the presentation contains the details about hydrological parameters of rainfall including tests procedures.

1. HYDROLOGICAL
PARAMETER
Rainfall

2. RAINGAUGE
• A Raingauge is a meteorological tool which has been designed to
measure the amount of rainfall reaching the ground.
• The raingauge is made up of a collection container, a retaining
container, a measuring cylinder and is located in an open space,
where rainfall is unobstructed by surface effects.
• Rainfall is recorded as the height of the precipitated water which
has accumulated in the retaining container. This value is
expressed in mm.
• 1mm of measured precipitation is equivalent to 1L of precipitated
rainwater volume per metre square.
For information about EFFECTIVE RAINFALL:
http://www.fao.org/3/X5560E/x5560e02.htm

3. TYPES OF RAINGAUGES

4. TYPES OF RAINGAUGES
• NON-RECORDING RAINGAUGE
• Manually operated – daily measurements are taken
• Tipping Bucket Raingauge
• A tipping bucket rain gauge consists of a pair of rainwater collecting buckets. It is covered by a funnel, with an
open collector area at the top where A is the area of collection. The tipping bucket rain gauge is an automated
rain meter that uses a "tipping bucket" mechanism to measure rainfall. It is used by professional weathers
services' remote weather stations, and is also popular and widely used in home weather stations.
https://www.sciencedirect.com/topics/engineering/rain-gauge

5. TYPES OF RAINGAUGES
• Manually operated – daily measurements are taken
• Weighing Precipitation Gauge
• A weighing precipitation gauge consists of receiving bucket
mounted on a weighing device, usually a mechanical
mechanism such as a spring. The rain accumulates in the
container, and the increased weight compresses the springs.
The amount of compression gets measured and used to
calculate the weight of the water. The measurement can be
recorded manually with a pen on a drum, or electronically
with a data-logger, and send to the base weather station via
landline or wireless connection. Most modern systems are
also self-emptying, reducing the amount of
maintenance required on this type of rain gauge.
https://ownyourweather.com/what-is-a-rain-gauge/

6. TYPES OF RAINGAUGES
• RECORDING RAINGAUGE
• Automatically operated – hourly
(at times every 15 minutes)
measurements are taken

7. RAINGAUGES

8. RAINGAUGES

9. FACTORS TO BE CONSIDERED WHILE SITING A
RAINGAUGE

10. GOVERNING FACTORS
• Accessibility to the site – (Non recording/daily measurement)
• Non recording raingauge – when easy to take daily measurements, human resources
mobilization
• Recording raingauge – Site is not easily accessible, but regular measurements
needed.
• Obstructions
• Obstructions deflect rainwater – buildings, trees, shaded places, corridors created by
presence of many high rise buildings
• Potential use of the rainfall data
• For flood analysis – need rainfall data every 15 minutes
• For water demand of crops – need rainfall data every hour at least
• For water resources development – need weekly or monthly rainfall data

11. GOVERNING FACTORS
• Higher Accuracy
• Daily measurements relies on human resources – one may miss collecting the
data
• One can make error while recording the data (non recording raingauge)
• Nearby disturbing activities:
• People – temptation to tamper with the raingauge
• Irrigation Systems nearby
• Topography & Landuse within the study area
• Rainfall patterns influenced by topography
• Rainfall patterns also influenced by landuse activities – wind direction

12. ESTIMATING AVERAGE RAINFALL
https://file.scirp.org/Html/5-9402024_43175.htm

13. ARITHMETIC MEAN

14. Limitations of the Arithmetic Mean
• If the area under study has a flat topography and landuse is uniform over the
study area, then we can safely expect that the recorded rainfall values will fal
within a close range.

15. Weightage Average – Thiessen Polygon methods
(Graphical)
• Weightage Average: By area
• This method applies to study areas
characterized by changes in topography &
landuse.
• It requires a dense network of raingauge
• Each raingauge reflects the rainfall patterns
around it, not for the entire catchment.

16. Weightage Average – The Method of Isohyets (Graphical)
• Also weighted by area
• This method applies to study areas characterized by
changes in topography & landuse.
• It requires a dense network of raingauge
• The rainfall recorded by each raingauge are
interpolated over the whole study area. In so doing a
series of isohyets – rainfall of equal intensity are
drawn. The study area is then subdivided into
subareas under the influence of respective isohyets.
https://www.biologydiscussion.com/hydrology/estimation-of-
the-average-depth-of-rainfall/73328
https://www.yourarticlelibrary.com/water/precipitation/calcula
ting-average-depth-of-precipitation-3-methods/60452

17. GENERATION OF MISSING DATA

18. PRE RAINFALL DATA ANALYSIS
• Prior to using the rainfall data at a given station, the data have to be checked for
Continuity and Consistency. Simple statistics such as long term mean, daily mean, long
term monthly/annual mean (normal precipitation) are calculated.
• Any missing data can be generated using rainfall data from nearby rainfall stations.
• N = Normal annual precipitation. (Mean of 30 years of annual precipitation data)
• If the normal precipitations at the different rainfall stations are within 10% of the
normal precipitation at the suspect station, the simple average - method of Normal
Ratio can be used to generate the missing data, else the weighted average will be
used:

19. NORMAL RATIO METHOD

20. NORMAL RATIO METHOD - Example

21. CHECKING THE RELIABILITY OF A SUSPECT
STATION

22. SUSPECT STATION
• A rainfall station can for some reasons record wrong rainfall data.
• This can be detected when analysis the statistical parameters of nearby
other stations.
• Data from other nearby stations can be used to check whether the
consistency/validity of the recorded rainfall data from the ‘suspect’
station.
• The method is referred to as the Double Mass Curve Method.

23. The Double Mass Curve Method

24. POTENTIAL REASONS FOR INCONSISTENCY OF RAINFALL DATA RECORDED
• The rainfall station is damaged.
• There are interferences nearby (irrigation systems, a building has
been constructed, or a tree is shading the station)
• Other reasons may be due to a change in the landuse pattern of the
surrounding physical environment.
• Someone may have moved the station from its original position.

25. ADDITIONAL READING MATERIALS
• Measurement of Precipitation: [http://www.jma.go.jp/jma/jma-eng/jma-
center/ric/Our%20activities/International/CP6-Precipitation.pdf]
• Precipitation Sensors: [https://observator.com/instruments/meteo-and-
hydro/meteorological-products/rain-gauges-precipitation-sensors/]
• Generation of Spatial Variation of Rainfall:
[https://ewater.org.au/archive/crcch/overview/archive/pdfs/working200201.pdf]