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Functional Specification for the Planning Model The most fundamental level of output from the Planning Model will be the flows in the arcs of the network,. The flow of water through arcs of the network is governed by the flow capacities specified for each arc and the "rank" assigned to each arc, Arc capacities represent the upper and lower bound flow limits of the arc; the upper bound capacity of an arc typically represents a physical or legal limitation on flow through the arc. Arc rank is the unit value of flow in the arc and is used to portray water rights and operational priorities; the underlying moqel solution procedure allocates water flows to arcs with higher ranks before allocating it to arcs with lower ranks. Most model modifications will be m;lde by changing capacities and ranks of arcs in the network, although the model will allow the addition and deletion of arcs, The model will consist of an underlying network representation of the basin as described above and, "in front" of this network, a software interface to facilitate user modification of the network and specification of inputs and outputs for model runs. Both the underlying network structure and the nature of the user interface are described in detail in this Specification. ! Model Runs In broad terms, the model will take a set of inflows and carryover reservoir storage levels for a particular month and compute a corresponding allocation of that water throughout the basin network, inCluding diversions, consumptive use, return flows, evaporation from reservoirs, changes in reservoir storage levels and outflow from the Gunnison River to the Colorado River. Input to the model will include time-series data for inflows, inflow forecasts, and'demands reflecting historical climatic and hydrologic conditions for the 1952-1990 period, althpugh users will have the option of specifying the use of subsets of this time-series data, OutVut from the model will include the resulting allocations of water to demands, reservoir stor~ge, and streamflow throughout the basin. A model run, or "scenario", comprises both the inputs to the model and the resulting model outputs for the monthly time series selected by the user. Rules of administration and operation will not be permitted to change during a model run. The "Baseline" and User-Defined Scenarios . The Planning Model is intended to provide users both with the flexibility to specify situations they wish to explore and with a completely defined set of assumed conditions for the basin to use as a common starting point. The common starting point is referred to as the "Baseline," and any set of user-defined additions and modifications to the Baseline assumptions is referred to as a "scenario". The Baseline assumptions will be taken as the default in every scenario where a user has not specified a different value or configuration. In the following sections, there will be references both to the initial assumptions represented in the Baseline and to the kinds of changes that users will be able to make in those assumptions when specifying a ne~ scenario. The Baseline will approximate "present conditions" with respect to the level of development in the basin. Existing facilities and levels of demand for water will be included in the Baseline. For the most part, the operations of individual projectsin the Baseline will also follow current practices, e.g. the use of inflow forecasts and target storage levels. With respect to water rights administration, the Baseline will represent "textbook" administration of water rights in accordance with priority. A special, pre-defined scenario referred to as the "Historical Scenario" will also be included with the model to represent certain agreements and informal practices that are extensions or variations upon strict textbook administration of water rig~ts. 3 e e .