Baseflow Separation Techniques First assumption is that the direct precipitation and interflow components are inconsequential (but this should be re-evaluated in extreme situations) Overland flow is assumed to end at some fixed time after the storm peak.
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First assumption is that the direct precipitation and interflow components are inconsequential (but this should be re-evaluated in extreme situations)
Overland flow is assumed to end at some fixed time after the storm peak
Fetter: “graphical separation techniques are convenient fiction”
Isotope budgets and tracers can often help improve accuracy..
Gaining (effluent) stream - baseflow entering stream
- typical in humid regions
- as you move down stream, more water in stream even though no tributaries exist
Losing (influent) stream
- water table lower than bottom of stream channel
- water loss as you go down stream
- rate of loss is a function of the depth of water and hydraulic conductivity of the underlying alluvium
- In some cases (mountainous arid regions), you start with a gaining stream and move into a losing stream..
- During baseflow recession a stream may be gaining, but become a losing stream during floods
- ground-water pumping near a stream can drop the water table locally and cause a section of stream to be losing, while it is gaining up and downstream
Understanding of baseflow recession is necessary before we can look at hydrograph separation
- baseflow of a stream decreases during a dry period because as ground water flows into the stream the water table falls
Baseflow recession equation is:
where Q is the flow at some time t after recession has started
Q0 is the flow at the start of the recession
a is a recession constant for the basin
t is the time since recession began
e is base of natural logs
-plotting Q vs t on semilog paper should yield a straight line (with t on the linear scale)
- If more than one straight line apparent, there may be two groundwater sources
- in most watersheds groundwater depletion characteristics are ~ stable since they closely match watershed geology..
- If the amount of baseflow in the reservoir is calculated at the end of a recession and then the beginning of the next recession the amount of recharge can be obtained by the difference
- hence the amount of baseflow remaining at any time after baseflow recession begins is:
- the above assumes no consumptive use during the time period of interest..
Recharge is calculated as:
T1 = 45 days for this chart
- rational equation assumes constant rainfall and infiltration rate
- best used for small (200 acres or less) watersheds
Q is peak runoff rate
I is average rainfall intensity
A is the drainage area
C is a runoff coefficient (gotten from a table)
Lower range of C is used for low intensity storms
Higher range for high intensity storms..
- often want to know how often a stream flows at a lesser or greater discharge than some value
- duration curves usually daily or annual flow
1. Rank flow records (m) starting with 1 for highest flow and n for lowest over the period of interest (if two are equal, they each get their own rank...no ties)
2. The probability (P) that a given flow will be equaled or exceeded is given by:
3. If comparing multiple rivers reduce Q to discharge per unit area of basin (e.g. m3/s/km2
4. On probability paper (or in spreadsheet) Plot Q as Y-axis and P as x-axis
- distribution of runoff is caused by geology of drainage basin
- steeper curves have thinner soils, lower hydraulic conductivity, less overall baseflow..
2. Thick sand deposits
3. Glacial till with silt and clay
4. Multiply each original hydrograph by 1 over value in obtained in 3.
6. Plot several unit hydrographs for similar duration rains in this way
7. Construct composite unit hydrograph: take peak as average in both x and y, and adjust until area under curve is 1" of runoff
With say a 2.5 hour unit hydrograph can then take a forecast precipitation of say 2" and just double the height of your 1" inch unit hydrograph to come up with prediction of stream response to forecast storm.
- urbanization generally increases total quickflow for a given rainfall
- faster time to peak (lower time of concentration) and higher peak
- lower rates of ground water recharge in area of urban centers
- serious in areas where ground-water is big portion of supply..