1. Introduction to Precipitation
CEE 4420 – Engineering Hydrology
(Prepared by Abebe Gebregiorgis)

2. 2.1 Introduction
All forms of water that reach the earth from the atmosphere is called Precipitation.
The usual forms are rainfall, snowfall, frost, hail, dew. Of all these, the first two contribute significant amounts of water.
Rainfall being the predominant form of precipitation causing stream flow, especially the flood flow in majority of rivers. Thus, in this context, rainfall is used synonymously with precipitation.

3. Introduction…. In nature water is present in three aggregation states:
solid: snow and ice;
liquid: pure water and solutions;
gaseous: vapors under different grades of pressure and saturation
The water exists in the atmosphere in these three aggregation states.

4. Introduction…. Types of precipitation Rain:
Rain, snow, hail, drizzle, glaze, sleet
Rain:
Is precipitation in the form of water drops of size larger than 0.5 mm to 6mm
The rainfall is classified in to
Light rain – if intensity is trace to 2.5 mm/h
Moderate – if intensity is 2.5 mm/hr to 7.5 mm/hr
Heavy rain – above 7.5 mm/hr

5. Introduction…. Snow: Hail (violent thunderstorm)
Snow is formed from ice crystal masses, which usually combine to form flakes
Hail (violent thunderstorm)
precipitation in the form of small balls or lumps usually consisting of concentric layers of clear ice and compact snow.
Hail varies from 0.5 to 5 cm in diameter and can be damaging crops and small buildings.

6. 2.2 Temporal and Spatial Variation of Rainfall
Rainfall varies greatly both in time and space
With respect to time – temporal variation
With space – Spatial variation
The temporal variation may be defined as hourly, daily, monthly, seasonal variations and annual variation (long-term variation of precipitation)

9. 2.3. Measurement of Rainfall
Rainfall and other forms of precipitation are measured in terms of depth, the values being expressed in millimeters.
One millimeter of precipitation represents the quantity of water needed to cover the land with a 1mm layer of water, taking into account that nothing is lost through drainage, evaporation or absorption.
Instrument used to collect and measure the precipitation is called raingauge.

10. Rainfall measurement…
Precipitation gauge
1 - pole
2 - collector
3 - support- galvanized metal sheet
4 – funnel
5 - steel ring
1. Non recording gauge

11. 2. Recording gauge / graphic raingauge
The instrument records the graphical variation of the fallen precipitation, the total fallen quantity in a certain time interval and the intensity of the rainfall (mm/hour).
It allows continuous measurement of the rainfall.
The graphic rain gauge
1-receiver
2-floater
3-siphon
4-recording needle 5-drum with diagram
6-clock mechanism

12. 3. Tele-rain gauge with tilting baskets
The tele-rain gauge is used to transmit measurements of precipitation through electric or radio signals.
The sensor device consists of a system with two tilting baskets, which fill alternatively with water from the collecting funnel, establishing the electric contact.
The number of tilting is proportional to the quantity of precipitation hp

13. Tele-rain gauge …… The tele-rain-gauge
1 - collecting funnel 2 - tilting baskets
3 - electric signal
4 - evacuation

14. 4. Radar measurement of rainfall
The meteorological radar is the powerful instrument for measuring the area extent, location and movement of rainstorm.
The amount of rainfall overlarge area can be determined through the radar with a good degree of accuracy
The radar emits a regular succession of pulse of electromagnetic radiation in a narrow beam so that when the raindrops intercept a radar beam, its intensity can easily be known.

15. Raingauge Network
Since the catching area of the raingauge is very small as compared to the areal extent of the storm, to get representative picture of a storm over a catchment the number of raingauges should be as large as possible, i.e. the catchment area per gauge should be small.
There are several factors to be considered to restrict the number of gauge:
Like economic considerations to a large extent
Topographic & accessibility to some extent.

