Rain fall or “precipitation” may be defined as the total condensation of moisture that reaches the earth in any form. It includes all forms of rains, ice, snow, hail or sleet etc. “Evaporation” represents practically that entire portion of the rainfall that does not reach the point of ultimate use as stream flow. So, evaporation includes all the rainfall that is returned to the atmosphere from land and water surfaces. The intensity of rain fall is expressed as an amount of precipitation in a stated period. It varies widely minute to minute during heavy rain storm and can only be able to measure with a continuously recording gauge
The measurement of rainfall: All forms of precipitation are measured on the basis of vertical depth of water which would accumulate on the level surface if all precipitation remained where it fell. The rainfall is generally measured with the help of rain- gauge. Their types are as follows:
1.Non recording type: I the water from the rainfall is collected before the losses take place then the depth of the water over the small area can be accurately determined to find out the rainfall in mm. The area selected for setting the gauge should represent the large area whose meteorological characteristics are the same.it consists of funnel mouth that directly discharging into a receiver large enough to hold the maximum possible day’s rainfall. The funnel and receiver are placed in a metal casing with suitable packings. At regular intervals, the water is collected into a measuring vessel which is so calibrated in relation to the area of the funnel mouth that directly reads the rainfall in cm or mm. The base of the gauge is permanently leveled & fixed in the concrete block.
2.Recording type: The recording type of gauge has a funnel on the top of rectangular box as shown in figure. The float is adjusted in same box. The float is connected to a pin point by means of a float rod. The pin point touches the graph paper mounted on a rotating drum as shown in figure. The rotating drum kept rotating continuously with the help of electric motor. The drum makes generally one rotation during 24 hours. When the rainfall occurs, water passes into the rectangular box. The float rises as the water level in the box rises. As the float rises, the pin point moves upon the graph paper to plot the mass curve of rainfall.
Mean depth of rain fall: The collection of mean monthly and annual rainfall depth over the catchment area. When the rain gauge are more than one for a particular basin, We are always interested to find out only one value of rainfall which will represent the whole catchment area, The following three methods use for calculating average of rainfall depending upon the area of basin.
a. Arithmetic mean method
b. Thiesson method
c. Iso-hyetol method