This volume of the report outlines the purpose and scope of the return flow research and specifically explains the capabilities of the conjunctive use model for predicting the mineral quality of irrigation return flow. The purpose of the research was to develop a conjunctive use model which would (1) predict the salinity contribution from new irrigation projects and (2) predict the change in return flow salinity that would result from operational changes on existing projects. The model developed and described herein describes the chemical quality in terms of eight ionic constituents and total dissolved solids. A description of the Vernal Field Study which describes the physical setting for the model testing is included. A narrative describing the problems encountered with the original data is included. Tables and figures showing the computed-observed comparisons from the verification are included. Results of model operations for the Cedar Bluff and Grand Valley areas are also described. It is concluded that the model can satisfactorily be used to simulate irrigation return flows if sufficient data are available, especially groundwater hydrology and chemistry.

Prepared under contract no. EPA-IAG-D4-0371. Includes user's manuals, v. 3 and 5. Issued Aug. 1977. V. 5 prepared by Marvin J. Shaffer, Richard W. Ribbens, and Charlse W. Huntley. Includes bibliographies and appendices.

v. 1. Summary report and verification.--v. 2. Vernal field study.--v. 3. Simulation model of conjuctive use and water quality for a river system or basin.--v. 4. Data analysis utility programs.--v. 5. Detailed return flow salinity and nutrient simulation model.