Presentation on theme: "Analyses of Rainfall Hydrology and Water Resources RG744"— Presentation transcript:
1 Analyses of Rainfall Hydrology and Water Resources RG744 Institute of Space TechnologyOctober 05&11, 2013
2 Mean RainfallMean Annual Rainfall: determined by averaging the total rainfall of several consecutive years at a placeMean Monthly Rainfall: by averaging the monthly total rainfall for several consecutive yearsMean annual rainfall: minimum of years of continuous records is desirable.Deficit or surplus rainfall comparing with mean annual rainfall
3 Interception and Net Precipitation Vegetation influence on pattern of deposition and amount of precipitation reaching the soil surfaceInterception losses are less in arid and semiarid regions that have sparse vegetationNet precipitation is gross precipitation (measured by gauge) minus interception lossNet precipitation is the amount available either to replenish soil water deficits or to become surface, subsurface, or ground water flow.
4 Effective RainfallRainfall component causing perceptible change in the stream flowThat is only a portion of the total rainfall recorded over the catchmentRainfall that does not infiltrate the soil and moves into the stream as overland flowInfiltration capacity: High in dry soilNo overland flow if precipitation is lower than infiltration capacity (but interflow may occur).With persistence precipitation , the soil becomes wet and infiltration rate decreases causing runoff.4
5 Flood/Storm Frequency Flood frequency: number of times a particular flood/storm can be expected in a river/watershed in a given number of yearsExample: 100 year flood – a flood that can be expected once in 100 years. (usually worked out by extrapolating the observed data)Frequency is the percentage of years, during which a storm of a given magnitude may be equaled or exceededRecurrence interval/return period: Time interval after which a similar flood can be forecasted or expected.Return Period: Time interval after which a storm of given magnitude is likely to recur.
6 Frequency Analysis Objective: to develop a frequency curve Precipitation frequency curves can be developed to evaluate maximum eventsUsed for planning water resources structuresRelationship between the magnitude of events and either the associated probability or the recurrence intervalWeather systems vary year to year and hence magnitude of future events can not be predicted accuratelyHave to rely on statistical analyses of rainfall amounts over certain periodFrequency distribution of past eventsProbability or likelihood of having certain events occurring over a specified period is estimatedSource: Hydrology and the management of watersheds By Kenneth N. Brooks
7 Recurrence Interval of a Storm Number of years within which a given storm may equal or exceed once - Also known as return periodMeans this precipitation value or more than that occurs m times in n yearsRanking: The serial number of a specific value of precipitation in the descending order .Source: Hydrology Principles
8 Probability of Exceedance The probability of occurrence of a flood (having a recurrence interval T-year) in any year,Probability: Reciprocal of the return periodp = 1/ TExample source: Elementary Engineering Hydrology By Deodhar M. J.Frequency: Inverse of return period multiplied with 100 and expressed as %.
9 Frequency Frequency: Probability expressed in terms of percentage Frequency of a rainfall of a given magnitude = the number of times the given event may be expected to be equaled or exceeded in 100 yearsExample source: Elementary Engineering Hydrology By Deodhar M. J.Frequency: Inverse of return period multiplied with 100 and expressed as %.
10 Example:Example source: Elementary Engineering Hydrology By Deodhar M. J.
12 Hydrology and Management of Watershed by Kenneth. Frequency curve of daily rainfall for a single stationThe probability of having a 24 hr rainfall event of 100mm or more in any given year is about 0.02 (or a 50 year recurrence interval)
13 Prob (no occurrence in N years) = (1-p)N Once the frequency curve is developed, the probability of exceeding certain rainfall amount over a specified period can be determinedThe probability that an event with probability p will be equaled or exceeded x times in N years is determined by:If x=0 (no occurrence in N years) thenProb (no occurrence in N years) = (1-p)NTherefore;Prob (at least 1 occurrence in N years) = 1-(1-p)NExample: the probability of having a 24 hr rainfall event of 100mm or greater over a 20 year period is determined by (see solution in the end)
14 The percentage probabilities of floods (or rainfall) of different recurrence intervals (T) to occur in particular periods (N) are given in Table 8.4Source: Hydrology Principles
15 Intensity Duration Analysis Study of intensity and its duration is called Intensity Duration AnalysisUsually most intense storms last for shorter durationAs intensity reduces duration increases
16 Intensity Duration Curve Graph of duration vs. intensity for an areaNormally follows the following equationWhere;I = intensity in mm/hrt = duration in minutesC, a, b = constants for the specific areaAlso known as intensity duration graph.Source: Elementary Engineering Hydrology By Deodhar M. J.
