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Step 3: Structures Represented in Previous Modeling Efforts <br />In Task 3 of the RGDSS surface water modeling effort, previous efforts to model surface and <br />groundwater resources in the San Luis Valley were evaluated and documented (see Task 3.2 <br />Memoranda: Review Previous Modeling Efforts in the San Luis Valley). An objective of the <br />RGDSS study was to explicitly represent in the Rio Grande Surface Water Model all diversion <br />and reservoir structures that have been explicitly represented in previous models. As such, a list <br />of structures that have been represented in previous models was developed and compared to the <br />list developed from Step 2. Any structures represented in previous models that were not found on <br />the Step 21ist were then added, and will be explicitly represented in the Rio Grande model as key <br />structures. <br />Step 4: Information from Basin Interviews <br />Between February and May, 1999, numerous interviews were held with water users and water <br />managers in the Rio Grande Basin to review the preliminary key structure list and to discuss, <br />among other things, the most important diversion structures with respect to the volume of water <br />diverted and the administration of water rights. The information gathered during these interviews <br />was documented in several memoranda and was used to refine the list of key structures. <br />Interviews with Division 3 and district Water Commissioners provided information concerning <br />day to day administration of water rights. Commissioners identified specific structures within <br />their District that should be considered key because they play an important role in water <br />allocation. For example, Commissioners identified important "swing" ditches or priorities. <br />These discussions also led to the identification of several structures that should not be considered <br />key because they have been abandoned, have transferred their water rights or have their rights <br />carried in another structure. In addition, water commissioners identified structures located on <br />drainage and seepage ditches that should not be considered key because they are not tributary to <br />the mainstem and are therefore not administered. <br />Step 5: Final Selection Process <br />The list of structures that met one or more of the defining criteria was used to develop a <br />preliminary model network diagram for the Rio Grande. This process required establishing the <br />geographic location (water source) of each surface diversion structure, reservoir and instream <br />flow water right. This was accomplished using straight line diagrams provided by Division 3, <br />decreed headgate locations, and personal communications with Commissioners and others with <br />familiarity of the systems. Once locations were established, several rivers and streams were <br />identified that supplied only one or two structures that met the key criteria or were located on <br />minor tributaries with no stream flow records. Structures on these relatively small tributaries <br />(approximately 35 structures) were estimated to divert, on the average, significantly less than <br />their total decreed water right. Therefore, they were removed from the list of key structures <br />(modeled explicitly), and are expected to be modeled in an aggregated fashion along with other <br />water rights in a later modeling phase. <br />Final Key Structures List <br />The results of applying the evaluation criteria, discussions with Water Commissioners, and <br />applying several additional refinement criteria to all surface diversion structures are shown in <br />Attachment 1 for each of the eight Water Districts in Division 3. The Attachment indicates <br />C:Acdss\Task6-2.doc Finalize Key Structures July 27, 1999 -Page 3 of 4 <br />