Boron transport within an agricultural field: Uniform flow versus mobile-immobile water model simulations

The transport of boron in soil is important to agriculture because boron concentrations in soil water are beneficial to plants only over a limited range (0.37 to 1.39 mmol L-1 for tolerant crops). Irrigation water in the San Joaquin Valley, California, commonly has elevated B concentrations, and soil water B can reach phytotoxic levels as a result of the concentrating effects of evapotranspiration. Because the constant capacitance model was successful in computing B speciation in soil water and on mineral surfaces, it was incorporated into a multicomponent solute transport code, and a 2-year field test of the model was performed for 43 sites within a 65-ha field in the San Joaquin Valley. The model predicted the adsorbed B (XOB(OH)(3)(-)) concentration successfully with a median scaled root mean square error (SRMSE) of 11% for 43 sites. The median SRMSE was 36% for prediction of total B and 46% for solution B. The higher SRMSE for solution B may be caused by lack of detail in specifying the lower boundary condition. A steady increase in SRMSE from east to west in the field, the same trend as the seven tile drains, suggests an unknown E-W systematic variation in the lower boundary condition. A mobile-immobile water transport model failed to exhibit significant improvement over the standard uniform flow model (UFM) and, thus, the simpler UFM was preferred. The change in total B mass at all sites generated was accurately predicted with a relative error of only 4.1%. This work has potential practical application in the study of the effect of water management practices on soil B.