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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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slide1

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

Fetter: “graphical separation techniques are convenient fiction”

Isotope budgets and tracers can often help improve accuracy..

slide2

Gaining and Losing streams

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..

slide5

- 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

slide6

Baseflow Recession

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..

slide9

Determining Baseflow Storage and Ground-water Recharge from Baseflow

  • Seasonal Recession Method
  • Assumes no dams or other regulation and minimal snowmelt
  • Need hydrographs from two or more consecutive years
  • using approximations derived from the baseflow recession equation we find:
  • Where: Vtp is the total potential volume of groundwater discharge (i.e. volume of water discharged during complete recession)
  • Qo is baseflow at the start of recession
    • t1 is the time for baseflow to go from q0 to 0.1q0..
slide10

- 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..

slide11

10,000

1000

cfs

100

10

0

  • Recession Curve displacement method
  • More suited to areas without strong seasonal dry periods
  • t1 is the time for recession to cover one log cycle
  • t1 needs to exceed D from equation
  • Where D = days between storm peak and
  • end of overland flow
  • A = basin area (mi2)
  • tc is a critical time which is 0.2144t1

Recharge is calculated as:

T1 = 45 days for this chart

slide12

Rainfall-Runoff Relationships

  • - basic goal is to predict amount of runoff that will occur from a given storm
  • - need to design structures and neighborhoods based on peak discharges
  • Rational Equation is simplest
  • if it rains long enough, peak Q from basin will be the average rate of rain times
  • the basin area (adjust by a coefficient to account for infiltration)
  • time of precip has to exceed time of concentration for rational equation to apply
  • - time of concentration is time necessary for water to flow from the most distant part of watershed to point of discharge
  • - conceptually time of concentration is the average velocity of the longest stream channel times the length of the channel, plus time for overland flow to reach the channel..
slide13

- 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..

slide14

Duration Curves

- 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

Steps are:

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..

slide15

Connect the stream with the

Basin character:

2. Thick sand deposits

3. Glacial till with silt and clay

slide16

Unit Hydrographs

  • - a unit hydrograph is the characteristic response of a given watershed to a unit volume (depth) of effective water input applied at a constant rate
  • - used to forecast response of a watershed to a given input of water
  • - hydrograph of direct runoff (excludes baseflow)
  • To develop unit hydrograph (1")
  • 1. Collect as much streamflow and precip data as possible
      • Best storms are: a) individual
  • b) uniform temporal and spatial distribution
      • c) rainfall duration should be ~10-30% of basin lag time
  • d) direct runoff should range from 0.5 to 1.75 inches
  • 2. For each storm, separate quickflow and baseflow
  • 3. Calculate depth of direct runoff (quickflow) per hour (from beginning of quickflow)..
slide17

4. Multiply each original hydrograph by 1 over value in obtained in 3.

5.

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.

Urban Hydrology

- 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..