Use of simulation-optimization modeling to assess regional ground-water systems

Abstract

Increasing—and often competing—demands on the Nation’s ground-water resources are creating a need for improved scientific information and analysis techniques to better understand and manage ground-water systems. Since the 1960s, numerical simulation models have been important tools for the assessment of ground-water flow systems and ground-water development strategies. Commonly, these models are used to test specific water-resource management plans, or, in a trialand-error approach, to select a single plan from a few alternative plans that best meets management goals and constraints. Because of the complex nature of ground-water systems, however, and the large number of engineering, legal, and economic factors that often affect ground-water development and management, the process of selecting a best operating procedure or policy can be extremely difficult. To address this difficulty, ground-water simulation models have been linked with optimization-modeling techniques to determine best (or optimal) management strategies from among many possible strategies. Optimization models explicitly account for water-resource management objectives and constraints, and have been referred to as management models (Ahlfeld and Mulligan, 2000). The use of combined simulation-optimization models greatly enhances the utility of simulation models alone by directly incorporating management goals and constraints into the modeling process (fig. 1). In the simulation-optimization approach, the modeler specifies the desired attributes of the hydrologic and water-resource management systems (such as minimum streamflow requirements or maximum allowed ground-waterlevel declines) and the model determines, from a set of several possible strategies, a single management strategy that best meets the desired attributes. In some cases, however, the model may determine that none of the possible strategies are able to meet the specific set of management goals and constraints. Such outcomes, while often not desirable, can be useful for identifying the hydrologic, hydrogeologic, and management variables that limit water-resource development and management options. Because of their usefulness for evaluating complex hydrogeologic and water-resource management systems, simulation-optimization models have been developed to assess various types of regional ground-water management problems, such as • Ground-water-level declines and aquifer-storage depletions