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Return flows to the system from irrigation, municipal use and other non-consumptive <br />uses of water <br />Given the relatively large size of the South Platte River basin, time of travel also represents a <br />significant factor in the variation of streamflows throughout the basin. <br />Historical streamflow data are a very important component of understanding and modeling of <br />surface and groundwater resources. Historical records are used to (1) establish the baseline <br />hydrology of the basin, (2) define the available surface water supply under a range of <br />hydrological conditions and flood/drought cycles, (3) provide boundary conditions for <br />groundwater modeling, and (4) calibrate water resources models. <br />Real-time streamflow data are used by the Division Engineers to quantify the flows throughout <br />the basin at any given time. DWR also maintains and operates a number of real-time satellite- <br />monitoring gages on major diversions in the South Platte basin. These diversion gages are used <br />in conjunction with the streamflow gages to more accurately administer water rights. Based on <br />both the flows and the priority of diverting rights, the Division Engineer administers the water <br />rights by identifying those rights that are junior in priority that must be curtailed in order to <br />satisfy rights more senior. Understanding the relationships between flows at various locations <br />along the river and the effect of curtailing water rights diversions on downstream water rights <br />enters into the administration decision making process. <br />The greater the coverage of streamflow data available, both in terms of spatial coverage and <br />period of record, the greater the understanding of the hydrologic system and the ability to <br />effectively administer water rights and model the operation of the surface water and groundwater <br />systems for purposes of managing South Platte water resources. <br />3.2.1 Description and Inventory of Available Data <br />HydroBase is a database that includes streamflow, diversion, reservoir and water rights records <br />that is available through the Division of Water Resources' (DWR) Colorado Decision Support <br />Systems (CDSS). Diversion, reservoir, water rights, and climate data also contained in <br />HydroBase are discussed in subsequent sections of this chapter. HydroBase currently includes <br />records through 2000, and is updated annually by DWR. Table B-1 in Appendix B presents a <br />summary of information for stream gages in the South Platte and North Platte (including the <br />Laramie) River basins. The gages are summarized first by Water District and second by station <br />name. The streamflow data in HydroBase are available on a daily basis. <br />3.2.2 Data Assessment <br />The streamflow data assembled in HydroBase are obtained from the U. S. Geological Survey <br />(USGS) and the DWR. Inspection of Table B-1 shows that some of the gages include surface <br />water diversions, return flows, transbasin diversions, storage facilities, or precipitation gages, <br />due to the querying processes used to extract streamflow data from HydroBase. The available <br />p:data\gen\spdss~final report\chapter 3.doc 3-2 <br />October 31, 2001 <br />