Stream Flow and Discharge Measurement

Stream Flow and Stream Water

Introduction to Stream Flow

  • Overview of the topic and structure of the lecture.
  • Two-part lecture intended to cover:
    • Basic components of a water budget.
    • Methods for quantifying and displaying stream discharge.
  • Importance of understanding stream flow for ecology and system functioning.

Hydrologic Cycle

  • Definition of the hydrologic cycle.
  • Key components of the hydrologic cycle:
    • Gaseous water (vapor)
    • Precipitation (rain and snow)
    • Stream flow component (focus of this lecture)
    • Surface water and groundwater.
  • Two source pathways for streamflow:
    1. Surface runoff (overland flow)
    2. Groundwater (shallow or deep)
  • Importance of precipitation as a driver of streamflow.

Evapotranspiration

  • Definition: Combination of evaporation and transpiration from plants.
  • Role of evapotranspiration in the water cycle and streamflow:
    • Not all precipitation directly contributes to streamflow.

Types of Flow into Streams

  • Description of different flows:
    • Overland flow:
    • Occurs with saturated soils or hydrophobic conditions.
    • Groundwater flow:
    • Variants include shallow groundwater (near surface) and deep groundwater (subsurface).
    • Direct input to streams (generally minimal).

Water Budget Equation

  • Components of water budget:
    • Water inputs:
    • Precipitation (P)
    • Groundwater (G)
    • Water outputs:
    • Evapotranspiration (ET)
    • Discharge (Q)
  • Equation representation:
    P + G = Q + ET
  • Definition of discharge (Q): Represents the volume of water moving through a system over time.
  • Concept of mass balance in watersheds:
    • All inputs must balance with all outputs in the system.

Characterizing Discharge (Q)

  • Units of measurement for volume and time:
    • Common volume units: liters (small systems), cubic meters (larger systems), cubic feet (used by USGS).
    • Time metrics: usually measured in seconds.
  • Flux definition:
    • Volume or mass per unit time.
    • Commonly expressed as cubic meters per second (m³/s) or liters per second (L/s).

Hydrographs

  • Definition of a hydrograph: graphical representation of stream discharge over time.
  • X-axis: time (could represent days, months, or years).
  • Y-axis: discharge (in cubic feet per second, or m³/s).
  • Description of temporal variation in discharge:
    • Definition of peak discharge and lag time relative to rainfall events.
    • Discussion of flashy systems:
    • Characterized by rapid increases in discharge following precipitation.

Measurement Methods

Direct Measurement in Streams

  • Directly measuring discharge involves measuring:
    • Velocity of water flow and channel morphology.
  • Conceptualization of stream cross section for calculations:
    • Breakdown of stream into small sections for total discharge estimation.
    • Measurements taken at various depths to ensure mean velocity is captured.
  • Measurement example:
    • At 60% of depth captures mean velocity effectively due to variations in flow speed across the vertical profile.

Solute Dilution Methods

  • Purpose: Estimate discharge when direct measurements are impractical.
  • Plateau Release Method:
    • A non-reactive tracer (e.g., salt) is introduced at a constant rate.
    • Measure concentration downstream to estimate discharge.
    • Example:
    • Inject 100 mg/L at 50 mL/min; after reaching plateau, concentration measured at 40 mg/L downstream leads to discharge calculations based on dilution.
    • Assumptions include constant discharge during the measurement period.
  • Pulse Release Method:
    • Introduce a surge of tracer into the stream instead of a constant rate.
    • Monitor concentration changes over time to estimate discharge.
    • Suitable for times with minimal discharge changes to ensure accuracy.

Other Methods of Discharge Measurement

Weirs

  • Definition: Structures to measure flow rate, e.g., v-notch weirs.
  • Relationship between water height and discharge can be established empirically.
  • Advantages:
    • Simple relationship for continuous discharge measurement.

Stream Gauges

  • Institutional standards (like USGS) for estimating discharge.
  • Use rating curves to relate water height at gauging stations to discharge.
  • Importance of regular updates to rating curves due to dynamic channel conditions.

Acoustic Doppler Technology

  • Emerging technology for estimating discharge using acoustic Doppler current profilers.
  • Provides simultaneous depth and velocity measurements across a stream channel.
  • Advantages include integrated measures and greater accuracy during flow changes.

Estimation Techniques for Discharge Without Gauging

  • Model based on precipitation inputs:
    • Assessing runoff based on land use (forest, urban, wetland).
    • Refined estimations of discharge based on known hydrology of the watershed.
  • Area-to-discharge relationships:
    • Use known discharge from gauged sites to estimate discharge in unmonitored areas.
    • Requires knowledge of watershed areas for accurate predictions.

Conclusion

  • Recap of various methods for estimating and measuring stream discharge.
  • Importance of understanding hydrologic concepts for ecological impacts and water management practices.