Talk, International Workshop on Preferential flow and transport processes in soil, Monte Verità, Ascona, Switzerland: 2006-11-04 - 2006-11-09
Abstract:
Dye tracer experiments are often performed to study water flow and solute transport in soils (Flury et al. 1994; Ghodrati and Jury 1990). However, dye patterns are mostly used as qualitative proof for preferential flow. Schlather and Huwe (2005) proposed to quantify the risk of groundwater contamination by solutes using the form parameter ξ of the Pareto distribution. They called it risk index and estimated it by fitting the tail distribution 1-H to dye tracer distributions obtained form digital images of stained profiles, H being the generalised Pareto distribution: H(d,ξ,s)=1-(1+ξd/s), where s > 0, ξ € R and (1 + ξd/s) > 0. The parameter ξ determines the form of the distribution and therefore the contamination risk: 1. If ξ < 0: H has an upper end point; the dye tracer does not exceed a certain depth. 2. If ξ > 0: H has an infinite upper end point; the water table is surely reached. 3. If ξ = 0: H decreases exponentially; the water table is reached, but the transported mass might be negligible. In our study, we evaluated the behaviour of the risk index ξ. We performed five infiltration experiments at a forest site at different irrigation rates and initial soil moisture conditions. As tracers, we used Brilliant Blue dye and potassium iodide. The latter served as reference tracer and was visualised by a spray method applying Fe(NO3)3•9H2O and starch directly on the profile as described by Lu and Wu (2003). The excavated stained profiles were photographed and the pictures segmented by image analysis. The risk index was calculated with SoPhy (Schlather 2005), a contributed package to R, the free software environment for statistical computing (Ihaka and Gentleman 1996). We found that the risk index is not invariant to initial hydrological conditions. A successful estimation of the risk index needs a monotonically decreasing dye tracer distribution within the soil profile, in order to fit the Pareto distribution. This requirement was fulfilled for dry initial conditions but not for all moist profiles, in which case the estimation failed. However, the risk index showed some persistence against changes of irrigation intensity. References: Flury M, Flühler H, Jury WA, Leuenberger J (1994) Susceptibility of soils to preferential flow of water: a field study. Water Resour Res 30(7):1945-1954 Ghodrati M, Jury WA (1990) A field-study using dyes to characterize preferential flow of water. Soil Sci Soc Am J 54(6):1558-1563 Ihaka R, Gentleman R (1996) R: a language for data analysis and graphics. J Comput Graph Stat 5(3):299–314 Lu JH, Wu LS (2003) Visualizing bromide and iodide water tracer in soil proriles by spray methods. J Environ Qual 32(1):363-367 Schlather M (2005) SoPhy: some soil physics tools for R. URL: http://www.r-project.org/, contributed extension package Schlather M, Huwe B (2005) A risk index for characterising flow pattern in soils using dye tracer distributions. J Contam Hydrol 79(1-2):25-44