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<br />001161 <br /> <br />24 <br /> <br />Demands <br /> <br />, <br />Demand arcs withdraw water from the network up LO an amount equal to' the user-supplied <br />capacitY of the demand arc. It is possible that less than this amount will be withdrawn if watcr is not <br />physically available at the demand point or if other demands having higher priorities call physically <br />ayailable water past the demand point. The priorities on demand arcs usually rellect the water rights <br />priorities associated with those demands. <br /> <br />Decrees <br /> <br />A decre~ is an arc which allows a finite amount of water to be delivered in each modeled year. <br />A common example of a decree is a reservoir with no refill rights. The reservoir may store its decreed <br />amount only once in a year, though this can be accomplished in one or over several months. Once the <br />decree capacity is fe,ached, no additional diversions through that arc are allowed. There arc nine <br />decrees in the Yampa Basin Model,'each representing a filling right to an existing or potential reservoir. <br /> <br />Un ks <br /> <br />Link arcs are used for a variety of purposes, the most common being representation of river <br />reaches or conveyance facilities. In these cases links do nOL have specific priorities but serve mainly to <br />impose capacity limitations on flow rates. River reaches, -of course, generally do not have capacity <br />limitations. <br /> <br />Links may be assigned ranks in order to selectively send water a preferred direction in the <br />network. In the Yampa model a rankcd link is used to rcprcsent instream Oow rights (Link 121 in the <br />network diagram). Link 121 has a monthly capacity dcfincd by the Junipcr coctcmplated draft (W. W. <br />Wheeler, 1989) and a priority higher than all the future dcmand incrcmcnts. Hence the model will <br />allocate water to this instream flo~ link rather than to junior (lower priority) demands up to the point <br />wherc the instream flow link is running at capacity. Whcn thc Oow in Link 121 reaches thc value of the <br />contcmplated draft for a givcn month, any additional water in cxccss of upstream demands will flow in <br />Link 117, an unrankcd link paralic I to Link 121. <br /> <br />Another important usc of link arcs is the dcfinition of rcscrvoir storage targets. Parallcllinks arc <br />used in the representation of reservoirs to show multiple storage decrees or to impose differential <br />priorities on various storage pools. For example, if a reservoir has a minimum recreation pool which <br />should always be prescrved, a link rcprescnting this targct would have a capacity equal to the dcsired <br />storagc volumc and a priority high cnough to induce allocation of watcr to that arc rather than to othcr <br />demand arcs elsewhere in the system that have lower pri,?ritics. <br /> <br />Reservoirs <br /> <br />A reservoir is a point where water may be stored in the system to be allocated in the next time <br />step. The simplest reservoir is represented by two links, a "stan" link representing the volume of water <br />carried over in the reservoir from the previous time step and an -"end" link representing the amount of <br />watcr left in the reservoir at thc end of thc currcnt period. Additional links can be uscd in parallel to <br />represent reservoir storage or release 'targets each with its own volume and priority. For example, if a <br />rescrvoir has a minimum pool which should bc protected from withdrawals, a link reprcscnting this <br />target would have a capacity equal to the dcsircd storage volumc and a priority high enough to allocate <br />watcr to that link instead of to other demand links with lower priorities. <br /> <br />~ : <br /> <br />11 <br />11 <br /> <br />(;~ <br />