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t.l) t.l) t.l) ,.... the water with the two principal salts being CaC03 (calcium carbonate) and CaS04.2H20 (gypsum), 2) concentration of salts within the soil water as the water is lost by evapotranspiration, and 3) spatial and temporal dIfferences that impart spatial and temporal variability to the salt content of the soil water. CHAPTER 2, REVIEW OF PROCESSES CONTRIBUTING TO SALINITY IN RIVER SYSTEMS Introduction In order to quantify how the salinity management options would affect the stream flow quantity and quality in a particular river basin, the physical processes control- ling salt and water movement within the basin would have to be identified, evaluated, and represented in the management model. The model would have to represent these processes sufficiently well to indicate how they would respond to the range of alternatives. The following review of chemical processes that occur in soil water systems, mineral weathering in the groundwater system, and salt loading in streams, provides the theo- retical background for an overview of the available hydrosalinity models discussed later in this report. Processes occurring in the soil water system Levels of soil water salinity vary by location and over time in irrigated soils with differences in the quality, quantity, and application patterns of irrigation and natural waters and in' the chemical, geohydro- logic, and biological properties of the soil. Salt pickup from the soil varies with interrelated physical, chemical, and biologi- cal factors. The physical factors include the soil type, quantity of water percolating through the soil, travel path within the soil prof lIe, depth of groundwater table, and the hydraulic gradient causing the flow. The salt transport depends upon mass of flow, ionic diffusion, and dispersion in the soil. Chemical factors are the partial pressure of C02, complex inorganic and organIC chemical reactions involving ion exchange, ionic adsorption, dissolution and precipitation,. and formation of complex ion pairs changing the ionic strength of water and the concen- tration gradient. Biological activity depends on the extent and nature of micro- organisms and their substrate present in the soil. All in all, many physical processes interact in determining soil salinity levels, and the dominating ones vary greatly with local conditions. Generally, however, the three major processes are 1) precipitation of the salt content of the water within the soil or d i ssolut ion of the salts in the soi 1 by Precipitation of CaC03 and gypsum in the soil. Both the qualitatlve ana-quantIta- tIve aspects of the salt precipitation phenomena occurring within the soil profile under various conditions of leaching have been described (Willardson et a1. 1979 and Swarez and Rhoades 1977). Willardson et a1. (1979) showed that chemical precipitation occurs in the soil profile during cycles of evaporation and water additions which reduce the effluent salt content below what one would expect theoretically from the leaching fraction (LF), The data in Table 2.1 indicate that when irrigation water is applied, CaC03 and gypsum precipitate in the soil in varying amounts depending upon the leaching fraction, depth within the root zone, and the type of irri- gation water. The precipitation of salts in the root zone was less when the applied irrigation water was initially undersaturated with CaC03 as compared with the salt pre- c ipitation when the applied water was satu- rated. with CaC03, Swarez and Rhoades (1977) contend that salt is deposited in the soil with high leaching (LF=O.4) because CaC03 is dissolved in the upper layers and is deposited in the lower layers of the root zone, although there is no net deposition in the soi 1. Concentration of salts within the soil-~ter-system:---The-saIt-content-oI the soil-water system is concentrated through consumptive use by irrigated crops and phreatophytes. The amount of increase in concentrat ion depends on the LF, the type of soil, and the quality of the applied wa.ter (Table 2.4). Reduced leaching would: 1) increase precipitation of CaC03 and CaS04 in the soil, 2) reduce soil mineral weathering and dissolutions of salts previously de- posited in the soil, and 3) increase the amount of soluble salt in the soil profile because less salt would be returned in the dr ainage water. At high leaching fractions, salt is added to percolating waters, passing through 3