Development of Procedures to Evaluate Salinity Management Strategies in Irrigation Return Flows

The salinity added 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 judged from reliable information on their actual effects on the salinity in rivers receiving the drainage water and the water withdrawn from the river by downstream users.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 of 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 practices. A review of the state-of-the-art of hydrosalinity models identified one of the major gaps in modeling as inadequate understanding and representation of the quantity and quality interrelationships between surface water, drainage water, and groundwater. Most models predict relatively constant levels of salinity over time in surface drains during the irrigation season and an increase in concentration in similar drains at other locations during the nonirrigation season. The study also identified taht a site specific equilibrium threshold conentration (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 interrelationships 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 managment 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