Quantification of the heterogeneity in water transport through the unsaturated zone of sandy soils using environmental isotopes

A new method, which combines mathematical modelling with environmental and hydrological data, was investigated to estimate the heterogeneity of the unsaturated soils by separation of preferential and matrix flows, quantify both fluxes and determine their transit times. Finally, the transit time distribution functions were used to construct vulnerability diagrams of different soils without plants. The model complexity was simplified using a lumped parameter approach that combines an input and output function of environmental tracer contents with hydraulic measurements. Assuming a two parallel flow-paths model the environmental deuterium (2H) with its seasonal variation in precipitation and in lysimeters outflow was taken to estimate the mean transit times as well as the amount of preferential and matrix flow and enabled to quantify the heterogeneity of seven sandy lysimeters (L = 2 m, A = 0.125 m2) installed at the area of the GSF, Germany. The calculations were performed using weekly 2H contents in precipitation and discharge during an eight year period. The fraction of preferential flow directly appearing in the outflow within one week, varied between 17% and 30%. The amount was practically independent from the texture and flow rates. The crucial parameter influencing the fraction of preferential flow was found to be the saturated hydraulic conductivity (Ks). The vulnerability diagrams yielded different patterns for all soil materials depending on the mean water content and the saturated hydraulic conductivity. Coarser material with low mean water content and high Ks showed a short mean transit time for the matrix flow (about 10 weeks) and mean preferential flow equal to or higher than 20%. Finer sand with lower Ks and higher mean water contents resulted in mean transit times of approximately 30 weeks and preferential flow of about 20%. (author)