River basin network model for integrated water quantity/quality management

The MODSIM river basin network flow model is extended to directly incorporate constraints on concentrations of conservative water quality constituents. The extended model MODSIMQ is linked with the US Environmental Protection Agency's QUAL2E streamflow water quality model and a model for estimating quality of irrigation return flows. An iterative procedure based on the Frank–Wolfe nonlinear programming algorithm links MODSIMQ and the water quality models to assure convergence to solutions satisfying water right priorities, while attempting to maintain minimum water quality requirements. Irrigation return flows, canal seepage, reservoir seepage, deep percolation, and river depletion due to groundwater pumping are modeled using stream depletion factors from the US Geological Survey. Application of MODSIMQ to the lower Arkansas River basin in Colorado successfully models the complex legal and administrative issues under Colorado water law and the Arkansas River Compact, including the many water exchange mechanisms governing use of off-stream reservoirs in the basin. Model calibration exercises conducted for the case study area confirm that MODSIMQ reasonably reproduces both historical flows and salinity levels for the calibration period. Results from various management scenarios indicate that appropriate conjunctive use of surface and groundwater can simultaneously satisfy water demands for users while enhancing control of salinization.