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<br />Basic Concepts <br /> <br />~. lor- ",.:'1' _"\.... <br />Inflow ' ;.' ~ .", oJ <br />. I, ~ <br />" <br /> <br />3.4 Inflow <br />Inflow at a station consists of: <br />Runoff or Local Flow <br />Flows from upstream <br />Return flows <br />Other gain <br />These components are more fully described below, <br /> <br />3.4.1 Runoff: Runoff is the incremental station gain (or loss). It is derived from the <br />Flow File by taking the pristine flow at this station and subtracting the pristine flow of each station <br />immediately upstream of this station. Conceptually, this is the precipitation and ground water accrual <br />(or loss) in the reach above the station. <br />Incremental runoff is not allowed at reservoir stations. See release notes, V3.01.1. <br />(Appendix F [Page 98]). <br />Runoff is somewhat ambiguous. If a station is at or immediately below the junction of two <br />streams, and there are stations on each branch of the stream above our station, then the runoff is sum <br />of the branch accruals and we have no idea what proportion of the gain or loss occurred in the <br />individual reaches. <br />This may be a problem for water quality studies where the runoff from different upstream <br />branches has significantly different concentrations of the pollutant under consideration. In that case, <br />the only way to mitigate the problem is to put intermediate stations immediately above the stream <br />junction and have someone estimate the split between the two upstream reaches; HYDROSS has no <br />rational way to determine the split. <br /> <br />3.4,2 Local Flow: Local flow replaces the incremental runoff. <br />Local flow is a new feature in HYDROSS V3.0. A pointer to a local flow table (col 28-31 <br />on the ST A card) tells the program to ignore the incremental runoff at this station and use the local <br />flow data instead. <br />Flow data is still required at the station when you use local flow, since the flow data is used <br />in deriving not only the runoff at this station, but also the next station downstream. <br />Local flow enters the system as natural water. <br />The motivation for local flow is the case where the "headwater" of the study area is below a <br />regulating dam. In this case the "runoff' to the model is affected by upstream regulation, and the <br />pristine flows are not the correct values for the model. <br />Local flows may be negative, just as the incremental runoff calculated from the Flow File <br />may also be negative. <br />Local flows are not allowed at reservoir stations. See release notes for V3.01.1 (Appendix F <br />[Page 98]). <br /> <br />3.4.3 Flows From Unstream: These are natural and project outflows from upstream <br /> <br />stations , <br /> <br />3.4.4 Return Flows: Return flows are flows from diversions or reach losses which <br />are now returning to the stream. Return flows may appear as natural or project water, as specified by <br />coI. 5 on the RFL card. <br /> <br />March 25, 1991 <br /> <br />HYDROSS 4.1 <br /> <br />Page 17 <br />