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Last modified
9/26/2011 8:33:25 AM
Creation date
7/2/2008 8:51:24 AM
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Decision Support Systems
Title
RGDSS Task 4 - Basin Interviews
Description
Memo documenting interview with DWR, Division 3.
Decision Support - Doc Type
Task Memorandum
Date
6/28/1999
DSS Category
Surface Water
DSS
Rio Grande
Basin
Rio Grande
Contract/PO #
C153863
Grant Type
Non-Reimbursable
Bill Number
HB98-1189, SB99-173
Prepared By
Leonard Rice Engineering
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total flows over this same period estimated in Step 1. The curtailment percentage <br />is then adjusted up or down using engineering judgement to account for ungaged <br />gains and losses and administered in each river basin to meet the Compact <br />obligation by reducing the amount of water available for diversion in Colorado. <br />November 1St through December 31St <br />Both the Rio Grande and the Conejos River are typically curtailed 100 percent, <br />i.e. no diversions are allowed except for storage by pre-Compact reservoirs. <br />Exceptions may allow diversions under winter recharge programs on the Rio <br />Grande and for late season irrigation if Compact obligations are expected to be <br />satisfied. <br />Because the Compact is river specific, this process for determining curtailment percentages <br />occurs independently for both the Rio Grande and Conejos Rivers and different curtailment <br />percentages typically result for the two systems. It is important to note this process relies heavily <br />on forecasted inflows. Actual flows are not, and cannot be, known until the end of the year. <br />While Division 3 attempts to match the delivery requirement on an annual basis, over (credit) and <br />under (debit) deliveries can and do result from inaccuracies associated with inflow forecasts and <br />uncertainties associated with natural stream systems. These credits or debits then carry forward <br />and are considered in the following year's curtailment calculations. <br />The forecasted inflows that are relied upon by the Division Engineer are developed and provided <br />by the National Resources Conservation Service (NRCS) in cooperation with the National <br />Weather Service. These forecasts are published monthly, typically beginning in January and <br />ending in May or June. Because Division 3 administers the Compact (i.e. adjusts curtailment <br />calculations) every 10 days, there is often a need for more up-to-date information. Division 3 <br />regularly has discussions with the NRCS concerning trends and intermediate forecasts if <br />available prior to the release of updated monthly forecasts. <br />As previously described, the effect of applying a curtailment to the Rio Grande and Conejos <br />River is to make a percentage of the water flowing past the Index gages unavailable for diversions <br />such that it can be delivered at the state-line. As curtailment information is developed during the <br />irrigation season, the calculated percentages are communicated to the appropriate Water <br />Commissioners, who use this data in their water rights administration. Compact administration <br />primarily effects those water rights that divert from the Rio Grande, Conejos River, Rio San <br />Antonio, and Rio Los Pinos (Water District 20 and 22). Normal surface flows on creeks in Water <br />Districts 21 and 35 (including the Alamosa River, La Jara Creek and Trinchera Creek) are not <br />subject to Compact administration. <br />Reservoir Storage, Trans-Basin Diversions, and Compact Accounting <br />Most reservoirs within the Rio Grande Basin in Colorado were constructed prior to signing and <br />ratification of the Rio Grande Compact. As such, storage and releases by these reservoirs are not <br />reflected in the Compact accounting performed by the State of Colorado. By contrast, reservoirs <br />constructed after 1939 ("post-Compact" reservoirs) are subject to special Compact restrictions on <br />when they can store water and require adjustments to observed flows at several stream flow gages <br />during the accounting procedures. For example, operations at Platoro Reservoir, which is the <br />largest post-Compact reservoir in the Basin, affect the flows in the Conejos River at the Mogote <br />Index Gage. Observed flows at the Mogote gage must therefore be adjusted (upward when the <br />C:\cdss\Div3_Mem2.doc Division 3 Interview June 28, 1999 -Page 7 of 8 <br />
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