CHAPTER 3 PRECIPITATION 1 Hydrology (CE 424) Instructor: Dr. Saleh AlHassoun

2 The term precipitation denotes all forms of moisture that reach the ground from the atmosphere(clouds). The usual forms are : rainfall, snowfall, hail, glaze, rime, and ice pellets. Precipitation

3 (i) The atmosphere must have moisture (ascending humid air ), (ii) There must be sufficient nuclei ( Mm) present to aid condensation( example : Salt, oxides, CO 2 ), (iii) Weather conditions (cooling & pressure change) must be good for condensation of water vapor to take place, (iv) The products of condensation must reach the earth, i.e. droplet weight > buoyancy force Formation of precipitation :

4 A fine sprinkle of numerous water droplets of size less than 0.5 mm, and intensity less than 1 mm/h. The drops are so small that they appear to float in the air. FORMS OF PRECIPITATION Drizzle

5 The term rainfall is used to describe precipitation in the form of water drops of sizes larger than 0.5 mm. The maximum size of a raindrop is about 6 mm Types : Light( 7.6) Rainfall intensity ~ 2.5 mm/hr – 7.6 mm/hr FORMS OF PRECIPITATION Rain ( 0.5 mm < size < 6 mm )

6 Snow is another important form of precipitation. Snow consists of ice crystals which usually combine to form flakes. When new, snow has an initial density varying from 0.06 to 0.15 gm/cm 3. It is usual, to assume : an average density of 0.1 gm/cm 3. FORMS OF PRECIPITATION Snow

7 When rain or drizzle comes in contact with cold ground at around 0ºC, The water drops freeze to form an ice coating called glaze or freezing rain. Sp. Gravity = 0.8 FORMS OF PRECIPITATION Glaze Sleet (Rime) * It is frozen raindrops of transparent grains which form when rain falls through air at subfreezing temperature. * In Britain, sleet denotes precipitation of snow and rain simultaneously.

10 It is a showery precipitation in the form of irregular pellets or lumps of ice. size more than 5 mm, Sp. Gravity = 0.8 Hails occur in violent thunderstorms in which vertical currents are very strong. FORMS OF PRECIPITATION Hail

11 A front is the interface between two distinct air masses. Under certain favorable conditions when a warm air mass and cold air mass meet, the warmer air mass is lifted over the colder one with the formation of a front. The ascending warmer air cools adiabatically with the consequent formation of clouds and precipitation. WEATHER SYSTEMS FOR PRECIPITATION Front

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13 Lifting of moist air converging into a large low pressure area with circular wind motion. Two types of cyclones are recognized: 1.tropical cyclones, and 2. extra tropical cyclones. 1. Cyclonic Precipitation WEATHER SYSTEMS FOR PRECIPITATION

14 In this type of precipitation a packet of air which is warmer than the surrounding air due to localized heating rises because of its lesser density. Air from cooler surroundings flows to take up its place thus setting up a convective cell. The warm air continues to rise, undergoes cooling and results in precipitation. WEATHER SYSTEMS FOR PRECIPITATION 2. Convective Precipitation

15 Warm front Convective Precipitation

16 The moist air masses may get lifted-up to higher altitudes due to the presence of mountain barriers and consequently undergo cooling, condensation and precipitation. Such a precipitation is known as Orographic precipitation WEATHER SYSTEMS FOR PRECIPITATION 3. Orographic Precipitation

17 Orographic Precipitation

Rainfall characteristics 1. Size and shape : Raindrops have sizes ranging from 0.5 to up to approximately 6 mm mean diameter, above which they tend to break up. 2. Intensity, duration, and depth Intensity (i) : amount of rain in a certain time ( (depth/time) i.e. (mm/hr Duration (t) : time at which rain occurs.(hr). Depth (d) : Volume of rain over an area (d= Vol./A) d = i * t 18

The( i and t ) are usually inversely related, i.e., high intensity storms are likely to be of short duration, and low intensity storms can have a long duration. 3. Intensity and area We can expect a less intense rainfall over a large area than we can over a small area. 4. Intensity and drop size High intensity storms have a larger drop size than low intensity storms. 19

20 Precipitation is expressed in terms of the depth to which rainfall water would stand on an area if all the rain were collected on it. Thus 1 cm ( 10 mm) of rainfall over a catchment area of 1 km 2 : represents a volume of water equal to 10 4 m 3 The precipitation is collected and measured in a rain gage. MEASUREMENT

21 For setting a rain gauge the following considerations are important: 1. The ground must be level and in the open and the instrument must present a horizontal catch surface. 2. The gauge must be set as near the ground as possible to reduce wind effects. 3. The instrument must be surrounded by an open fenced area of at least 5.5 m x 5.5 m. 4. No object should be nearer to the instrument than 30 m or twice the height of the obstruction. Rain gage Setting :

22 Non-recording Gauges

23 Rain Gauge A rain gauge is a weather tool used to collect rain. Using measurements on the side of the rain gauge, you can see how many inches (or mm) it rained.

