Analysis of Field-scale Salinization: a Comparison of Different Modeling Approaches

Soil and groundwater salinization are major problems for irrigated agriculture in many arid and semiarid areas of the world. Studies addressing such problems require accurate estimation of salt loadings from irrigated areas through the vadose zone to underlying groundwater. We studied Cl transport in the vadose zone at 45 locations in a field in the San Joaquin Valley, California, through a combination of soil sampling at six depths (0–1.8 m) and numerical modeling using a coupled water flow and solute transport code (Unsatchem). Our purpose was to assess water and salt loadings from the heterogeneous field to groundwater over a two-year period, and to test applicability of the code to the data. Soil sampling in November, 1995, defined the initial water content and the Cl concentration, and the soil hydraulic properties. Four more sampling periods, ending in November 1997, provided data for evaluating model performance. Cl distributions in 1997 exhibited a variety of shapes including monotonically increasing or decreasing distributions versus depth, and profiles with maxima, minima or and sigmoidal shapes. The standard modeling approach, based on the Richards equation and the convection-dispersion equation, predicted more Cl leaching than was observed in the field. Somewhat improved predictions were obtained when the potential transpiration rate was increased by a factor of 1.5. Better leaching predictions were also obtained when the model included separate mobile and immobile water fractions, mostly by improving the profile shapes. Our study shows the importance of accurate descriptions of the lower boundary conditions, spatial variability in the water infiltration rate, and estimation of soil surface evaporation and transpiration rates.