Hydro Lecture 3

Lecture 3: Baseflow Recession & Hydrograph

  • baseflow: groundwater moving into surface water
    • can occur over 3 to 7 months generally, does not usu occur all year

*ground water level CAN be higher than the stream stage, this happens near begining of baseflow
* after groundwater loses water to the stream, the water table drops, sometimes far lower than the stream elevation
* once water table is below stream stage, the water from the stream will drain into the water table
* if the flood stage and flood water table are higher than the baseflow water table, the stream will be recharged during this flooding

Hydrograph
hydrograph.gif

  • at the beginning of the baseflow recession, the baseflow will be the highest amount
  • starting point of the baseflow is located at the 1st drop (or second peak) after the wet season dropoff
    • there will always be a 2nd peak

Baseflow Equation
Q = Qo e -at

  • Q-knot comes from hydrograph
  • a depends on drainage basin, but can be found in data from past measurements
  • t you can figure out yourself, in days
  • will be used in hw 1 & 2

Determining Groundwater Recharge
Important to know so that we don't overpump groundwater resources.
The baseflow equation is aka Meyboom Method and assumes:

  • no dams
  • negligible snowmelt runoff contribution
  • have to use two or more consecutive years of records, usu 3 years

Approach for Estimating Groundwater Recharge

  1. gather your data (at least 2 yers of hydrograph data…)
  2. calculate Total Potential groundwater discharge for entire groundwater recession of the 2nd year of data
    • Basically, you want to know how much total water was available
    • Insert complicated equation here
  3. Calculate the discharge at end of recession of the first year
    • vt comes from previous step
    • vtp is the total potential baseflow of first year
  4. Last, groundwater recharge = V tp2 - V tp1 OR V tp - V t
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