Precipitation

Hydrologic cycle: Various components are rainfall, runoff, infiltration, evaporation, transpiration etc.

• Sun is the main source of energy for this cycle.
• The total water resource of the earth is constant.

Precipitation: Denotes all forms of water that reach the earth from atmosphere (rainfall. snowfall, dew, hail etc.)

Rainfall is used synonymous with precipitation.

The term “rainfall” is used to describe precipitations in the form of water drops of sizes larger than 0.5mm. The maximum size of a raindrop is about 6mm.

Type	Intensity
Light Rain	Trace to 2.5mm/hr
Moderate Rain	2.5   mm/hr to 7.5 mm/hr
Heavy Rain	>7.5 mm/hr

Drizzle: Size of drop is Less than 0.5mm and intensity is very small. Drops float in air.

Snow: Ice crystals, combine to form flakes.

Hail: Lumps of ice more than 8mm in size.

Types of Precipitalions:                

1. Cyclone
2. Convective                            
3. Orographic

Cyclone:

• It is a large low pressure region with circular wind motion.
• Isobars are closely spaced and winds are anticlockwise in the northern hemisphere.
• The center of the storm is called ‘eye’, which will he relatively quiet.
• Outside the eye, very strong winds with speed decreasing towards outer edge.
• Pressure increases outwards.
• Rainfall will be heavy over larger area.

 

Anticyclone:

• It is a region of high pressure.
• Cause clockwise wind in the northern hemisphere.
• At outer edges precipitation exists.

Convective Precipitation : Due to heat, warmer air rises, undergoes cooling and gives precipitation. This type is called as convective Precipitation.

 Orographic Precipitation : Moist air mass gets lifted up due to the presence of mountain barriers and consequently undergoes cooling and gives precipitation. This type is called as  Orthographic

precipitation. The windward slope gets heavy precipitation and leeward slope gets light rainfall.

South- West Monsoon:

• It is principal rainy season in India. 
• Originates in Indian Ocean, appears first in Kerala.

The average annual rainfall for the entire country (India)  is  119 cm and average run Off is 55 cm

 Measurement  of  Precipitation  :  By rain gauges.

• The rain gauge is also known as Pluviometer, Ombrometer, hyetometer.
• No object should be nearer to the instrument than 30 m or twice the height of obstruction.
• Snowfall is measured by an equivalent depth of water.

Types of rain gauges:

1. Non Recording Type
2. Recording Type

Non Recording Raingauge: It gives total depth of precipitation.

Ex: Symon’s rain gauge (commonly used in India).

• Diameter of Symon’s  gauge is 12.7 cm and the rim of the  collector is at a height of 30cm above G.L. Symon’s gauge can also be used to measure snow fall by removing funnel and receiving bottle.

Recording Type Gauge : It gives rainfall depth with respect to time. (i.e intensity of

rainfall, mass curve of rainfall

Ex : Tipping Bucket type, weighing bucket type, natural siphon’s type.

• Tipping type gives intensity of rainfall.
• Weighing bucket  type and natural siphon type  give mass curve of rainfall.
• Natural siphon type also called float type and it is the standard recording type gauge adopted by Indian standards.
• Tipping bucket type (Telemetric gauge) is ideally suitable for mountainous and other inaccessible places.

Rain gauge network: The ratio of total area of the catchment  to the total number of

gauges in the catchment is defined as the rain gauge density or the network density. Thus the rain gauge density gives the average area served by each gauge.From the view point of accurate hydro logical analysis and forecasting, therefore, the rain gauge density should be as small as possible. The actual gauge density to be adopted,  however,  will be governed by many factors such as topography, climate, the state of development of the region and largely by the finances available to install and maintain the network.

From practical consideration of the Indian conditions, the Indian Standard Institution, has recommended the following densities:

• One gauge per 520 km2 in plain areas, with denser network for the areas lying in the path of low pressure systems.
• One gauge per 260 to 390 km2 in regions with an average elevation of 1000m above the mean sea level.
• One gauge per 130 km2 in predominantly hilly regions with heavy rainfall.
• It is also recommended that at least 10 percent of the gauges are of recording type.

The network arrived at based on these recommendations may be located that all the gauges will have more or less equal Thiessen weights.

Adequacy of rain gauge stations:

The optimum number of rain gauges required, n is given by:

where 

• e = allowable degree of error in the measurement, in %.
• Cv = coefficient of variation of rainfall, in %.

If there are ‘m’ stations with rainfall values P1,P2 ,…….Pm.

Where ‘Pa’ is average rainfall= P / m,

‘σ’ is Standard Deviation.