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The following paragraphs give definitions of essential terms used in this manual.

Water Resources System, System Load, System Element

The term water resources system includes all water-related transport and storage processes within a limited area, whereas it is irrelevant if it is a real-world system, or it represents a potential future or a planning state. The water-related processes are integrated into a model as individual components or elements. The simulation of water resources systems requires an abstract representation of the real-world processes as mathematical equations to carry out the calculation of hydrological and hydraulic processes. In other words, the system should perform the abstraction and mapping of the spatial and temporal distribution of water. To completely determine a water resources system, the definition of system boundaries is necessary. These boundaries are not only of spatial nature due to catchment area boundaries, but they are also a distinction between system loads and system results. The system loads - water supply and water demand - affect the system from the outside and trigger processes within the system, i.e. they do not directly belong to the system itself. It is assumed that there is no feedback between the system and system load. However, this assumption becomes less and less valid the more a water management system interferes with the water balance. Consequently, a water resources system is the sum of components or elements, which mathematically represent the water-related processes. The representation of the flow relationships between the elements is also part of a water resources system. Depending on the respective objective, a multitude of spatial resolutions can be achieved. Considering all processes, taking place in water management systems, is neither meaningful nor possible. Generally, it is advised to record all relevant processes and to represent them as accurately as necessary. Sometimes, this requires the abstraction and combination of different transport and storage processes. Integrating several processes into the system as one combined element results in a representation of reality through individual calculation units. These units will be called system elements in the following. A system element always delivers the same results facing the same conditions. System elements undergo a classification, which will be explained later on. The size and structure of a system element are determined either by geography, water management processes, or by both. For example, a storage is delimited by its storage space and the structure itself, with all comprised processes influencing each other. For this reason, operating facilities such as spillways, bottom discharge, and operating discharge are part of the system element storage. Geography and water resource processes are thus responsible for the design of the system element storage.

System Data, System States, Parameters

System data includes all values necessary to describe the system elements and their flow relations (arrangement of the system elements, parameters, and characteristics). Using system data, system loads generate system states and resulting system responses. System states describe momentary conditions within the system and are variable over time. States and responses are assigned to individual system elements. The terms parameter and characteristics have different meanings. Characteristics are determinable features of system elements, e.g., the geometry of a pipeline or a dam. For the simulation, they are constant unless being the subject of an investigation themselves. Parameters are also characteristics of system elements, but their unambiguous determination cannot be achieved sufficiently by measurement. They can only be measured at points but are related to larger areas (e.g., kf-value of soils) or represent a multitude of natural processes, e.g., a retention constant to describe the effluent concentration from a catchment area. They are subject to calibration and verification. To determine the behavior of system elements and the overall system, it is thus necessary to know characteristics and parameters.

Variable System, Uses, Storage Operation

Systems are variable systems if transport and storage processes can be influenced by operating control elements such as slides, gates, weirs, or valves.

Control elements interfere with natural flow patterns and therefore are not an end in itself, but sometimes rather inevitable to meet the demands placed on the water. Uses among others are:

• Water supply / service water usage
• Flood protection
• Preservation of minimum water volumes
• Low water elevation
• Irrigation
• Energy generation
• Recreational use

The approach of operating a storage or regulating elements to directly or indirectly facilitate or influence a use is called storage operation. For each use, the optimal condition is expressed as a target and the targets of several uses might be competing. E.g., to safely operate a water supply from a storage, it is optimal to store as much water as possible. However, to use the storage as a flood protection measure, an empty storage is optimal. Models can help to optimize the system states considering competing uses. A well-adjusted balance is gained by defining the storage operation.

Simulation Model, Storage Operating Model

A simulation model is an abstraction of reality. It provides the calculation of system elements and their mutual dependencies at given system loads. Thereby, system behavior is determined by calculating all relevant hydrological and hydraulic processes. If it is a variable system and the model can represent the artificial interventions in the discharge processes, it becomes an operational model. If interventions on the water balance are carried out via storages, the operating model becomes a storage operating model. A description of physical processes, such as the outflow from openings - which corresponds to the uncontrolled discharge from a bottom outlet - does not yet constitute a storage operating model.

Operating Plan, Operating Rule

To regulate water management systems, operating rules are necessary, which, depending on present system states, define the extent of interference in transport and storage processes of water. The sum of these instructions is called an operating plan. In Germany, there are some synonyms for this term, i.a., operating rules, water management plan, or just management plan. In this wiki, the term operating plan is used.

The operating plan usually consists of several individual regulations. In the context of operation plans, individual regulations are called operation rules.

Operating plans are available in different forms differing in their complexity and temporal extent. In most cases, there are operating rules with a long-term or medium-term period of validity, i.e., they are optimized so that the needs are satisfied in the long run, whereas short-term disadvantages for individual uses may occur. Such operating plans are usually determined using long periods that include as many different system loads as possible. Short-term operating plans - so-called real-time control - are, on the other hand, adjusted to individual events. Once this event is over, the short-term plan loses its validity.