Understanding Rain Gauge Density as per IS 4987-1968

The IS 4987-1968 standard provides guidelines for the design and planning of hydrological networks, specifically focusing on the density of rain gauges required in different regions and conditions.

The density of rain gauges is crucial for accurately measuring rainfall distribution over an area, which is essential for various hydrological studies, water resource management, and flood forecasting.

Rain Gauge Density in Hilly Regions with Heavy Rainfall

The IS 4987-1968 standard specifies different recommended areas to be covered by one rain gauge depending on the geographical characteristics and rainfall intensity of a region. For areas classified as predominantly hilly regions with heavy rainfall, a specific density is recommended.

Let's look at the recommendations for different types of regions according to the standard (approximate values often cited):

• In plains: One gauge for 520 $\text{km}^2$
• In areas with average elevation: One gauge for 260 $\text{km}^2$
• In predominantly hilly regions with heavy rainfall: One gauge for 130 $\text{km}^2$
• In dry regions of sparse rainfall: One gauge for 650 $\text{km}^2$ to 1000 $\text{km}^2$

The question specifically asks for the recommendation in "predominantly hilly regions with heavy rainfall" according to IS 4987-1968.

Based on the standard guidelines, in predominantly hilly regions experiencing heavy rainfall, one rain gauge is recommended for every 130 square kilometers.

Comparing with the Options

Let's compare this standard recommendation with the given options:

• Option 1: 390 $\text{km}^2$
• Option 2: 520 $\text{km}^2$ (This is typically for plains)
• Option 3: 130 $\text{km}^2$
• Option 4: 260 $\text{km}^2$ (This is typically for areas with average elevation)

The value recommended by IS 4987-1968 for predominantly hilly regions with heavy rainfall is 130 $\text{km}^2$ per gauge.

Therefore, the correct option is 130.