Type of Precipitation

Transcription

1 Precipitation Type of Precipitation Measurement of rainfall Location of rain gauges Categorisation Climate Estimation of basin rainfall Finding Average rainfall, Standard deviation, and Coefficient of variation(%) in a basin Rainfall analysis Recurrence interval or return period Depth Area and duration curves Mass curve and Hyetograph Maximum Depth-Duration & intensity curves

2 Type of Precipitation Rain Rain occurs when tiny cloud droplets collide to form bigger droplets. This keeps happening until the droplet is two heavy for the air to support it. The droplet then begins to fall, colliding with more cloud droplets as it gains in Hail size. Hail is a product of very intense thunderstorms. Drops of water will rise up with the upward directed wind as they collide with other droplets and grow larger. This will eventually result in the droplet freezing into a hailstone. Snow Snow occurs when the layer of the atmosphere from the surface of the earth through the cloud is entirely below freezing. The precipitation falls from the cloud as snow and does not melt at all while

3 Type of Precipitation The precipitation may be due to i. Thermal convection: The air close to warm earth gets heated and rises due to its low density and suddenly cools and falls as a thunder storm.

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5 Measurement of rain Non Recording

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7 Recording/ Automatic rain gauges Radio reporting rain gauges Radars

8 Location of rain gauges Rain gauges must be located to avoid exposure to wind effect or interception by trees or buildings. The best location is an oen plan ground like air ports Density of rain gauges Plains Elevated regions one in 520 km^2 one in 520 km^2 Hilly and very heavy rainfall areas one in 130 km^2 and 10% should be automatic raingauges In India on an average In developed countries 1 on 500 km^2 1 in 100 km^2

9 Categorisation Climate If the average rain fall < 40 cm : Arid climate If the average rainfall 40 to 75 cm :Semi arid climate If the average rainfall is more than 75 cm : humid climate The Normal rainfall is for a period of 35 years

10 Estimation basin rainfall Estimation of Areal Precipitation A single point precipitation measurement is quite often not representative of the volume of precipitation falling over a given catchment area. A dense network of point measurements can provide a better representation of the true volume over a given area. Various mean areal precipitation computation techniques yield the following results. 2.21" = Arithmetic Mean 1.90" = Isohyetal Analysis 2.03" = Thiessen Polygon 1.63" = Distance Weighting 1.94= AVERAGE of averges

11 (1) Arithmetic Mean - This technique calculates areal precipitation using the arithmetic mean of all the point or areal measurements considered in the analysis Arithmetic Mean Station RF( ) a 0.55 b 0.87 c 2.33 d 5.4 e

12 2) Isohyetal Analysis This is a graphical technique which involves drawing estimated lines of equal rainfall over an area based on point measurements. The magnitude and extent of the resultant rainfall areas of coverage are then considered versus the area in question in order to estimate the areal precipitation value 1. Measure full squares 2. Measure ½ 1 Take a graph paper squares 2. Trace the Area on the graph 3. Measure ¼ squares paper 4. Total all squares 3. If the scale of the map is and estimate the 1:50000 area ie 1cm = cm = 500 m ie sq.cm = 500*500= sq.m =25 ha = 0.25 sq.km

13 Weighted average(isohyetal Method) RF RF(') Range(") Area(sq. km) Weighted average

14 3) Thiessen Polygon - This is another graphical technique which calculates station weights based on the relative areas of each measurement station in the Thiessen polygon network. The individual weights are multiplied by the station observation and the values are summed to obtain the areal average precipitation. Station AREA RF Weighted a b c d e

15 4) Distance Weighting/Gridded - This is another station weighting technique. A grid of point estimates is made based on a distance weighting scheme. Each observed point value is given a unique weight for each grid point based on the distance from the grid point in question. The grid point precipitation value is calculated based on the sum of the individual station weight multiplied by observed station value. Once the grid points have all been estimated they are summed and the sum is divided by the number of grid points to obtain the areal average precipitation. Statio Distanc n e(d) 1/d^2 weight RF a b c d e Weighted average rf

16 Finding Standard deviation, and Coefficient Station No of Rain gauges(n)=(cv/p)^2 N= No of rain gauges required Cv= Coefficient of variation(%) Rainfall (x) cm A 88 B 104 C 138 D 78 E 56 Average 92.8 p= percentage of error allowed (%) N= 11 of variation(%) Calculation of average, standard deviation and coefficient of variation Station B C D E Total Average Standard deviation (sigma) Rainfall (x) cm Avera ge x- xavera ge 92.8 No of readings= Coefficient of Variation ( Cv) % (x-xaverage)^2 A SD= sqrt((xxaverage)^2/( n-1)) CV(%)= (SD/ xaverage)*

17 Estimation of Missing Data STATION NORM Year AL 1975 RF(N) A B C D M 3 Nx/M Variation from Normal ( %) Ratios ,=SUM(E3:E5),=B8*E6,=B6/(B7) if the variation of normal rainfall of the missing station is less than 10%, then Px=(1/M)*(p1+p2+p3) if the variation of normal rainfall of the missing station ismore than 10%, then Px=(Nx/M)*(p1/n1+p2/n2+p3/n3)

