Question

As per Indian Standard IS 4987-1968 code the number of raingauges to be provided for $1000\text{ km}^2$ hilly area with heavy rainfall are}

• A. 2
• B. 4
• C. 8
• D. 10

Hint:

Remember the three major IS 4987 density values: 520 (plains), 260-390 (moderate elevation), and 130 (hilly with heavy rain). Simple division of the area by 130 yields the answer.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
To obtain representative precipitation data over a catchment, a minimum network density of rain gauges must be maintained.
The Bureau of Indian Standards (BIS) prescribes these density guidelines in the code IS 4987-1968.
Key Formula or Approach:
According to IS 4987-1968, the recommended minimum density of rain gauges depends on the topography and rainfall patterns:
1. In plains: $1\text{ station per } 520\text{ km}^2$.
2. In regions with average elevation of $1000\text{ m}$ above sea level: $1\text{ station per } 260\text{ to } 390\text{ km}^2$.
3. In hilly areas with heavy rainfall: $1\text{ station per } 130\text{ km}^2$.

Step 2: Detailed Explanation:
Let us apply the recommended density for hilly areas with heavy rainfall to the given catchment:
Given:
\[ \text{Catchment Area } (A) = 1000\text{ km}^2 \]
The recommended density is 1 station per $130\text{ km}^2$.
We calculate the minimum number of rain gauges (\(N\)) as:
\[ N = \frac{\text{Total Area}}{\text{Recommended Area per Gauge}} \]
\[ N = \frac{1000}{130} \approx 7.69 \]
Rounding up to the nearest whole number to ensure adequate density gives:
\[ N = 8 \]
Therefore, a minimum of 8 rain gauges must be established in this hilly area.

Step 3: Final Answer:
The minimum number of rain gauges to be provided is 8.