17 Intensity Duration Curve Intensity Duration Curve prepared for a specific study
18 Example: A storm occurred over a catchment area as under: Time (min) Precipitation (mm) Plot maximum duration intensity curve?
19 Intensity Frequency Duration Analysis When sufficient data (say 50 years) for a catchment area is availableAnalyze data for each stormAnalyze for:IntensityFrequencyDurationDifferent graphs for different catchment areas depending on their hydrologic character
20 Intensity-Frequency-Duration Curve Source: Hydrology by Das and SaikiaTo know rainfall intensities of different duration and different return period
21 Isopluvial MapsCombined map for large area for maximum rainfall depth for various combination of a return period and durationThese are Isohyets shown on regional rainfall map
23 Depth-Area-Duration (DAD) Analysis Average depth of storm and its duration for a specific areaAlso called DAD studyHorton’s EquationPa =Poe-(KA)^nAreal spread of rainfall within watershed. Amount of high rainfall that may be expected over the catchment.Frequency analysis discussed before is based upon rainfall characteristics at a point or a specific location. The design of storage reservoir and other water resources related structures required that the watershed area to be taken into account. Because rainfall is usually not uniform, increasing the area of watershed the depth of rainfall with a certain probability will decrease.Every storm has a center having maximum precipitation.Po= maximum precipitation at the center of storm.Pa=Average Precipitation over a specific area.(Po-Pa) is positive value and its value increases with bigger catchment and decreases with smaller catchment.K & n are regression constants varies with duration of storm.
24 DAD Curve Based on records of several storms on an area Maximum areal precipitation for different durations corresponding to different areal extentsSource: Hydrology Principles
27 Depth-Area-Duration (DAD) Curve Based on records of several storms on an areaMaximum areal precipitation for different durations corresponding to different areal extents
28 Index of wetnessRatio of rainfall in a given year and average annual precipitationWhen it isLess than 1 - bad year/deficient year/ dry yearMore than 1 – good year / surplus year / wet yearEqual to 1 – normal yearSource: Deodhar chapter 4
29 Probable Extreme Rainfall Events (Standard Design Storms) Probable Maximum PrecipitationStandard Project StormFrequency based stormImportant for water resources engineering
30 Probable Maximum Precipitation (PMP) Assumed physical upper limit of rain that will fall over a specified area in a given time (that’s physically possible)Rainfall for a given area and duration that can be reached or exceeded under known meteorological conditionsUsed to provide an estimate of the Probable Maximum Flood (PMF) hydrographsFor design of major structure with the threat of loss of life PMP is used (where no risk of failure accepted)Source: usfAmount of rainfall over a region that can not be exceeded over at that place. The greatest depth that can occur in a given duration at a given locationHigher in hot humid regions.PMP is obtained by studying all the storms that have occurred over the region and maximizing them for the most critical atmospheric conditions (assuming maximum air moisture and maximum air inflow occurs simultaneously) .PMF estimated after appropriate adjustment for infiltration losses.
31 Standard Project Storm (SPS) Storm which is reasonably capable of occurring over the basin under consideration (actually occurred)The heaviest rain storm occurred in the region during the period of rainfall recordsUsed for design project with economic considerations and low riskSPS or SPF - Standard Project Flood
32 Frequency Based Stormi: Frequency analysis of long term stream flow data at a site of interestOrii: Frequency analysis of rainfall data coupled with rainfall- runoff model to get design flood (if flow data is not available)ii) When i data is not available
33 Solution to exampleSource: Hydrology and the Management of Watersheds by Brooks