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w w w::. CJ1 conditions and considering TDS as a conser- vative constituent. Neither the chemical reactions nor the precipitation/dissolution of salt within the soil profile is considered in these models, which generally operate on a monthly time increment. Models depicting chemical processes These models synthesize both hydrologic (water flow) and chemical (solute flow) CHAPTER 3, REVIEW OF HYDROSALINITY MODELS Introduction Recent advances in the state-of-the-art of hydrosalinity modeling have been signifi- cant as many computer simulation models have been developed to predict salinity in return flows. Walker (1978) and Fifield (1979) have provided preliminary evaluations of different categories of models. Lewis (1976) demon- strated an evaluation procedure by applica- tion to many water quality models, some of which include conservative constituents. The above evaluations, however, did not state the assumpt ions made in the representat ions of salt pickup phenomena in the reviewed models. Since irrigation waters usually undergo significant quality deterioration in passing through the soil profile, the capability of a model can be better understood by evaluating 1) the basic assumptions in the model, and 2) the representations of the various processes that occur within the soil profile, that affect the salinity status of the soil-water. The review of hydrosalinity models discussed in this section focuses on these assumptions and representations as well as the criteria listed in Table 3.1. Only deterministic models that portray the hydrology and chemistry of soil-water and groundwater regimes were reviewed. Available models predict water salinity (TDS and/or constituent ion) for specific situations and represent a wide range of capability and applicability. There exist many other models and vers ions of models, but those reviewed are representative of the range of those cur- rently available for hydrosalinity modeling. General Description of Models Two major categories of hydrosalinity models exist, namely, 1) the simple water and salt budget models and 2) the more complex models that also depict chemical processes within the soil-water system. Water and salt budget models Lane (1975) and Oixon (1978) provide an extensive discussion of these mass balance models. A brief description of character- istics of eight pertinent models is outlined in Table 3.2. Both one-dimensional instream models and two-dimensional river basin models have been developed assuming steady state Table 3.1. Criteria for evaluating salinity management models. 1. Model Capabilities Applicable situations Constituents rodeled 2. ~del Assumptions Wi thin root zone Wi thin the unsaturated zone below root zone Within the saturated groundwater zone 3, Salt Pick Up Methodology 4. Representation of Groundwater Salt Component 5. Data Requirements For lOOdel inputs Additional, for model verfication 6, ~del Costs Initiation costs Utilization costs 7. Model accuracy Representation of physical system Numerical accuracy Sensitivity to input errors Sensitivity of management options 8. Ease of Application Adequacy of available documentation Output fonn and content Updateability of data decks Mbdification of source decks 9. ~del Credibility Adapted research areas Adapted practical locat~ons 17