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VNUF - 2016 Lecture #6 Runoff and stream flow Principles of Watershed management Dr Bui Xuan Dung- Department of Environment Management Stream or river flow measurement Q Streamflow • Streamflow, discharge, runoff all mean the same • Discharge is the most important info for the water resources manager • Peak flow data are needed for flood control planning • Low-flow data are needed for estimating the dependability of water supply • Total runoff needed for design purposes (e.g reservoir storage) Amount of runoff measured as flow, streamflow, discharge (all the same thing) Discharge • Q = Discharge (volume/time) = flow, streamflow, stream runoff • m3/s = 35.32 cfs • Large river discharge – 180 m3/s (6358 cfs) • Small stream flow = 0.01 m3/s = 10 L/s (0.35 cfs) Streamflow measurement • Stage (height of water) is measured readily at some point on a stream reach with staff gauge or water level logger (pressure transducer) Staff Gauge Streamflow measurement • Stage data are used for calculating discharge • Common methods that use stage data for discharge calcs are: • Stream gauging • Flumes and weirs • Empirical equations Parshall flume Field methods Equipment collection Rain gauge monitor precipitation Water stage data logger monitor water level at Parshall flume Q = k*Hn Where: H is water level; k, n are constants Parshall flume monitor runoff Tell me the dominant runoff mechanism: Runoff from convergent versus planar hillslopes Runoff: tussock (bunchgrass) vs pine (GH2) Watershed condition and discharge • As watershed area increases, discharge increases and lag time increases • Bigger watersheds have larger water storage capacity Importance of runoff pathways decreases with increasing basin size Water spends more time in the channel and less time getting to the channel; Channel processes dominate in basins larger than about 100 km2; How morphology of watershed affect runoff patterns? Watershed morphometry and discharge Max elevation: 30 m Min elevation: 10m Max elevation: 1000 m Min elevation: 20 m km km • Which basin would have a ‘flashier’ hydrograph? Why? Watershed morphometry and discharge • What is the primary difference between these two basins? • How much potential hydrograph differed ... Discharge (volume/time) = flow, streamflow, stream runoff • m3/s = 35. 32 cfs • Large river discharge – 180 m3/s (6358 cfs) • Small stream flow = 0.01 m3/s = 10 L/s (0.35 cfs) Streamflow measurement •.. .Stream or river flow measurement Q Streamflow • Streamflow, discharge, runoff all mean the same • Discharge is the most important info for the water resources manager • Peak flow data... Area of saturation overland flow grows during storm – Increasing area contributing runoff during storm Definitions Baseflow; Stormflow; Separation of baseflow and stormflow is an arbitrary