From Mays 2011 Ground and Surface Water Hydrology A D hour unit hydrograph is the characteristic response of a given watershed to a unit volume eg 1 in or cm of effective water input usually rain applied at a constant rate for D hours ID: 1027201
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1. Rainfall – Runoff Analysis From Mays, 2011, Ground and Surface Water Hydrology
2. A D hour unit hydrograph is the characteristic response of a given watershed to a unit volume (e.g. 1 in or cm) of effective water input (usually rain) applied at a constant rate for D hoursRunoff (mm/hr)Flow (m3/s)TimeRunoff and Flow
3. Hydrologic Response from the unit HydrographTimePrecipitationInfiltration CapacityExcess PrecipitationExcess Precipitation
4. ExampleThe 1- hr unit hydrograph for a watershed is given below. Determine the runoff from this watershed for the storm pattern given. The abstractions have a constant rate of 0.3 in/ h.Time ( hr) 123456Precipitation ( in) 0.511.50.5Unit hydrograph ( cfs) 1010020015010050
5. Calculating a Hydrograph from a Unit Hydrograph...... for n=1 ... N From Mays, 2011, Ground and Surface Water Hydrology
6. AssumptionsExcess rainfall has constant intensity within effective durationExcess rainfall is uniformly distributed over watershedThe base time of the direct runoff hydrograph from an increment of excess rainfall is constantThe ordinates of all direct runoff hydrographs are proportional to the amount of direct runoffFor a given watershed the hydrograph resulting from a given excess rainfall reflects the unchanging characteristics of the watershed
7. ExampleDetermine the 1- hr unit hydrograph for a watershed using the precipitation pattern and runoff hydrograph below. The abstractions have a constant rate of 0.3 in/ hr, and the baseflow of the stream is 0 cfs.Time (h)12345678910Precipitation (in)0.511.50.5Runoff (cfs)22712229238530018580100