Presentation on theme: "Introduction to Precipitation"— Presentation transcript:
1Introduction to Precipitation CEE 4420 – Engineering Hydrology(Prepared by Abebe Gebregiorgis)
22.1 IntroductionAll 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.
3Introduction…. 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 saturationThe water exists in the atmosphere in these three aggregation states.
4Introduction…. Types of precipitation Rain: Rain, snow, hail, drizzle, glaze, sleetRain:Is precipitation in the form of water drops of size larger than 0.5 mm to 6mmThe rainfall is classified in toLight rain – if intensity is trace to 2.5 mm/hModerate – if intensity is 2.5 mm/hr to 7.5 mm/hrHeavy rain – above 7.5 mm/hr
5Introduction…. Snow: Hail (violent thunderstorm) Snow is formed from ice crystal masses, which usually combine to form flakesHail (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.
62.2 Temporal and Spatial Variation of Rainfall Rainfall varies greatly both in time and spaceWith respect to time – temporal variationWith space – Spatial variationThe temporal variation may be defined as hourly, daily, monthly, seasonal variations and annual variation (long-term variation of precipitation)
92.3. 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.
112. 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 gauge1-receiver2-floater3-siphon4-recording needle 5-drum with diagram6-clock mechanism
123. 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
13Tele-rain gauge …… The tele-rain-gauge 1 - collecting funnel 2 - tilting baskets3 - electric signal4 - evacuation
144. 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 accuracyThe 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.
15Raingauge NetworkSince 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 extentTopographic & accessibility to some extent.
16World Meteorological Organization (WMO) recommendation: Raingauge Network…..World Meteorological Organization (WMO) recommendation:In flat regions of temperate, Mediterranean and tropical zonesIdeal 1 station for 600 – 900 km2Acceptable 1 station for 900 – 3000 km2In mountainous regions of temperate , Mediterranean and tropical zonesIdeal 1 station for 100 – 250 km2Acceptable 1 station for 250 – 1000 km2In arid and polar zone1 station for 1500 – 10,000 km210 % of the raingauges should be self recording to know the intensity of the rainfall
172.4 Preparation of DataBefore using rainfall data, it is necessary to check the data for continuing and consistencyMissing dataRecord errorsEstimation of Missing DataGiven annual precipitation values – P1, P2, P3,… Pm at neighboring M stations of station X 1, 2, 3 & m respectivelyThe normal annual precipitation given by N1, N2, N3,…, Nm, Ni… (including station X)To find the missing precipitation, Px , of station X
18Test 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 selectedArrange 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 figureA 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
19Pcx – corrected precipitation at any time period t1 at stationX Test for consistency record….acPcx – corrected precipitation at any time period t1 at stationXPx – Original recorded precp. at time period t1 at station XMc – corrected slope of the double mass curveMa – original slope of the mass curve
202.5 Mean Precipitation over an area Raingauges rainfall represent only point sampling of the areal distribution of a stormThe important rainfall for hydrological analysis is a rainfall over an area, such as over the catchmentTo convert the point rainfall values at various stations to in to average value over a catchment, the following methods are used:arithmetic meanthe method of the Thiessen polygonsthe isohyets method
21Applicable rarely for practical purpose Arithmetic Mean MethodWhen 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
22The influence zones are represented by convex polygons. Method of Thiessen polygonsThe 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
25Thiessen polygons ……….Generally for M stationThe ratio is called the weightage factor of station i
26An isohyet is a line joining points of equal rainfall magnitude. Isohyetal MethodAn isohyet is a line joining points of equal rainfall magnitude.10.08Da56C129.212a4a37.0B47.2AEa210.09.14.0a1a1F864
27P1, P2, P3, …. , Pn – the values of the isohytes Isohyetal MethodP1, P2, P3, …. , Pn – the values of the isohytesa1, a2, a3, …., a4 – are the inter isohytes area respectivelyA – the total catchment area- the mean precipitation over the catchmentNOTEThe isohyet method is superior to the other two methodsespecially when the stations are large in number.
282.6 Intensity – Duration – Frequency (IDF) Relationship Mass Curve of Rainfall1st storm, 16 mm2nd storm, 16 mm
29HyetographIDF ….is a plot of the accumulated precipitation against time, plotted in chronological orderTotal depth = 10.6 cmDuration = 46 hr
30IDF ….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