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iii Co\) Co\) l\) ~ ABSTRACT The salinity added by irrigation return flows is a major problem in rivers draining agricultural lands throughout the arid regions of the world, and many irrigation water management alternatives have been proposed for reducing downstream salinity problems. The merits of these alternatives, however, can only be jud~ed from reliable information on their actual effects on the salinIty in rivers re- ceiving the drainage water and the water withdrawn from the river by downstream users. Hydrosalinity models are widely used to estimate these effects to guide the selection of a policy on management ot irrigation return flows. The purpose of this research was to assess the state-of~the-art of hydrosalinity modeling in order to develop a practical management tool for predicting how the salt outflow from irrigated agriculture is affected by various farm management prac- tices. A review of the state-of-the-art of hydrosalinity models identi- f ied one of the major gaps in modeling as inadequate understanding and representation of the quantity and quality interrelationships between sur face water, drainage water, and groundwater. Most models predict relatively constant levels of salinity over time in sur face drains during the irrigation season and an increase in concentration in similar drains at other locations during the nonirrigation season. The study also identified that a site specific equilibrium IIthreshold concentration" (TC) of dissolved solids can be adequately estimated and represented in a model. Salt concentration above the TC would result in precipitation of salts within the soil profile. Higher TC values would, however, exist in the unsaturated soil. Based on these new concepts, salinity in the return flows was modeled as a composite of indiVIdual component outflows from the unsaturated zones and the saturated groundwater zone, and represents the inter- relationships among surface water, drainage water, and groundwater. The -model termed BSAM-SALT was tested using field data from irrigated areas in Grand Valley, Colorado, and the Circleville sub- basin of the Sevier River Basin in Utah. A set of management rUns was made to demonstrate the utility of the model in predicting the salt loading caused by irrigated agriculture in the Grand Valley, Colorado, area.