**6.1 Types of applications**

Water development purposes are a key consideration in formulating a modeling approach. The distinction between flood control and conservation purposes such as hydroelectric power and water supply is particularly important. Hydrologic analyses of floods focus on storm events, and analyses of droughts are long-term time series oriented. Modeling flow attenuation is important for flood control. Evaporation is important for conservation operations. Flood control operations are typically modeled using a daily or smaller time step. Conservation operations are sometimes modeled with a daily interval, but monthly or weekly time steps are more common.

All four of the alternative modeling systems are designed to simulate flood control, hydropower, water supply, environmental flows, and other reservoir management purposes. However, whereas development of the other three models was motivated primarily by conservation purposes, ResSim is flood control oriented. ResSim is limited to daily or shorter time steps and provides greater flexibility for flood routing and simulating flood control operations. RiverWare and WRAP have been recently expanded to increase their flexibility for modeling flood control.

In addition to the basic water accounting computations, the modeling systems include various optional features for reliability and frequency analyses, economic evaluations, water quality, and surface/groundwater interactions. These features may involve either computations performed during the simulation or additional post-simulation computations performed using simulation results. WRAP has particularly comprehensive options for reliability and frequency analyses. The relative priorities represented by the objective function coefficients in MODSIM and the RiverWare LP option may optionally be economic costs or benefits. MODSIM and WRAP simulate salinity. RiverWare options include various water quality constituents. Groundwater sources and channel losses are included in the models. Surface/ground water interactions have been approximated in various ways. MODSIM has a groundwater routine, and has been linked with the U.S. Geological Survey MODFLOW groundwater model.

System analysis models are often categorized as being prescriptive or descriptive. With the exception of the optimization option in RiverWare, the four models are essentially descriptive simulation models that demonstrate what will happen if a specified plan is adopted. Prescriptive optimization models automatically determine the plan that will best satisfy the decision criteria. Although it may be desirable for models to be as prescriptive as possible, real-world complexities of reservoir system operations typically necessitate model orientation toward the more descriptive end of the descriptive/prescriptive spectrum.
