Calculating Areal Average Precipitation

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Questions and Answers

What is the purpose of constructing isohyets in the isohyetal method?

To plot the accumulated rainfall against time

To draw lines of equal precipitation (correct)

To smooth out high frequency fluctuations in a time series

To calculate the areal average of rainfall

What is the formula used to compute the areal average in the isohyetal method?

∑(Pi) / ∑Ai

∑(Pi / Ai)

∑(Ai) / ∑Pi

∑(Pi * Ai) / ∑Ai (correct)

What is the name of the method that estimates the mean precipitation by drawing lines of equal precipitation?

Thiessen Polygon Method

Isohyetal Method (correct)

Arithmetic Mean Method

Inverse Distance Weighting

What is the purpose of a hyetograph?

To plot the accumulated rainfall against time Signup and view all the answers

What is the name of the method that uses the concept of polygons to estimate the mean precipitation?

Thiessen Polygon Method Signup and view all the answers

What is the formula used to compute the areal average in the Thiessen polygon method?

(∑Pi) / n Signup and view all the answers

What is the purpose of a mass curve of rainfall?

To plot the accumulated rainfall against time Signup and view all the answers

What is the name of the technique used to smooth out high frequency fluctuations in a time series?

Moving Average Signup and view all the answers

What is a common cause of inconsistency in rainfall records?

Shifting of a raingauge station to a new location Signup and view all the answers

What is the principle behind the Double-Mass Curve Technique?

That each recorded data comes from the same parent population Signup and view all the answers

What is the purpose of plotting ΣPx against ΣPav in the Double-Mass Curve Technique?

To check for breaks in the slope Signup and view all the answers

What happens when a raingauge station undergoes a significant change?

The rainfall data becomes less consistent Signup and view all the answers

What is a possible reason for a change in ecosystem due to calamities?

All of the above Signup and view all the answers

What is the corrective measure taken when a break in the slope is detected in the Double-Mass Curve Technique?

Correcting precipitation values using the relation Signup and view all the answers

What is the primary objective of calculating areal rainfall?

To determine the average rainfall over a catchment area Signup and view all the answers

In the Inverse Distance Weighting method, what is the weightage factor based on?

Distance from the ungauged site to the gauged sites Signup and view all the answers

Which method is used when rainfall at various stations shows little variation?

Arithmetic Mean Method Signup and view all the answers

What is the formula used to compute the areal average in the Thiessen Polygon Method?

P = (P1 / d1 + P2 / d2 + ... + Pm / dm) / (1/d1 + 1/d2 + ... + 1/dm) Signup and view all the answers

What is the purpose of drawing perpendicular bisectors in the Thiessen Polygon Method?

To form representative areas for the gauges Signup and view all the answers

Why is the Inverse Distance Weighting method used to calculate the precipitation at an ungauged site?

Because it takes into account the distance from the ungauged site to the gauged sites Signup and view all the answers

What is the primary advantage of the Thiessen Polygon Method?

It provides a more accurate representation of areal rainfall Signup and view all the answers

Study Notes

Methods for Areal Average Computation

Theissen Polygon Method: estimates the mean precipitation across an area by assigning a weightage to each rainfall station based on its surrounding area.
Steps:
- Draw lines joining adjacent gauges
- Draw perpendicular bisectors to the lines created in step 1
- Extend the lines created in step 2 in both directions to form representative areas for the gauges
- Compute the representative area for each gauge
- Compute the areal average using the formula:
Isohyetal Method: estimates the mean precipitation by drawing lines of equal precipitation and computing the average precipitation for each pair of adjacent isohyets.
Steps:
- Construct isohyets (rainfall contours)
- Compute area between each pair of isohyets (Ai)
- Compute the average precipitation for each pair of adjacent isohyets (Pi)
- Compute areal average using the formula:
Arithmetical Mean Method: can be used if rainfall at various stations show little variation.
Formula:

Presentation of Rainfall Data

Mass Curve of Rainfall (Hyetograph): a plot of accumulated rainfall against time, plotted in chronological order.
Derived from float type and weighing bucket type gauges.
Moving Average: a technique to smooth out high frequency fluctuations in a time series and enable the trend to be noticed.

Inverse Distance Weighting Method

Formula:
Used to estimate precipitation at an ungauged site based on precipitation data at surrounding gauged sites.

Testing for Consistency of Records

Causes of inconsistency: shifting of a raingauge station, change in neighborhood, ecosystem change, and observational errors.
Double-Mass Curve Technique: used to check for inconsistencies in rainfall data.
Steps:
- Sort data in descending order, starting from the latest year.
- Calculate the cumulative precipitation for both precipitation of a specific station and the average of the group of base stations.
- Plot the values of ΣPx against ΣPav.
- Check for the break in the slope, indicating a change in precipitation regime.
- Correct precipitation values at the station beyond the period of change of regime using the relation:

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Description

Learn how to calculate the areal average precipitation using the Isohyetal Method and Theissen Polygon Method. Understand the steps involved in constructing isohyets and computing average precipitation.