16. World Meteorological Organization (WMO) recommendation:
Raingauge Network…..
World Meteorological Organization (WMO) recommendation:
In flat regions of temperate, Mediterranean and tropical zones
Ideal  1 station for 600 – 900 km2
Acceptable 1 station for 900 – 3000 km2
In mountainous regions of temperate , Mediterranean and tropical zones
Ideal  1 station for 100 – 250 km2
Acceptable  1 station for 250 – 1000 km2
In arid and polar zone
1 station for 1500 – 10,000 km2
10 % of the raingauges should be self recording to know the intensity of the rainfall

17. 2.4 Preparation of Data
Before using rainfall data, it is necessary to check the data for continuing and consistency
Missing data
Record errors
Estimation of Missing Data
Given annual precipitation values – P1, P2, P3,… Pm at neighboring M stations of station X 1, 2, 3 & m respectively
The normal annual precipitation given by N1, N2, N3,…, Nm, Ni… (including station X)
To find the missing precipitation, Px , of station X

18. Test for consistency record
(Double mass curve techniques)
Let a group of 5 to 10 base stations in the neighbourhood of the problem station X is selected
Arrange the data of X stn rainfall and the average of the neighbouring stations in reverse chronological order (from recent to old record)
Accumulate the precipitation of station X and the average values of the group base stations starting from the latest record.
Plot the against as shown on the next figure
A decided break in the slope of the resulting plot is observed that indicates a change in precipitation regime of station X, i.e inconsistency.
Therefore, is should be corrected by the factor shon on the next slide

19. Pcx – corrected precipitation at any time period t1 at stationX
Test for consistency record…. a c
Pcx – corrected precipitation at any time period t1 at stationX
Px – Original recorded precp. at time period t1 at station X
Mc – corrected slope of the double mass curve
Ma – original slope of the mass curve

20. 2.5 Mean Precipitation over an area
Raingauges rainfall represent only point sampling of the areal distribution of a storm
The important rainfall for hydrological analysis is a rainfall over an area, such as over the catchment
To convert the point rainfall values at various stations to in to average value over a catchment, the following methods are used:
arithmetic mean
the method of the Thiessen polygons
the isohyets method

21. Applicable rarely for practical purpose
Arithmetic Mean Method
When the area is physically and climatically homogenous and the required accuracy is small, the average rainfall ( ) for a basin can be obtained as the arithmetic mean of the hi values recorded at various stations.
Applicable rarely for practical purpose

22. The influence zones are represented by convex polygons.
Method of Thiessen polygons
The method of Thiessen polygons consists of attributing to each station an influence zone in which it is considered that the rainfall is equivalent to that of the station.
The influence zones are represented by convex polygons.
These polygons are obtained using the mediators of the segments which link each station to the closest neighbouring stations

23. Thiessen polygons ……….

24. Thiessen polygons ………. P7 P6 A7 A6 P2 A2 A1 A8 A5 P1 P8 P5 A4 A3 P3 P4

25. Thiessen polygons ……….
Generally for M station
The ratio is called the weightage factor of station i

26. An isohyet is a line joining points of equal rainfall magnitude.
Isohyetal Method
An isohyet is a line joining points of equal rainfall magnitude.
10.0 8 D a5 6 C 12
9.2 12 a4 a3
7.0 B 4
7.2 A E a2
10.0
9.1
4.0 a1 a1 F 8 6 4

27. P1, P2, P3, …. , Pn – the values of the isohytes
Isohyetal Method
P1, P2, P3, …. , Pn – the values of the isohytes
a1, a2, a3, …., a4 – are the inter isohytes area respectively
A – the total catchment area
- the mean precipitation over the catchment
NOTE
The isohyet method is superior to the other two methods
especially when the stations are large in number.

28. 2.6 Intensity – Duration – Frequency (IDF) Relationship
Mass Curve of Rainfall
1st storm, 16 mm
2nd storm, 16 mm

29. Hyetograph
IDF ….
is a plot of the accumulated precipitation against time, plotted in chronological order
Total depth = 10.6 cm
Duration = 46 hr

30. IDF ….
In many design problems related to watershed such as runoff disposal, erosion control, highway construction, culvert design, it is necessary to know the rainfall intensities of different durations and different return periods.
The curve that shows the inter-dependency between i (cm/hr), D (hour) and T (year) is called IDF curve.
The relation can be expressed in general form as:
i – Intensity (cm/hr)
D – Duration (hours)
K, x, a, n – are constant for a given catchment

31. IDF ….
k = 6.93
x = 0.189
a = 0.5
n = 0.878