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<br />then be used to determine the flood control release to make a time <br />period T following a given index flow. <br /> <br />In order to understand this procedure, it is important to remember <br />that: <br /> <br />a. There is a delay, t, between the time that releases are made <br />and the time that they reach the damage center. It is therefore neces- <br />sary to match current releases with local flows expected to occur at <br />the damage center at a time t later. A refinement to the procedure <br />would be to account for routing effects by obtaining the translated local <br />flows by a reverse routing process. rather than pure lagging. <br />b. Maximum correlation between some index-station flows and local <br />flows is obtained if local flows are offset in time so that the peak <br />local and peak index flows coincide. If this offset time is greater <br />than t, there will be some advance warning time using index flows to <br />forecast local flows t time periods later. If this offset time is less <br />than t, maximum correlation between local and index flows must be sacri- <br />ficed in order to forecast early enough, so index flo.ls are correlated <br />with local flows that occur t later, even though higher correlation <br />could be obtained with some different offset. <br />Where records are not available for the index station or for com- <br />puting local runoff, an approximate relation can be obtained in the <br />same manner if balanced hypothetical floods for the index station and <br />local runoff were computed for several sizes of floods using procedures <br />described in Volume 5. An error allowance for forecasting local flow <br />from index flow should be added to the local flow. The amount would <br /> <br />2-06 <br />