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

Precipitation 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.

Formation of precipitation : (i) The atmosphere must have moisture (ascending humid air ) , (ii) There must be sufficient nuclei (0.1- 10 Mm) present to aid condensation( example : Salt , oxides, CO2 ) , (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

Drizzle FORMS OF PRECIPITATION 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.

Rain ( 0.5 mm < size < 6 mm ) FORMS OF PRECIPITATION Rain ( 0.5 mm < size < 6 mm ) 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(<2.5) - Moderate - Heavy(>7.6) Rainfall intensity ~ 2.5 mm/hr – 7.6 mm/hr

Snow FORMS OF PRECIPITATION 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/cm3 . It is usual , to assume : an average density of 0.1 gm/cm3.

FORMS OF PRECIPITATION Glaze 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 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.

Hail FORMS OF PRECIPITATION 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.

WEATHER SYSTEMS FOR PRECIPITATION Front 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.

1. Cyclonic Precipitation WEATHER SYSTEMS FOR PRECIPITATION 1. Cyclonic Precipitation 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.

2. Convective Precipitation WEATHER SYSTEMS FOR PRECIPITATION 2. Convective Precipitation 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.

Warm front Convective Precipitation

3. Orographic Precipitation WEATHER SYSTEMS FOR PRECIPITATION 3. Orographic Precipitation 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

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

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.

MEASUREMENT 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 km2 : represents a volume of water equal to104 m3 The precipitation is collected and measured in a rain gage .

Rain gage Setting : 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. The instrument must be surrounded by an open fenced area of at least 5.5 m x 5.5 m. No object should be nearer to the instrument than 30 m or twice the height of the obstruction.

Non-recording Gauges

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.

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

RAINGAGE NETWORK 1. In flat regions of temperate, Mediterranean and tropical zones (According to WMO) : Ideal – 1 station for 600 – 900 km2 Acceptable – 1 station for 900 – 3000 km2 2. In mountainous regions of temperate, Mediterranean and topical zones: Ideal - 1 station for 100 —250 km2 Acceptable - 1 station for 250 —1000 km2 3. In arid and polar zones: 1 station for 1500—l0,000 km2 4. In Islands 1 station for 25 km2 ( depending on the feasibility .)

Pi = precipitation magnitude in the ith station. Adequacy of Rain gauge Stations Pi = precipitation magnitude in the ith station. σ = standard deviation.

EXAMPLE Station A B C D E Rainfall (mm) 82.6 102.9 180.3 98.8 136.7 A catchment has 5 rain gauge stations. In a year, the annual rainfall recorded by the gauges are as follows:- Station A B C D E Rainfall (mm) 82.6 102.9 180.3 98.8 136.7 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

ANALYSIS OF PRECIPITATION DATA Estimation of Missing Data Given the annual precipitation values, P1, P2, P3,. . ., Pm 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:

Adequacy of Rain gauge Stations 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)

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

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.

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

PRESENTATION OF RAINFALL DATA Hyetograph

MEAN PRECIPITATION OVER AN AREA 1. Arithmetical—Mean Method

MEAN PRECIPITATION OVER AN AREA 2. Thiessen-Mean Method

MEAN PRECIPITATION OVER AN AREA 3. Isohyetal Method

DEPTH-AREA—DURATION RELATIONSHIPS Depth-Area Relation where = average depth in (cm.) over an area (A in km2 ), Po = highest amount of rainfall in (cm.) at the storm center , and K and n are constants for a given region

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

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 Pr,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

FREQUENCY OF POINT RAINFALL Plotting Position Method P California m/N Hazen (m-0.5)/N Weibull m/(N+1) Chegodayev (m-0.3)/(N+0.4) Blom (m-0.44)/(N+0.12) Gringorten (m-3/8)/(N+1/4)

FREQUENCY OF POINT RAINFALL

For a station A, the recorded annual 24 hr maximum rainfall are given below. 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.

TABLE 2.3 ANNUAL MAXIMUM 24 h RAINFALL AT STATION A Year 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 Rain-Fall cm 13.0 12.0 7.6 14.3 16.0 9.6 8.0 12.5 11.2 8.9 7.8 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 9.0 10.2 8.5 7.5 6.0 8.4 10.8 10.6 8.3 9.5

m Rainfall (cm) Return period T=1/P Years 1 16.0 0.043 23.00 12 9.0 0.522 1.92 2 14.3 0.087 11.50 13 8.9 - 3 13.0 0.013 7.67 14 0.609 1.64 4 12.5 0.174 5.75 15 8.5 0.652 1.53 5 12.0 0.217 4.60 16 8.4 0.696 1.44 6 11.2 0.261 3.83 17 8.3 0.739 1.35 7 10.8 0.304 3.29 18 8.0 0.783 1.28 8 10.6 0.348 2.88 19 7.8 0.826 1.21 9 10.2 0.391 2.56 20 7.6 0.870 1.15 10 9.6 0.435 2.30 21 7.5 0.913 1.10 11 9.5 0.478 2.09 22 6.0 0.957 1.05 Probability Probability

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

INTENSITY-DURATION-FREQUENCY RELATIONSHIP

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