The direct routing computation is a method based on accounting for storage, incremental with time; time is incremented, inflow at the current time multiplied by the time interval is added to storage, storage and the basin geometry are used to compute a water elevation, the elevation and the structure data are used to compute a discharge value, that discharge value is then subtracted from storage, and time is incremented again, etc. The program runs each hydrograph until there is no inflow, and no storage, or a time limit is reached.
The direct modeling above has the advantage of making discharge values easy to manually verify. However, if the time increment is too big, and the incremental volume creates a large disparity in storage and discharge between iterations, accuracy suffers. Therefore, the program computation stops and gives an error message if the incremental volume is too large, and a smaller time increment should be used.
In cases where outlet structures give very large incremental flows, such as with large weirs, the output may also be skewed. This is usually evident by large fluctuations in the routing curves. In such cases the storage indication method should be used..
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