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002043 . 2. INPUT DATA AND DEFINITIONS 2.1 System Definition - Node and Element Concepts The configuration of the river system is defined by means of element data cards (Type 3-1)*. Only tree-type branching systems are permissible, resulting in a single downstream boundary. Model nodes which correspond to an exact geographical location on the river are first located. They are chosen to coincide with existing gaging stations, to locate points at which model output is desired, to delimit portions of the river system such as reservoirs or river reaches, and to subdivide lengths of the river to obtain the desired resolution for water use inputs. Nodes are consecu- tively numbered using integer values starting at 1. However, there is no restriction on their order: Node 1 can be downstream from Node 20 and upstream from Node 7. . Elements are defined as that portion of the river system between geographically adjacent nodes. The only exception is for upstream boundary elements which have no upstream node. Consequently, nodes represent a single location and elements embody the dimen- sions of length or area. By convention, elements assume the number of the downstream node. All inputs for an element must reference this element number. Five types of elements are allowed, with the type specified by the numeric code ITYPE on the element data Card 3-1. They are: ITYPE Description 1 2 3 4 5 Upstream boundary Downstream boundary River reach Junction Reservoir . Both water and salt are introduced to the system at the upstream boundary elements. These generally provide the main driving force or input to the system. Desired flows at the downstream boundary are specified as target values which mayor may not be realized. River reach elements are assumed to have no storage properties (surface or bank) and merely route flows from the upstream to downstream nodes encompassing the element. Reservoir elements provide for storage of water (and salt) and may be operated in a variety of ways as specified on input. Both evaporation and bank storage are included. The confluence of several river branches is accomplished through use of the junction elements. * See footnote for Section 4.5.