Double Mass Curve Technique in Hydrology
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Questions and Answers

What does a decided break in the slope of the resulting plot indicate?

  • A change in the precipitation regime (correct)
  • A consistent rainfall pattern
  • Insufficient data for analysis
  • An error in data collection

    • Which formula is used to correct the precipitation values at station X after a regime change?

  • $P_{cx} = P_{x} / rac{M_{c}}{M_{a}}$
  • $P_{cx} = P_{x} + rac{M_{c}}{M_{a}}$
  • $P_{cx} = P_{x} - rac{M_{a}}{M_{c}}$
  • $P_{cx} = P_{x} imes rac{M_{c}}{M_{a}}$ (correct)

    • What type of data is used to plot against the accumulated precipitation of station X?

  • Monthly precipitation values from station Y
  • Accumulated values of average rainfall from a group of base stations (correct)
  • Raw data collected without any corrections
  • Average daily rainfall from surrounding regions

    • What is the purpose of arranging the rainfall data in reverse chronological order?

    <p>To facilitate accumulated calculations from the latest record</p> Signup and view all the answers

    What technique helps in evaluating data consistency between the precipitation of a station and averaged data from nearby stations?

    <p>Double Mass Curve Technique</p> Signup and view all the answers

    Which method is primarily used to determine areal average precipitation in a watershed?

    <p>Arithmetic mean method</p> Signup and view all the answers

    What characterizes the Thiessen polygon method in rainfall measurement?

    <p>It assigns weights based on proximity to each rain gauge.</p> Signup and view all the answers

    What is a significant factor influencing the reliability of a single gauge reading for average precipitation?

    <p>Distance from the gauge to the center of the area</p> Signup and view all the answers

    In the moving average technique, what does the variable 'm' represent?

    <p>The length of the time range selected for averaging</p> Signup and view all the answers

    Which of the following is NOT one of the methods used to measure average precipitation over an area?

    <p>Standard deviation method</p> Signup and view all the answers

    Which characteristic is essential for using the mass curve technique effectively?

    <p>Availability of rainfall data from multiple stations</p> Signup and view all the answers

    How does topography affect precipitation measurement reliability?

    <p>It can create microclimates that alter rainfall patterns.</p> Signup and view all the answers

    What is typically the minimum value for 'm' in the moving average method?

    <p>3 years</p> Signup and view all the answers

    What does the Normal Ratio Method estimate in rainfall data?

    <p>The annual rainfall at a station using a weighted average</p> Signup and view all the answers

    Which stations recorded annual precipitation in the year 2019 despite station D being inoperative?

    <p>Stations A, C, and B</p> Signup and view all the answers

    What is a possible cause of inconsistency in rainfall data recorded at a station?

    <p>Shifting of the raingauge station to a new location</p> Signup and view all the answers

    In the example provided, what is the normal annual rainfall at station B?

    <p>67.59 cm</p> Signup and view all the answers

    What is the first step in applying the Double Mass Curve Technique?

    <p>Select a group of base stations nearby</p> Signup and view all the answers

    How is annual rainfall at station D estimated in 2019 based on the given data?

    <p>By using the Normal Ratio Method</p> Signup and view all the answers

    Which factor does NOT contribute to the inconsistency of rainfall data?

    <p>Addition of more recording equipment</p> Signup and view all the answers

    What was the normal rainfall value at station A?

    <p>80.97 cm</p> Signup and view all the answers

    Study Notes

    Double Mass Curve Technique

    • Used to check for inconsistency in a raingauge station's record.
    • The technique relies on the principle that consistent rainfall data originates from the same parent population.
    • Steps:
      • Select a group (5-10) of base stations near the station in question (X).
      • Arrange the annual (or monthly/seasonal) rainfall data for station X and the average rainfall data for the base stations in reverse chronological order.
      • Calculate the accumulated precipitation for station X (∑ Px) and the accumulated average precipitation for the base stations (∑ Pb) starting from the latest record.
      • Plot ∑ Px against ∑ Pb for consecutive time periods.
      • A break in the plot's slope indicates a change in the precipitation regime at station X.
      • Correct precipitation values beyond the regime change using the formula: Pcx = Px * (Mc/Ma), where:
        • Pcx = corrected precipitation at time period t1 at station X.
        • Px = original recorded precipitation at time period t1 at station X.
        • Mc = corrected slope of the double mass curve.
        • Ma = original slope of the double mass curve.

    Causes of Inconsistency in Rainfall Record

    • Shifting of a raingauge station to a new location.
    • Significant changes in the station's surrounding environment.
    • Ecosystem changes due to events like forest fires or landslides.
    • Observational errors from a specific date onward.

    Mean Precipitation Over an Area

    • Determines the average precipitation depth over a specific area.
    • Factors affecting reliability of rainfall measurements at a single gauge:
      • Distance from the gauge to the area's center.
      • Area size.
      • Topography.
      • Nature of the rainfall event.
      • Local storm pattern characteristics.

    Methods to Calculate Areal Average Precipitation

    • Arithmetic Mean Method:
      • Simplest method.
      • Considers only stations within the watershed boundary.
      • Average precipitation (P) = ∑(Pi) / N
        • Pi = rainfall at the ith raingauge station.
        • N = total number of raingauge stations.
    • Thiessen Polygon Method:
      • Assigns weightage to each station's rainfall based on its corresponding area.
      • Procedure:
        • Draw the catchment area to scale.
        • Mark raingauge station positions on the map.
        • Connect stations to form triangles.
        • Construct perpendicular bisectors for each triangle side, forming polygons around each station.
      • P = (A1 * P1 + A2 * P2 + ... + An * Pn) / (A1 + A2 + ... + An)
        • Ai = Area of the polygon around station i.
        • Pi = Rainfall at station i.
    • Isohyetal Method:
      • Draws isohyets (lines of equal rainfall) based on rainfall data.
      • Calculates the area enclosed between each pair of isohyets.
      • Average precipitation is determined by weighing the precipitation at the center of each area by the area's size.
    • Inverse Distance Weighting:
      • Uses the inverse of the distance between a station and the area's point of interest to weight the station's rainfall value.
      • Closer stations have a higher weight.

    Additional Terms

    • Point rainfall: Rainfall measured at a single station.
    • Moving average: A technique to smooth out high-frequency fluctuations in a time series.
    • Hyetograph: A graphical representation of rainfall intensity over time.
    • Mass Curve of Rainfall: A graph that shows the cumulative rainfall over time.
    • Normal Ratio Method: A method used to estimate the annual rainfall at a station when normal annual precipitation values vary significantly.

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    Related Documents

    Module 3 Precipitation PDF

    Description

    This quiz explores the Double Mass Curve Technique used to assess inconsistencies in rainfall data from gauging stations. Participants will learn the methodology for plotting accumulated precipitation data, identifying regime changes, and applying corrections to recorded values. Test your understanding of this vital hydrological analysis tool.

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