|Van Genuchten, Martinus|
Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2003
Publication Date: 3/20/2004
Citation: Abbasi, F., Feyen, J., Van Genuchten, M.T. 2004. Two-dimensional simulation of water flow and solute transport below furrows: model calibration and validation. Journal of Hydrology. 290:63-79. Interpretive Summary: Numerical models are increasingly used for predicting or analyzing water flow and chemical transport in soils and groundwater. The governing flow and transport equations are now reasonably well established, while a large number of numerical solutions have been published during the last 30 years. Still, accurate simulation of field-scale processes subject to natural boundary conditions remains a challenge since many input parameters are difficult to measure at the desired scale. Recent studies indicate that a combination of numerical models with inverse parameter estimation algorithms and detailed measurement of different variables is a promising approach for process and parameter identification. The main objective of this study was to simultaneously estimate soil hydraulic and solute transport properties from a large-scale transient furrow irrigation experiment using parameter optimization in combination with the HYDRUS-2D computer software package. The optimized parameters were subsequently used to predict water flow and solute transport for three other furrow irrigation experiments conducted at the same scale in the same field. Calculations assumed the presence of an equivalent homogeneous soil profile. The optimized parameters were very similar to those found earlier for short furrows at the same field site. Minor differences were related to the presence of soil heterogeneity across the field and versus depth, different observation scales, and different water and solute applications (opportunity) times. Agreement between measured and predicted soil water contents was satisfactory, except for one of the sites. Observed solute concentrations displayed more variability and were not predicted as well as the soil water contents. One major conclusion of this study was that soil hydraulic properties should be estimated and used in model predictions at the same scale. Estimated soil hydraulic parameters from small soil cores in the laboratory may not adequately describe field-scale soil water status in view of considerable field-scale variability, uncertainty, and a number of experimental complications (especially air entrapment and macropore flow). Considering temporal variability in the soil hydraulic and transport parameters may well improve the model predictions also further. Results of this study are important to better understand and predict the effects of alternative water management practices on water and agrochemical transport in irrigated soils.
Technical Abstract: In this study a two-dimensional numerical flow/transport model (HYDRUS-2D) was calibrated and experimentally validated using data from long furrow irrigation experiments. The model was calibrated using data from an experiment carried out assuming free-draining (FD) outlet conditions, and subsequently validated against data from three experiments assuming blocked-end conditions. The data were analyzed using the Richards' equation for variably saturated flow and either the traditional convection-dispersion equation (CDE) or the physical nonequilibrium mobile-immobile (MIM) model for solute transport. Optimization was accomplished by means of Levenberg-Marquardt optimization in combination with the HYDRUS-2D model. Simultaneous and two-step optimization approaches were used to estimate the soil hydraulic and solute transport parameters near the FD furrow inlet and outlet sites. First, the saturated hydraulic conductivity (Ks) and CDE or MIM solute transport parameters were estimated simultaneously. We also used sequential (two-step) estimation in which we first estimated the soil hydraulic parameters followed by estimation of the solute transport parameters. In the two-step method, the saturated soil water content; the n parameter in van Genuchten's soil water retention model, and Ks values were estimated during the first step, and the CDE or MIM solute transport parameters during the second step. Estimated soil hydraulic and solute transport parameters were found to vary substantially between the inlet and outlet sites. Estimated CDE and MIM transport parameters were very similar for both optimization approaches. The two-step method significantly improved predictions of the soil water content during model calibration, while the solute concentration predictions were nearly the same for both approaches, with both not providing a good description of the observed concentrations. Solute data were also analyzed using horizontal averages to somewhat lessen the effects of spatial variability. Horizontally averaged concentration distributions showed better agreement with the predictions. Soil water contents for the three blocked-end experiments during model validation were well predicted. The two-step method produced slightly better agreement with observed data. However, both optimization approaches produced relatively poor agreement between measured and predicted solute concentrations and deep percolation rates.