Rain Gauge Densities: The rain gauge for an area should be evenly and uniformly distributed within a given catchment. The total number of rain gauges in a given area should neither be too many because it will increase the cost nor be too low to give reliable results. A lesser density of rain gauge (less number of gauge per unit area) is used for studies of a general storm and the high density of rain gauge (high number of gauge per unit area) is used for studies of rainfall pattern of a thunderstorm. Hence optimum density of gauges should be considered. 
The Indian standard (IS: 4987-1968) recommends the following densities: 
In plain area  → 1 station per 520 km²
In the region of average elevation 1000 m  → 1 station per 260-390 km² 
In predominantly hilly areas with heavy rainfall  → 1 station per 130 km²