HYDRUS-1D Computer Session

Transcription

1 HYDRUS-1D Computer Session Nonequilibrium Water Flow and Solute Transport In this computer session with HYDRUS-1D we demonstrate the capability of HYDRUS-1D to simulate nonequilibrium water flow and solute transport using the dual-porosity model. The dual-porosity model is demonstrated using the ponded infiltration into a -cm deep soil profile. The soil hydraulic parameters of the macropore (mobile) domain are taken as follows: θ r =., θ s =., α=.41 cm -1, n=1.964, l=.5, K s =.722 cm s -1, while the (immobile) matrix domain is assumed to have a saturated water content, θ sim, of.15. Initial conditions are set equal to a pressure head of 1 cm. We assume that water mass transfer is proportional to the gradient of effective saturations in the two domains, with the mass transfer constant ω set at.1 s -1. For simplicity we consider only convective solute mass transfer between the two pore regions (i.e. no diffusive transfer), with the dispersivity again fixed at 2 cm. Results Discussion: While for ponded surface conditions water in the fracture domain quickly reached full saturation (see the figure below), the water content of the matrix increased only gradually with time. Consequently, the total water content, defined as the sum of the water contents of both the fracture and matrix domains, also increased only gradually. The total water content would be the quantity measured with most field water content measurement devices, such as a TDR or neutron probe. Pressure head measurements using tensiometers are, on the other hand, often dominated by the wetter fracture domain that reaches equilibrium relatively quickly. The dual-porosity model can therefore explain often observed nonequilibrium between pressure heads and water contents. Similar nonequilibrium profiles as for the water content are also obtained for the solute concentration (see the modeling results). References: Šimůnek, J., N. J. Jarvis, M. Th. van Genuchten, and A. Gärdenäs, Review and comparison of models for describing non-equilibrium and preferential flow and transport in the vadose zone, Journal of Hydrology, 272, 14-35, 3. Šimůnek, J., M. Šejna, H. Saito, M. Sakai, and M. Th. van Genuchten, The HYDRUS-1D Software Package for Simulating the Movement of Water, Heat, and Multiple Solutes in Variably Saturated Media, Version 4., HYDRUS Software Series 3, Department of Environmental Sciences, University of California Riverside, Riverside, California, USA, pp. 315, 8. 1

2 t = 18 s t = 3 s t = s t = 7 s Mobile Water Content [-] Fracture Domain t = 18 s t = 3 s t = s t = 7 s Immobile Water Content [-] Matrix Domain t = 18 s t = 3 s t = s t = 7 s Total Water Content [-] Both Domains t = 18 s t = 3 s t = s t = 7 s Mass Transfer [1/s] Water content profiles in the fracture (mobile) domain (top left), the matrix (immobile) domain (top right), and both domains combined (bottom left), as well as the water mass transfer term (bottom right) as calculated using the dual-porosity model. 2

3 Nonequilibrium Water Flow and Solute Transport Project Manager Button "New" Name: Nonequil Description: Nonequilibrium Water Flow and Solute Transport Button "Open" Main Processes Heading: Nonequilibrium Water Flow and Solute Transport Check Box: Water Flow Check Box: Solute Transport Radio Button: General Solute Transport Geometry Information Length Units: cm Number of Soil Materials: 1 Decline from Vertical Axes: 1 Depth of the Soil Profile: cm Time Information Time Units: Seconds Final Time: 7 Initial Time Step:.5 Minimum Time Step:.1 Maximum Time Step: Check Time-Variable Boundary Conditions Number of Time-Variable Boundary Records: 1 Print Information Check T-Level Information, Every n time steps: 1 Check Print at Regular Time Interval, Time Interval: Check Screen Output Check Press Enter at the End Number of Print Times: 4 Button "Select Print Times" Print Times: Water Flow Iteration Criteria Water Content Tolerance:.1 3

4 Lower Time Step Multiplication Factor: 1.1 Upper Time Step Multiplication Factor:.8 Water Flow Soil Hydraulic Model Radio button Dual-porosity (mobile-immobile water content mass transfer) Radio button - No hysteresis Water Flow Soil Hydraulic Parameters Residual water content in the mobile zone, Qr =. Saturated water content in the mobile zone, Qs =. Alpha =.41 n = Ks =.722 l =.5 Residual water content in the immobile zone, QrIm = Saturated water content in the immobile zone, QsIm =.15 Mass transfer coefficient, Omega = 1.e-5 Water Flow Boundary Conditions Upper Boundary Condition: Variable Pressure Head Lower Boundary Condition: Free Drainage Initial Conditions: In the Pressure Head Solute Transport General Information Leave default values, except for Radio Button: Dual-Porosity (Mobile-Immobile Water) Model (Physical Nonequilibrium) Solute Transport - Solute Transport Parameters Leave default values for tracer, except Bulk Density = 1.4 cm 3 /g Disp. = 2 cm Frac = 1 (fraction of sorption sites at equilibrium with the solution) ThImob = (immobile water content) Solute Transport - Transport and Reaction Parameters Leave default values for tracer Solute Transport Boundary Conditions 4

5 Upper Boundary Condition: Concentration Flux BC Lower Boundary Condition: Zero Concentration Gradient Time-Variable Boundary Conditions Time htop [cm] ctop cbot HYDRUS-1D Guide: Do you want to run Profile Application Profile Information Graphical Editor Conditions->Profile Discretization (or from the tool bar) Click the Number command from the Edit Bar and specify 61 nodes. Conditions->Initial Conditions->Pressure Head (or from the tool bar) Button "Edit condition" Select with the Mouse the entire soil profile Specify initial water content of -1 cm Include observation points at,, and cm Save and Exit Execute HYDRUS-1D OUTPUT: Observation Points Profile Information Mass Balance Information 5