River basin network model for integrated water quantity/quality management

The MODSIM river basin network flow model is extended to directly incorpora te constraints on concentrations of conservative water quality constituents . The extended model MODSIMQ is linked with the U.S. Environmental Protecti on Agency's QUAL2E streamflow water quality model and a model for estimatin g quality of irrigation return flows. An iterative procedure based on the F rank-Wolfe nonlinear programming algorithm links MODSIMQ and the water qual ity models to assure convergence to solutions satisfying water right priori ties, while attempting to maintain minimum water quality requirements. Irri gation return flows, canal seepage, reservoir seepage, deep percolation, an d river depletion due to groundwater pumping are modeled using stream deple tion factors from the U.S. Geological Survey. Application of MODSIMQ to the lower Arkansas River basin in Colorado successfully models the complex leg al and administrative issues under Colorado water law and the Arkansas Rive r Compact, including the many water exchange mechanisms governing use of of f-stream reservoirs in the basin. Model calibration exercises conducted for the case study area confirm that MODSIMQ reasonably reproduces both histor ical flows and salinity levels for the calibration period. Results from var ious management scenarios indicate that appropriate conjunctive use of surf ace and groundwater can simultaneously satisfy water demands for users whil e enhancing control of salinization.