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Link transfer thresholds Stimulus-response model Event driven simulation ' Internal control RSS Concepts and Melhodology ,i"'''' -;'. J 1.8 Data Tolerance Checking: When a link object is requested to transfer information from one object to another, such as a required inflow from a downstream object's inflow data slot to a release data slot from an upstream object, it transfers new values into those slots, It the difference between the existing and new values is less than a pre-defined toler- ance threshold, this difference is ignored. Clearly, decreasing tolerance levels could resun in increasing the number of simulation operations in each simulation time step. Tolerance checking is performed by the links. 1,9 Simulation Control: The simulation control of a model in RSS works in a stimulus-response mode, This implies that objects only 'do" something if their internal state is such that they can reset that internal state in accordance with informati.on required from other objects, Thus, input objects, objects which are only responsible for providing data to other objects, never have to wait for information from other objects, whereas output objects always need to wait for information provided to them. Water objects, on the other hand, get messages in the form of water, demands, and other parameters fed by other objects, upon which they mayor may not decide that their internal state needs to be reset. In each time step of a simulation run, whenever a water Object'S data changes, the object attempts to perform its mass balance (Equation 1) and other operations, If any of the slots involved in this operation are linked to slots in other objects, this in turn will change the data of one or more other objects, When this happens, these other objects, in turn, attempt to perform their operations, The process continues until there are no more data changes to be made, This event-driven simulation procedure is usu_ally initiated by the trimsfer of hydrologic flow data from time-series input tables to various water objects at the beginning of each simula- tion time step, In addition, in each simulation time step numerous bookkeeping operations are performed by run control functions or methods. Users of RSS do not need to know how the run controllers work or what bookkeeping activities these run controllers perform. This information is important to programmers, and is contained in the Programmer's Reference Manual. Users, however, will find it helpful to under- stand the interactions and operations that may take place among all the objects in the simulation model. These procedures will be discussed in more detail in the following paragraphs. 10 RSS Concepts and Methodotogy