Toward Improved Simulation of Operations in Integrated Hydrologic Modeling

Abstract:

Conjunctive management of groundwater and surface water (GW-SW) resources has received increased attention as the gap between freshwater supply and demand widens in arid and semi-arid regions. The modeling tools used by water managers and government agencies, among others, for distributing the available freshwater supply among a complex set of competing demands in a ‘prior appropriation’ context, rely on loosely-coupled (or ‘feed-forward’) model applications that fail to achieve true convergence among values common to both codes. Commonly, models used for quantifying supply, that is, watershed and groundwater models, are run separately from river operations models that specialize in distributing a finite supply among a wide range of demands. As a result, it may be difficult to achieve a proper water balance among the respective codes, especially during drought conditions. Taking a step closer to a truly integrated environmental modeling framework for conjunctive management of GW-SW resources, the river operations model MODSIM has been integrated with the hydrologic model MODFLOW. MODSIM is a river basin management decision support system that simulates administration of water rights and (or) operational rules within river basins. MODFLOW is a physically-based distributed-parameter finite-difference model historically used for simulating groundwater systems, though the streamflow routing (SFR2) and lake (LAK) packages are capable of simulating surface water systems in hydraulic connection with the underlying alluvial aquifer. On their own, these two packages cannot simulate diversions and (or) releases from reservoirs, instead requiring users to specify these quantities prior to model execution. Through the MODSIM-MODFLOW integration, however, hydrologic processes are simulated simultaneously with dynamic river operations. In this way, solutions are synchronized before moving to the next time step. The newly developed code provides water planners and managers with a more robust decision making support tool than is possible running either code independently or in a feed-forward coupling. Results illustrating the importance of the integration, over and against a feed-forward approach, are provided for a hypothetical test case while real-world applications continue to be developed.