Question

Normal ratio method is used ____________.

• A. To separate baseflow from hydrograph
• B. To find mean precipitation over an area
• C. To check consistency of raingauge station
• D. To estimate missing rainfall data

Hint:

To select the correct estimation method for missing rainfall:
- If the difference in normal annual precipitation is \(\le 10\%\) \(\rightarrow\) Use Arithmetic Average.
- If the difference is \(> 10\%\) \(\rightarrow\) Use Normal Ratio Method.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Raingauge stations occasionally experience technical failures or observer errors during storms, leading to gaps in precipitation records.
In hydrology, the Normal Ratio Method is an empirical technique used to estimate these missing rainfall records at a specific station using data from surrounding index stations.
Key Formula or Approach:
The Normal Ratio Method is used when the normal annual precipitation of any surrounding index station (\(N_i\)) differs from that of the missing station (\(N_x\)) by more than 10%.
The formula is:
\[ P_x = \frac{N_x}{M} \left[ \frac{P_1}{N_1} + \frac{P_2}{N_2} + \dots + \frac{P_M}{N_M} \right] \]
Where:
- \(P_x\) is the estimated missing precipitation at station \(x\).
- \(N_x, N_1, \dots, N_M\) are the normal annual precipitation values at the target and index stations.
- \(P_1, \dots, P_M\) are the observed storm precipitation values at the index stations.
- \(M\) is the number of surrounding index stations.

Step 2: Detailed Explanation:
Let us review the other options:
- Baseflow separation (A) is performed using methods like the straight-line method, fixed-base method, or curve-extension method.
- Mean precipitation over an area (B) is calculated using the Arithmetic Mean, Thiessen Polygon, or Isohyetal methods.
- Consistency of a raingauge station (C) is checked using the Double Mass Curve technique.
- Estimating missing rainfall data (D) is done using either the Arithmetic Average method (if normal values are within 10% difference) or the Normal Ratio Method.
Thus, Option (D) is the correct application of this method.

Step 3: Final Answer:
The correct option is (D).