Streamflow Observations and Statistics of the Streamflow

Streamflow observations at or near to the location of interest are the designer’s best index of how a watershed will behave under conditions existing in the watershed at the time of observation of the flow. These data serve the following purposes:
  • Calibration of statistical model. If available, long records of annual maximum streamflow permit
    flood frequency analysis
    and design flow determination.
  • Calibration and verification of conceptual model. Shorter records of runoff from individual floods permit calibration and verification of conceptual models of the rainfall to runoff transformation, if corresponding records of rainfall are available. In this process, model parameters are estimated, runoff from observed rainfall is computed, and the computed flows are compared to the observed. Parameters are adjusted if the fit is not acceptable.
  • Assessment of reasonableness of results. Records of annual maximum flows at a site for limited periods permit assessment of reasonableness of predicted design flows. For example, if a record of annual maximum flows for 12 years at a site includes six peaks that exceed the predicted 10% chance design flow, a designer can apply the
    binomial statistical distribution
    to determine that the probability is only 0.0005 that this could happen. This is so unlikely that it raises doubt about the estimated 10% chance design flow.