Contents preface (VII) introduction 1—37

Fig. 2.5 Rainfall record of bucket rain gauge (mass curve of rainfall)
( iii) Siphon rain gauge : This gauge (Fig. 2.6) is also called float type rain gauge as this gauge has a chamber which contains a light and hollow float. The vertical movement of float on account of rise in the water level in the chamber (due to rain water falling in it) is transmit-ted by a suitable mechanism to move a pen on a clock-driven revolving chart. The record of rainfall is in the form of a mass curve of rainfall and, hence, the slope of the curve gives the intensity of rainfall.

44

175 mm

Reinforcement

Wire gauge filter

Inlet tube
Glass cap
Siphon chamber
Washer
Discharge

tube
Lower tube hexagon nut

Bracket

IRRIGATION AND WATER RESOURCES ENGINEERING

203.4 or 128.6 mm




Collector

Funnel

Ventilation hole
Recording drum
Float rod
Pen
Upper guide
Clock 750 mechanism mm
Float chamber
Rainfall

recording ₆₇₅ mechanism _mm

Lower guide
Base
Adjusting screw
Cement concrete platform

Foundation bolt	75 mm

Ground

level

Fig. 2.6 Siphon rain gauge

Bureau of Indian Standards has laid down the following guidelines for selecting the site for rain gauges (IS : 4897-1968):

HYDROLOGY

45

1. 
   The rain gauge shall be placed on a level ground, not upon a slope or a terrace and never upon a wall or roof.
   

2. 
   On no account the rain gauge shall be placed on a slope such that the ground falls away steeply in the direction of the prevailing wind.
   
3. 
   The distance of the rain gauge from any object shall not be less than twice the height of the object above the rim of the gauge.
   
4. 
   Great care shall be taken at mountain and coast stations so that the gauges are not unduly exposed to the sweep of the wind. A belt of trees or a wall on the side of the prevailing wind at a distance exceeding twice its height shall form an efficient shelter.
   

5. 
   In hills where it is difficult to find a level space, the site for the rain gauge shall be chosen where it is best shielded from high winds and where the wind does not cause eddies.
   
6. 
   The location of the gauge should not be changed without taking suitable precautions. Description of the site and surroundings should be made a matter of record.
   

2.