Unit hydrographs Flashcards
What does a unit hydrograph do?
Allows forecasters to predict the timing and magnitude of peak flow
What dies a unit hydrograph measure?
It predicts the discharge response to one unit of precipitation.
Why can you only build unit hydrographs over small basins?
Because they are more likely to have uniform precipitation coverage which is what you need to model the hydrograph
How do you build a unit hydrograph with guaged data?
- Pick a storm that has a single peak, plot it’s hydrograph
- then remove baseflow.
How do we find the amount of water that’s been contributed from the storm in our unit hydrograph? (steps 3 and 4 of building unit hydrograph). What do we convert that volume too?
- Add up all the discharge amounts for each hour to get the total discharge outside of the baseflow
- Convert into effective precipitation by dividing the total discharge by the basin area .
What is the 5th step in building a unit hydrograph?
We take the discharge values (how much water came per hour) and divide that by the effective precipitation to see how much discharge we get per unit of effective precipitation.
What is the 6th step in building a unit hydrograph?
By determining the duration of the effective rainfall, do this by taking all the rainfall subtracting it by the effective rainfall and then seeing the amount of hours it was above that value.
Is storm duration very long? How do we mitigate short storm durations?
No it’s not so to extends short storm durations we can seperate rainfall events into two events and then combine them and divide it by two to get the unit hydrograph
What do you if the precipitation distribution is not uniform in your unit hydrograph? What’s the issue with this method?
break the basin up into sub-basins. Then for each of those sub-basins you’re going to need to derive a UH, issue is that you need a lot of gauges at the basin
How do you separate the baseflow from the unit hydrograph?
Usually see the steady state before and after the event, if not that use a straight line from the minimum flow, a concave line hitting a point in the falling limb, or use stable isotopes
What is an issue with the unit hydrographs regarding rainfall loss?
That it assumes the infiltration rates are constant but they’re not, a storage of water can be there as well
What are the three major assumptions of unit hydrographs?
Two storms with equal duration and equal rainfall excess produce the same hydrograph, regardless of intensity.
The amount of discharge is assumed to be directly proportional to amount of excess rainfall: if I get twice as much excess rain I should get twice as much discharge.
The timing of runoff is the same, regardless of antecedent conditions.
If we have a unit hydrograph for a 6 hour storm duration with 1 mm of effective precipitation, how can we determine the unit hydrograph for a 2.4 mm effective precipitation?
Just multiply all values by 2.4
What are synthetic unit hydrographs?
These are unit hydrographs we can estimate based on knowing the geomorphology of these basins and seeing how those effect the unit hydrograph shape