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19 1. Shifting rain gauge station 2. Neighbourhood changed markedly 3. Change of ecosystem due to calamities 4. Error in measuring due to change of people etc

20 year annual rf M (Pm) Average rainfall of 10 neighbouring stations P(av)

21 year annual rf M(Pm) Cummulaative RF Average rainfall of 10 Cummulaative (mm) neighbouring stations (Pav) (mm) RF (avg)

22 Cummulative RF of Station Relation ship between Average and Station Cummu_ RF Cummulative RF of 10 st (Avg) Chart Title y = x Cummu_ RF Linear (Cummu_ RF) y = x Series1 Linear (Series1)

23 year annual rf Cummula Average rainfall of 10 neighbouring CummulaativAdjusted Finalised M(Pm) (mm) ative RF stations (Pav) (mm) e RF (avg) values of Pm values of Pm

24 Sr.No Year Rainfall (c m) Year Rainfall (cm) Rainfall analysis and graphical presentation Let us calculate Mean Median Moving Average Return period /Frequency /Probability of certain rainfall Mean Median-42

25 Rainfall (cm) Year Rainfall (cm) 5 year moveing average Moving Average 5 Year Moving average of rainfall Year Rainfall (cm) 5 year moveing average average

26 Year Rainfal l(mm) average Difference Cum.Diffe rence Rainfall analysis Year Rainfall Year Rainfall Year Rainfall

27 Recurrence interval / return period/ Frequency of rainfall If there are 30 or 40 year rainfall data, they may be arranged in descending order of their magnitude. If there are n items and their order no or rank of any particular storm is m, then their recurrence interval T or return period can be calculated as under California method T= n/m Hazen s method T= n/(m-0.5) Kimbal s method T= (n+1)/m The frequency (f) ( as percentage of time) of that storm magnitude having recurrence period T) is given by F=1/T*100 (%) Year Rainfall (cm)

28 Annual Ranfall (cm) Year Rainfall (cm) n=21 Rank No (m). No of times >=P Return period (T) (n+1)/m Frequency (f=(1/t* Recurrence interval / return period/ Frequency of rainfall Rainfall frequency Vrs Rainfall y = Ln(x) Frequency (%) F(%) P T RF Series1 Log. (Series1)

29 Year Rainfall (cm) Rank No (m). No of times >=P Return period (T) (n+1)/m Frequency (f=(1/t)* Total 678 n= 11

30 Rainfall(cm) Rainfall Analysis (Mass curve) Time since Commence ent of storm (mts) Accumula ted Rainfall (cm) Mass curve Time since commencement of storm (mts) Accumulated Rainfall (cm)

31 Time since Commence ent of storm (mts) Accumulated Rainfall (cm) Intensity (cm/hr) delta t (mts) delta P (cm) Rate of Rainfall (cm/hr) delta p / delta t * Hyetograph Time (mts)

32 Maximum depth duration- Intensity curve Time Since Comm ncemet of stotm (mts) Rainfall analysis Accumul ated Rainfall (cm) Delta T(mt) Delta P(cm) Rate of Rain Fall (cm/hr) delta p/ delta t * Depth cm) Delta p/delta t *60 (cm/hr)-intensity Min Max 0.1 Maximum depth-duration precipitation(cm) of rainfall in minutes

33 Maximum Depth & Intensity of Rainfall (cm) Duration ( mts) Maximum Depth-Duration & intensity curves Max.Depth (cm) Max Intensity (cm/hr) Maximum Depth-Duration and Depth- intensity of precipitation Duration (mts) Max.Depth (cm) Max Intensity (cm/hr)

34 Y cm Maximum Depth-Duration & intensity curves Maximum Depth Duration Intensity curves 10 y = Ln(x) Max.Depth (cm) Max Intensity (cm/hr) Log. (Max.Depth (cm)) Log. (Max Intensity (cm/hr)) y = Ln(x) Time (mts)

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36 . Western Himalayan Region: J&K, HP, UP, Utranchal 2. Eastern Himalayan Region: Assam Sikkim, W.Bangal & all North-Eastern states 3. Lower Gangetic Plains Region: W.Bangal 4. Middle Gangetic Plains Region: UP, Bihar 5. Upper Gangetic Plains Region: UP 6. Trans-Gangetic Plains Region: Panjab, Haryana, Delhi & Rajasthan 7. Eastern Plateau and Hills Region: Maharastra, UP, Urissa & W.Bangal 8. Central Plateau and Hills Region: MP, Rajasthan, UP 9. Western Plateau and Hills Region: Maharastra, MP & Rajasthan 10. Southern Plateau and Hills Region: AP, Karnatak, Tamil Nadu 11. East Coast Plains and Hills Region: Urissa, AP, TN,& Pondicheri 12. West Coast Plains and Ghat Region : TN, Keral, Gowa, Karnatak, Maharastra 13. Gujarat Plains and Hills Region: Gujrat 14. Western Dry Region: Rajasthan 15. The Islands Region: Andman & Nicaobar, Lakshya Deep

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