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HYDRUS_1D Sensitivity Analysis Limin Yang Department of Biological Engineering Sciences Washington State University

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INTRODUCTION 1. To find the parameters of greatest importance in water flow simulation in vadose zone. 2. To allow users to budget resources so that the most important parameters can be determined with the greatest accuracy.

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HYDRUS-1D HYDRUS-1D is a commercial software package based on finite element model, for simulating the one-dimensional movement of water and solute in variably saturated media. This program was developed by U.S. Salinity Laboratory, U.S. Department of Agriculture, Agriculture Research Service (Simunek and van Genuchten, 1998).

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Governing Equations (2)

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INTEC SITE

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Soil distributions at INTEC

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Soil Properties There are two major types of soil, sediment and basalt, at INTEC. The average surficial alluvium samples saturated hydraulic conductivity K s is 4.1x10 -2 cm/sec. The average interbeds’ K s is 1.221x10 -4 cm/sec. α is between 0.0001 and 1.9868. n is between 1.1024 and 4.2289. θ r is between 0 and 0.0764. θ s is between 0.2247 and 0.6049.

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Methods 1.The sensitivity of model results to any given parameter can be described by the partial derivative of an output variable with respect to that parameter. 2. The change in cumulative bottom flux was calculated as: Where, ΔR is percent change in the result value of the testing function R t is result value for using the test parameter value R b is result value for using the base parameter value

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Basic settings Only consider water flow; Only one type soil will be considered with a depth of 500 cm, and there is no incline from vertical axis; Totally 512 min with each time step 1e-5, and the maximum time step is 25 min; The maximum number of iteration is 50, with all other iteration criteria default;

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Basic settings (cont’d) Hydraulic model is van Genuchten without air-entry value and no hysteresis; Soil properties based on sand; Upper boundary condition is constant pressure head; Lower boundary condition is free drainage; Initial condition is in the pressure head (10 cm water head on the top, -100 cm water head for other part of soil).

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Table 1. Outline

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NUMERICAL RESULTS α This experimental parameter was introduced for expressing the relationship of soil water content and the pressure head. It will influence the shape of the retention curve. In most case, since α is a small number and with a power of n, which is bigger than 1, it should not be a sensitive factor. Experiments’ results proved this true as shown in Table 2. The cumulative bottom flux changes less than 3% while α changes 25%.

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Table 2. Results

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Residual water content θ r It is not strange that θ r has only very limited influence on the cumulative bottom flux, since θ r is too small comparing to θ s. The cumulative bottom flux changes less than 3.5% while θ r changes 25%. Saturated water content θ s θ s determines θ e, the effective water content, which is a critical factor for solving governing equation. It is sensitive parameter to the bottom flux causing about 60% change in flux while itself changes only 25%. Notably, there is a negative linear relationship between θ s and the bottom flux. This can also be explained because that it occurs in the formula of θ e as a denominator.

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n Empirical parameter n occurs in formula (2, 3) as a power of h. It is also less sensitive to the flux although its influence to the bottom flux is bigger than those of α and θ r. It causes at most 12% changes in flux while itself changes 25%. Saturated hydraulic conductivity K s K s is of most sensitivity in all the parameters, as a key factor of equation (1). It also has a linear relationship with the cumulative bottom flux with a slope of about 3.2, which means that one unit change in K s will cause 3.2 unit changes in flux. l Empirical factor l is almost fixed as 0.5 according Simunek, J., M. Sejna, and M. Th. van Genuchten. (1998). In this study, it is the least important factor to the flux.

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Discussion and conclusions Hysteresis is an important phenomenon in soil physics and will have influence on the ground water flow. But comparisons of the runs of HYDRUS_1D indicate it has no significant effect on the cumulative flow flux (in most cases no effect). From the results, K s and θ s are very sensitive parameters to the vadose zone flow. In reality, K s and θ s are localized and cannot easily get with respect to the limitations of field methods. These will greatly hamper the solution of vadose zone flow and in turn influence of solute transport.

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Thank You

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References Simunek, J., M. Sejna, and M. Th. van Genuchten. 1998. The HYDRUS_1D Software Package for Simulating the One-Dimensional Movement of Water, Heat, and Multiple Solutes in Variably-Saturated Media. Version 2.0. US Salinity Laboratory, ARS/USDA. Riverside, California. Hull, L.C. et al, 1999, Draft Work Plan for the Waste Area Group 3, Operable Unit 3- 14, Tank Farm Soil and Groundwater, Remedial Investigation/Feasibility Study. INEEL. Hull, L.C. et al, 2002, Phase I Monitoring Well and Tracer Study Report for Operable Unit 3-13, Group 4, Perched Water. DOE. Jacomino, V.M.F., Fields, D.E. “ A critical approach to the calibration of a watershed model. ” 1997. American Water Resources Association. 33 (1), 143-154 van Genuchten, M. Th. 1980. A Closed-Form Equation for Predicting the Hydraulic Conductivity of Unsaturated Soils. Soil Science Society American Journal. Vol. 44, pp 892-898.

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