**6.2 Computational structure**

16 Current Issues of Water Management

The TVA applies RiverWare in optimizing the daily and hourly operation of the TVA system of multiple-purpose reservoirs and hydroelectric power plants. The USBR has used RiverWare as a long-term planning model and mid-term operations model of the Colorado River as well as a daily operations model for both the Upper and Lower Colorado Regions. The USBR has also applied the model in the Rio Grande, Yakima, and Truckee River Basins. The USACE has recently sponsored addition of features to RiverWare for simulating flood control reservoir operations. Other entities have also applied the model in various river

ResSim, MODSIM, WRAP, RiverWare, and other similar models provide flexible capabilities for analyzing multiple-purpose river/reservoir system operations. The models are water accounting systems that compute reservoir storages and releases and stream flows for each time step of a specified hydrologic period-of-analysis for a particular scenario of water resources development, management, allocation, and use. Though fundamentally similar, ResSim, MODSIM, WRAP, and RiverWare differ significantly in their organizational structure, computational algorithms, user interfaces, and data management mechanisms. The alternative modeling systems provide general frameworks for constructing and applying models for specific systems of reservoirs and river reaches. Each of the generalized modeling systems is based upon its own set of modeling strategies and methods and has its

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

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

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

basins for various purposes.

own terminology or modeling language.

weekly time steps are more common.

their flexibility for modeling flood control.

**6.1 Types of applications** 

**6. Comparative summary of modeling capabilities** 

The term *ad hoc* in Table 2 refers to computational strategies developed specifically for a particular model, as contrasted with linear programming (LP) which is a generic algorithm incorporated in numerous models. ResSim and WRAP are organized based upon ad hoc model-specific computational frameworks. MODSIM is based on network flow LP. RiverWare has two alternative solution options based on ad hoc algorithms and a third option that uses LP. The LP-based models have additional ad hoc algorithms used along with their LP solver, but the LP solver accounts for a major portion of the computations.

Repetitive loops and iterative solution procedures are incorporated in all of the models. Iterative algorithms are required for evaporation and hydropower computations. Evaporation depends upon end-of-period storage, but end-of-period storage depends upon evaporation. Reservoir storage volume versus surface area and elevation relationships are nonlinear. In the LP models, the entire LP solution of the whole system is repeated iteratively. With the ad hoc simulation procedures, the computations for an individual reservoir are repeated iteratively.


Table 2. Alternative development frameworks

ResSim and RiverWare generally follow an upstream-to-downstream progression in considering requirements for reservoir storage and releases, diversions, and hydropower generation. WRAP and MODSIM simulation computations are governed by user-specified priorities in considering water management requirements. The WRAP and MODSIM priority-based frameworks are beneficial in modeling complex water allocation systems.

Generalized Models of River System Development and Management 19

Data management efficiency, effective communication of results, documentation, and easeof-use are important factors in applying a modeling system. Documentation includes both instructions for using the software and detailed technical documentation for understanding modeling methods. The software should be as near error-free as possible assuming absolutely error-free software may be an idealistic goal yet to be achieved. Dealing with errors introduced by users in model input data is important. The modeling systems contain various mechanisms for detecting and correcting blunders and inconsistencies in input data. The organizations and individuals that originally developed the four modeling systems continue to improve the models and support their application. ResSim, MODSIM, and WRAP software and manuals are available free-of-charge at the websites listed in Table 1. Licensing fees and training required to implement RiverWare are described at its website. The HEC periodically provides training courses in the application of HEC-ResSim. The

RiverWare is designed for Unix workstations but is also used on personal computers with Microsoft Windows. The other three modeling systems are usually executed on personal computers with Microsoft Windows but can also be applied with other computer systems.

The evolution of computer modeling of systems of rivers and reservoirs that began in the 1950's is still underway and is expected to continue. Modeling systems continue to grow in response to advances in computer technology and intensifying water management and associated decision-support needs. The published literature on modeling reservoir systems is massive and complex. ResSim, MODSIM, WRAP, RiverWare, and other similar models, though continuing to be improved and expanded, are well established and significantly contributing to water management in the United States and throughout the world. These generalized modeling systems are readily available for application by water management

Generalized modeling systems reflect the types of applications that motivated their development. ResSim serves as the reservoir system operations component of the Corps Water Management System implemented in the USACE district offices nationwide to support real-time operations of multiple-purposes reservoirs and flood control and navigation projects. ResSim is also used in USACE planning studies. RiverWare was developed as a partnership between CADSWES and the USBR and TVA. The TVA uses ResSim to support real-time hydroelectric power system operations within the setting of multiple-purpose reservoir system operations. The USBR applies RiverWare for both longterm planning and short-term operational planning for its multiple-purpose reservoir systems. The network flow programming based MODSIM was developed at Colorado State University in collaboration with the USBR and has been applied primarily by university researchers in studies both in the United States and abroad. WRAP supports statewide and regional planning and water allocation regulatory activities in Texas that require detailed modeling of diverse and complex institutional water allocation arrangements and

ResSim, RiverWare, MODSIM, and WRAP provide general frameworks for constructing and applying models for specific systems of reservoirs and river reaches. Each of these four

TWRI periodically provides training courses in the application of WRAP.

professionals to river systems located anywhere in the world.

reservoir/river system management practices.

**7. Conclusions** 

RiverWare includes an optional prescriptive optimization feature that combines LP and goal programming. Computations are performed simultaneously for all the time intervals. Thus, model results show a set of reservoir storages and releases which minimize or maximize a defined objective function assuming all future stream flows are known as release decisions are made simultaneously during each period. With the exception of options for short-term flow forecasting, ResSim, MODSIM, WRAP, and the simulation options in RiverWare step through time performing computations at each individual time step. Thus, operating decisions are not affected by future inflows and future operating decisions.

Many other prescriptively oriented optimization models reported in the research literature, including the HEC-PRM Prescriptive Reservoir Model described earlier in this chapter, adopt the approach of optimizing an objective function while simultaneously considering all time steps of the entire hydrologic period-of-analysis. Thus, these models reflect perfect knowledge of future hydrology. Since the future is not known in the real-world, these models reflecting knowledge of the future provide an upper-limit scenario on what can be achieved. Descriptive simulation models are more realistic in that current operating decisions in the model are not affected by future hydrology and future operating decisions.