24 Tipping—Bucket Type Weighing—Bucket Type Float — Syphon Type Recording Gauges

25 1. In flat regions of temperate, Mediterranean and tropical zones (According to WMO) : Ideal – 1 station for 600 – 900 km 2 Acceptable – 1 station for 900 – 3000 km 2 2. In mountainous regions of temperate, Mediterranean and topical zones: Ideal - 1 station for 100 —250 km 2 Acceptable - 1 station for 250 —1000 km 2 3. In arid and polar zones: 1 station for 1500—l0,000 km 2 4. In Islands 1 station for 25 km 2 ( depending on the feasibility.) RAINGAGE NETWORK

26 Adequacy of Rain gauge Stations P i = precipitation magnitude in the i th station. σ = standard deviation.

27 EXAMPLE A catchment has 5 rain gauge stations. In a year, the annual rainfall recorded by the gauges are as follows:- StationABCDE Rainfall (mm) For a 10% error in the estimation of the mean rainfall, calculate the optimum number of stations (N) in the catchment Solution:- from first data

28 ANALYSIS OF PRECIPITATION DATA Estimation of Missing Data Given the annual precipitation values, P1, P2, P3,..., P m at neighboring M stations 1,2,3,.., respectively, It is required to find the missing annual precipitation Px at a station X not included in the above M stations 1. Normal Analysis : If the normal annual precipitations at various stations are within about 10% of the normal annual precipitation at station X:

29 where N = optimal number of stations, ε = allowable degree of error in the estimate of the mean rainfall, and Cv = coefficient of variation of the rainfall values at the existing m stations (in per cent) Adequacy of Rain gauge Stations

30 Ni = Normal Precipitation in i station. Estimation of Missing Data If the normal precipitations vary considerably (>10%) ANALYSIS OF PRECIPITATION DATA

31 PREPARATION OF DATA Test for Consistency of Record Some of the common causes for inconsistency of record are: (i) shifting of a rain gauge station to a new location, (ii) the neighborhoods of the station undergoing a marked change, (iii) change in the ecosystem due to calamities, such as forest fires, land slides, and (iv) occurrence of observational error from a certain date.

32 PREPARATION OF DATA Test for Consistency of Record – use : DMC Accumulated Annual Rainfall of 10 stations Mean ΣP in units of l0 3 cm Accumulated Annual Rainfall at x ΣP in units of l03 cm

33 PRESENTATION OF RAINFALL DATA Hyetograph

34 MEAN PRECIPITATION OVER AN AREA 1. Arithmetical—Mean Method

35 MEAN PRECIPITATION OVER AN AREA 2. Thiessen-Mean Method

36 MEAN PRECIPITATION OVER AN AREA 3. Isohyetal Method

37 DEPTH-AREA—DURATION RELATIONSHIPS Depth-Area Relation where = average depth in (cm.) over an area (A in km 2 ), P o = highest amount of rainfall in (cm.) at the storm center, and K and n are constants for a given region

38 DEPTH-AREA—DURATION RELATIONSHIPS Maximum Depth-Area-Duration Curves

39 FREQUENCY OF POINT RAINFALL If the probability of an event occurring is P, the probability of the event not occurring in a given year is q = (1 - P) where P r,n = probability of a random hydrologic event (rainfall) of given magnitude and exceedence probability P occurring r times in n successive years

FREQUENCY OF POINT RAINFALL example, (a) The probability of an event of exceedence probability P occurring 2 times in n successive years is (b) The probability of the event not occurring at all in, successive years is (c) The probability of the event occurring at least once in n successive years 40

MethodP Californiam/N Hazen(m-0.5)/N Weibullm/(N+1) Chegodayev(m-0.3)/(N+0.4) Blom(m-0.44)/(N+0.12) Gringorten(m-3/8)/(N+1/4) 41 FREQUENCY OF POINT RAINFALL Plotting Position

42 FREQUENCY OF POINT RAINFALL

43 For a station A, the recorded annual 24 hr maximum rainfall are given below. (a)Estimate the 24 hr maximum rainfall with return period of 13 and 50 year. (b) What would be the probability of a rainfall of magnitude equal to or exceeding 10 cm occurring in 24 hr at station A.

Year Rain-Fall cm Year Rain-Fall cm TABLE 2.3 ANNUAL MAXIMUM 24 h RAINFALL AT STATION A

45 Probability m Rainfall (cm) Return period T=1/P Years mRainfall (cm) Return period T=1/P Years Probability

46 INTENSITY-DURATION-FREQUENCY RELATIONSHIP where K, x, a and n are constants for a given catchment

47 INTENSITY-DURATION-FREQUENCY RELATIONSHIP

48 INTENSITY-DURATION-FREQUENCY RELATIONSHIP

PROBABLE MAXIMUM PRECIPITATION (PMP) where = mean of annual maximum rainfall series, σ = standard deviation it the series and, K = a frequency factor 49