Definitions of terms

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Following is an explanation of central terms as they will be used in the course of this manual.

Water resources system, System load, System element

Under the term water resources system all water-related transport and storage processes within a delimited area are summarized, whereas it is irrelevant if the system actually exists, or represents a future or possible planning state. The water-related processes are summarized in individual components or elements.The simulation of such a system requires the transformation of the actual running processes (reality) into mathematical equations for the calculation of the hydrological and hydraulic processes. In other words, it is the abstraction and mapping of the spatial and temporal distribution of water. For the complete coverage of a water resources system, the definition of the boundaries is necessary. These boundaries are, on the one hand, of a purely spatial nature due to catchment area boundaries. On the other hand, a distinction between system load and system results. The system loads - water supply and water demand - affect the system from the outside and trigger processes in the system, i.e. they do not directly belong to the system itself. It is assumed that there is no feedback between system and system load. However, this assumption becomes less and less valid the more the water management system interferes with the water balance.Consequently, a system is the sum of components or elements which in turn 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, different spatial resolutions can be achieved. A consideration of all processes taking place in water management systems is neither meaningful nor possible. The principle is to record all relevant processes and to represent them as accurately as necessary. This requires the abstraction and combination of different transport and storage processes. This integration of several processes results in a representation of reality by means of individual calculation units. These units will be called system elements in the following. A system element always delivers the same results under the same conditions. A classification of the elements is done later.The size and structure of a system element is determined by geography, by water management processes or by both factors together. For example, a dam - storage basin - is delimited by the storage space and the structure itself, because all processes taking place in it influence each other. For this reason, operating facilities such as spillways, bottom and operating drains are part of the system element of a dam. Geography and water ressources processes are thus responsible for the design of the system element dam.

System data, system states, parameters

All values necessary for the description of the system elements and their flow relations are summarized under the term system data (arrangement of the system elements, parameters and characteristics). The system loads generate - using the system data - at the respective elements certain system states and resulting reactions. System states describe the momentary conditions within the system and are variable over time. States and reactions are clearly assigned to individual system elements. The terms parameter and characteristic values have different meanings. Characteristics are clearly determinable features of system elements, e.g. the geometry of a pipeline or the dam height of a dam. In the sense of simulation they are considered to be unchangeable, unless they are subject of an investigation. Parameters are also characteristics of system elements, but their unambiguous determination cannot be sufficiently achieved by measurement. These are quantities that can only be measured at points but are related to larger areas (e.g. kf-value of soils) or are representative for a multitude of natural processes, e.g. a retention constant for the description of the effluent concentration from a catchment area. They are subject to calibration and verification. The knowledge of characteristics and parameters is necessary to clearly describe the behavior of the system elements and thus the entire system behavior.

Regulable system, uses, storage operation

If the transport and storage processes are influenced by the operation of control elements such as slides, gates, weirs or valves, these are controllable systems. Such interventions in the natural flow behavior are not an end in itself, but rather to meet the demands placed on the water. Claims arise among other things with regard to

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

If such a claim or use is present in a water ressources system, there is generally also the possibility of intervening in the water balance in a regulating manner. In many cases reservoirs are suitable structures for intervening in the water balance due to their balancing effect and practical control possibilities. If there is a use that is directly or indirectly controlled or at least influenced by a reservoir or by means of regulating organs at the reservoir, this is called reservoir operation. For each use an optimal condition exists, which can usually be expressed in the form of a target. These goals are partly competing. For example, for a safe water supply from a reservoir it is optimal to keep as much water as possible. Exactly the opposite is true for flood protection, which requires an empty reservoir to absorb flood water. It is the task of storage operations to find a moderate balance between competing uses.

Simulation model, memory operating model

The characteristic of a simulation model is the abstraction of reality as well as the calculation of the system elements, and their mutual dependencies at given system loads. Thereby, a certain system behaviour is determined by the calculation of all relevant hydrological and hydraulic processes. If it is a regulable system and the model is able to represent the artificial interventions in the discharge processes, it becomes an operational model. If the interventions on the water balance are carried out via reservoirs, the operating model becomes the reservoir 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.

Operation plan, operation rule

For the regulation of water management systems, regulations are necessary to define the influence on the transport and storage processes of water, depending on certain system states. The sum of these instructions is called an operational plan. In Germany there are some synonyms for the term operation plan. Among others, the operating rule, water management plan, management plan are to be mentioned. In the further course of the work the term operating plan is used. This usually consists of several individual regulations. An individual regulation is called a business rule. Operational plans are available in different forms regarding their complexity and their temporal validity. In most cases, there are rules with a long-term or medium-term period of validity, i.e. they have been defined in such a way that the needs are satisfied as well as possible in the long run, whereas short-term disadvantages for individual uses may occur. Such operational plans are usually determined on the basis of long periods of time 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 the special and usually exceptional event is over, the short-term plan loses its validity.