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I I I I I I I I I I I I I I I I I I I Total diverted volume in Table IV-2 accounts for all water diverted from the Poudre River into the canal. Records are kept on a continuous basis at the canal measuring flume located downstream of the main headgate. Water passing through the flume includes: direct diversion rights, reservoir calls from Fossil Creek Reservoir, exchanges with other irrigation companies, return flows, and excess storm flow. A breakdown of the individual components that account for the total diverted flow is not available. B. Firm Yield. The firm, or safe yield, of a canal is generally defined as "the maximum quantity of water which can be guaranteed during a critical dry period". The safe yield of diversion was not evaluated by analysis of the available flow of water in the Cache La Poudre River. It was evaluated only by review of the past history or record of flow into the main canal. From a review of Figures IV-2 through IV-7, it can be seen that the canal normally begins diverting water in May and reaches a peak in mid June, which corresponds to the peak runoff in the river. At the end of June, there is a substantial decline in the diverted flows. This typically starts the reservoir season of canal operation. Diverted flows during late July, August, and September are typically from release out of Fossil Creek Reservoir and from return flows and stormwater in the Poudre River. The amount of water that can be diverted is dependant on available flow in the Poudre. If direct runoff is available, water can be diverted by the Company on a reliable basis. However, during a drought period, the Company comes out of priority and cannot divert the desired flow rates. Over a 7-year period, the Poudre River has fluctuated in peak virgin river flows from a low of 1308 cfs in 1987 to a high of 3785 cfs in 1991. However, the peak diversion rate and the total amount of water diverted has increased between 1987 and 1992. This increase in total volume could be attributed to increased return flows entering the Poudre above the diversion structure. The Poudre River is very complicated in operation from the first diversion in the canyon to the NCLPIC diversion structure. Numerous irrigation diversions deliver water to the various irrigation companies. Many companies have agreements for exchanges whereby they divert at an upstream location and release it downstream. Also, return flows come from agricultural runoff as well as urban runoff from the developed areas in the City of Fort Collins. The safe yield of the canal could be considered to be the minimum diverted flow experienced by the canal for the year 1987, which is approximately 21,000 acre-feet. It is not certain what the safe yield would be with the present makeup of the system and could not be determined without analyzing the whole Cache La Poudre system. If the water storage record is studied for the years from 1987 to 1992, the results would indicate that a minimum diverted volume would yield approximately 21,000 acre-feet. PAGE - 5 - Chapter IV