UNIT II PRECIPITATION
UNIT IIPRECIPITATIONDR. P. RAMESHAssociate Professor and HoD,Dept of Civil EngineeringSree Vidyanikethan Engineering CollegeA.Rangampet, Tirupati-517102, A.P., INDIATable of contentsPRECIPITATION Measurement of rainfallRecording and non- recording type of rain gaugesErrors in measurementAnalysis and interpretation of rain fall dataMass curve of rainfallHyetographDouble mass curveMethods of calculation of mean precipitation over an area Depth-Area-Duration relationships. Rainfall measurement
The amount of precipitation is expressed as the depth in centimeters, which falls on a level surface, and is measured by rain gauge.Symons rain gaugeIt is the most common type of non automatic rain gaugeused by Indian meteorological department (IMD). It consists of cylindrical vessel 5 diameter with a base enlarged to 8 diameter. The top section is a funnel provided with circular brass rim exactly 5 internal diameter. The funnel shank is inserted in the neck of a receiving bottle, which is about 75 to 100 mm diameter. A receiving bottle of rain gauge has a capacity of about 75 to 100 mm of rain fall and as during a heavy rain fall this quantity is frequently exceeded, the rain should be measured 3 or 4 times in a day on day of heavy rainfall. A cylindrical gradated measuring glass is furnished with each instrument, which reads to 0.2mm. The rainfall should be estimated to the nearest 0.1mm. The rain gauge is set up in a concrete block 2x2x2. The rim should be 1 above the surface of the ground.
http://www.gitam.edu/eresource/environmental/murali/hyd_index.htm
Merits and Demerits of different types of automatic rain gauges
MERITS1. The rainfall is recorded automatically and there fore, there is necessity of any attendant.2. The recording rain gauge also gives the intensity of rainfall at any time.3. As no attendant is required it can be installed in far off places also.4. Possibility of human error is obviated.
DEMERITS1. It is costly in comparison with non automatic gauge.2. Fault may develop in electrical or mechanical mechanism or recording the rainfall.
Presentation of rainfall data
Mean precipitation over an area
Adjustments of precipitation data
Depth-Area-Duration relationships
Intensity-Duration-Frequency (IDF) curves
Exercise Problem
Recording and non- recording type of rain gauges
