River basin hydrosalinity modeling
ABSTRACT Tanji, K.K., (1981). River basin hydrosalinity modeling. Agric. Water Manage., 1981. Hydrologic-salinity modeling at the river sub-basin and basin spatial scales in the Western U.S.A. are evaluated. In the past two decades, significant advances have been made in basin hydrosalinity modeling. These models are providing increased understanding of the complexity of water and salt flows in large-scale systems. Most models, however, have been developed for specific problems and do not lend themselves for application to a wide variety of salinity problems. Much more work is needed in the areas of field verification of models and increasing their utility for management and policy-making decisions.
Evaluation of model complexity and space–time resolution on the prediction of long‐term soil salinity dynamics, western San Joaquin Valley, California
The numerical simulation of long‐term large‐scale (field to regional) variably saturated subsurface flow and transport remains a computational challenge, even with today's computing power. Therefore, it is appropriate to develop and use simplified models that focus on the main processes operating at the pertinent time and space scales, as long as the error introduced by the simpler model is small relative to the uncertainties associated with the spatial and temporal variation of boundary conditions and parameter values. This study investigates the effects of various model simplifications on the prediction of long‐term soil salinity and salt transport in irrigated soils. Average root‐zone salinity and cumulative annual drainage salt load were predicted for a 10‐year period using a one‐dimensional numerical flow and transport model (i.e. UNSATCHEM) that accounts for solute advection, dispersion and diffusion, and complex salt chemistry. The model uses daily values for rainfall, irrigation, and potential evapotranspiration rates. Model simulations consist of benchmark scenarios for different hypothetical cases that include shallow and deep water tables, different leaching fractions and soil gypsum content, and shallow groundwater salinity, with and without soil chemical reactions. These hypothetical benchmark simulations are compared with the results of various model simplifications that considered (i) annual average boundary conditions, (ii) coarser spatial discretization, and (iii) reducing the complexity of the salt‐soil reaction system. Based on the 10‐year simulation results, we conclude that salt transport modelling does not require daily boundary conditions, a fine spatial resolution, or complex salt chemistry. Instead, if the focus is on long‐term salinity, then a simplified modelling approach can be used, using annually averaged boundary conditions, a coarse spatial discretization, and inclusion of soil chemistry that only accounts for cation exchange and gypsum dissolution–precipitation. We also demonstrate that prediction errors due to these model simplifications may be small, when compared with effects of parameter uncertainty on model predictions. The proposed model simplifications lead to larger time steps and reduced computer simulation times by a factor of 1000. Copyright © 2006 John Wiley & Sons, Ltd.
Theory of the chemical properties of soil colloidal systems at equilibrium
The theories of model soil colloids are developed in the present work. The principal
 point of departure is thermodynamics, and in the first section, an effort is made
 to present a consistent system of chemical thermodynamics and pertinent related physical
 chemistry. The second section deals with the Donnan theory and some consequences of
 it which may have relevance for soil systems. Electrochemical measurements are also
 discussed. In the third section, the Gouy theory of the electrical double layer is
 reviewed and the theory is used to calculate ionic activities. In the fourth section,
 ion-exchange equations based on various models are developed. The significance of
 different models for soil systems is analyzed in relation to the available data. In
 the fifth section, a discussion is presented of the thermodynamics of soil moisture.
 The emphasis throughout is on the development of theory, and no effort has been made
 to make historical or literature surveys.