SIMULTANEOUS INVERSE ESTIMATION OF SOIL HYDRAULIC AND SOLUTE TRANSPORT PARAMETERS FROM TRANSIENT FIELD EXPERIMENTS HOMOGENEOUS SOIL

Authors: ABBASI, FARIBORZ - LEUVEN, BELGIUM; SIMUNEK, JIRI - UC RIVERSIDE, CA; FEYEN, JAN - LEUVEN, BELGIUM; Van Genuchten, Martinus; SHOUSE, PETER - 5310-20-05

Submitted to: American Society of Agricultural Engineers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2003
Publication Date: 9/20/2003
Citation: Abbasi, F., Simunek, J., Feyen, J., Van Genuchten, M.T., Shouse, P.J. 2003. Simultaneous inverse estimation of soil hydraulic and solute transport parameters from transient field experiments homogeneous soil. American Society of Agricultural Engineers. 46(4):1085-1095.

Interpretive Summary: While considerable progress has been made during the past several decades in describing and modeling water flow and solute transport processes under controlled conditions in the laboratory, detailed analyses of field-scale experiments remain limited, mostly because of labor and cost requirements, but also because of complications posed by field-scale heterogeneity. For example, the soil hydraulic conductivity (permeability) can easily change by several orders of magnitude, even over relatively short distances, thus making it difficult to assign values that are applicable at the field scale. In this study, we estimated field-scale water flow and solute transport parameters from several two-dimensional furrow irrigation experiments. The saturated hydraulic conductivity (Ks) and solute transport parameters in two transport models were evaluated from the field data. We used for this purpose both a traditional transport model (CDE) which assumes uniform water and solute fronts in the soil during infiltration, and a more sophisticated model (MIM) that accounts for the presence of immobile water pockets (e.g., in soil aggregates). The latter model leads to faster transport of the solutes (preferential flow) since the solute front bypasses some parts of the soil. The parameters in the two models were estimated from observed soil water content readings, cumulative infiltration data, and solute concentrations. Estimated immobile water contents were more or less constant at a relatively low average value of 0.025, thus suggesting relatively little preferential flow during our experiments. Agreement between measured and predicted infiltration rates was satisfactory, whereas soil water contents were somewhat overestimated, and solute concentrations were underestimated with both models. Differences between predicted solute distributions obtained with the CDE and MIM transport models were relatively small. This and the value of optimized parameters indicate that the observed data were sufficiently well described using the simpler CDE model, and that preferential flow did not play a major role in the transport process. Results of this study are important to better understand how alternative irrigation management practices can affect water and solute distributions in furrow-irrigated soils.

Technical Abstract: Inverse estimation of unsaturated soil hydraulic and solute transport properties has thus far been limited mostly to analyses of one-dimensional experiments in the laboratory, often assuming steady-state conditions. This is partly because of the high cost and difficulties in accurately measuring and collecting adequate field-scale data sets, and partly because of difficulties in describing spatial and temporal variabilities in the soil hydraulic properties. In this study, we estimated soil hydraulic and solute transport parameters from several two-dimensional furrow irrigation experiments under transient conditions. Three blocked-end furrow irrigation experiments were carried out, each of the same duration but with different amounts of infiltrating water and solutes resulting from water depths of 6, 10, and 14 cm in the furrows. Two more experiments were carried out with the same amounts of applied water and solute, and hence for different durations, on furrows with water depths of 6 and 10 cm. The saturated hydraulic conductivity (Ks) and solute transport parameters in the physical equilibrium convection-dispersion (CDE) and physical nonequilibrium mobile immobile (MIM) transport models were inversely estimated using the Levenberg-Marquardt optimization algorithm in combination with the HYDRUS-2D numerical code. Soil water content readings, cumulative infiltration data, and solute concentrations were used in the objective function during the optimization process. Estimated Ks values ranged from 0.0389 to 0.0996 cm/min, with a coefficient of variation of 48 percent. Estimated immobile water contents were more or less constant at a relatively low average value of 0.025, whereas the first-order exchange coefficient varied between 0.10 and 19.52 min-1. The longitudinal dispersivity (DL) ranged from 2.6 to 32.8 cm, and the transverse dispersivity (DT) ranged from 0.03 to 2.20 cm. DL showed some dependency on water level and irrigation/ solute application time in the furrows, but no obvious effect was found on Ks and other transport parameters, most likely because of spatial variability in the soil hydraulic properties. Agreement between measured and predicted infiltration rates was satisfactory, whereas soil water contents were somewhat overestimated, and solute concentrations were underestimated. Differences between predicted solute distributions obtained with the CDE and MIM transport models were relatively small. This and the value of optimized parameters indicate that observed data were sufficiently well described using the simpler CDE model, and that immobile water did not play a major role in the transport process.