PREFACEAlthough much progress has been made in the development ofregional grOlmdwater models and river basin simulation models, previous attempts at linking these two types of models into a workable conjunctive use decision support system for use in comprehensive river basin planning, management, and administration, have not been successful. With recent advances in computer hardware and software technology such as geographic information systems (GIS) and data base management system technology (DBMS), it is now possible to develop a computer based river basin decision support system for improved conjunctive use management ofgroundwater and surface water by linking a finite difference groundwater flow model with a river basin network model.A microcomputer based decision support system is presented for conjunctive stream-aquifer management mder prior appropriation This has been accomplished through a synthesis of existing technology ratherthan development ofnew models. The computer-aided design and drafting package, AUTOCAD, and a powerful, low-cost, raster GIS package for PC's called IDRISI, are used for preparing and processing grid-based spatial data. These data are processed for input into MODRSP, amodified version of1he USGS three-dimensional finite difference groundwater model, MODFLOW, to generate numerical groundwater response coefficients for considering distributive aquifer characteristics and realistic aquifer boundary conditions. These response coefficients are provided as input to the generalized river basin network mode~MODSIM, to simulate spatially varied and timelagged return/depletion flows from stream-aquifer interaction. The integration of GIS, DBMS, MODFLOW, and MODSIM allows analysis ofconjunctive use plans capable ofconsidering decreed flow and storage rights, river calls, exchanges, trades, and plans for augmentation. The groundwater hydrologic components provided with MODSIM include reservoir seepage, irrigation infiltration, well pumping, channel loss, channel routing, return flows, river depletion due to pumping, and aquifer storage.To demonstrate the capabilities ofthe Stream Aquifer Management Decision Support System (SAMDSS), a case study is presented for a portion ofthe Lower South Platte River Basin, Colorado. A 370 by 140 groundwater grid network (l000 ft x 1000 ft cell) was prepared for the case study area using GIS techniques. Groundwater response coefficients were generated using MODRSP for the 200 wells and over 30 recharge sites ofthe Bijou Irrigation Company groundwater augmentation plan. The water right return/depletion flow account for the Bijou augmentation plan was simulated using MODSIM. A separate MODSIM network was set up for a 70 mile section ofthe Lower South Platte River, Colorado, between the Kersey and Balzac river gage stations, under administrative control of State Engineer's Water District #1, to simulate daily administration of a river regulated under prior appropriation water right laws. The river system network model, which included 11 existing or proposed res...