showed a short mean transit time for the matrix flow (about 10 weeks) andmean preferential flow equal to or higher than 20%. Finer sand with lower K s andhigher mean water contents resulted in mean transit times of approximately 30 weeksand preferential flow of about 20%.