Lecture No. 2 - PRECIPITATION.pdf

CIEN 3123 ENGR. REN  In meteorology, the term precipitation is any product of the condensation of atmospheric water vapor that falls under gravity.  It includes all forms of water coming from the atmosphere to the earth.  Precipitation is one of the major elements of hydrology after it falls o...

CIEN 3123 ENGR. REN  In meteorology, the term precipitation is any product of the condensation of atmospheric water vapor that falls under gravity.  It includes all forms of water coming from the atmosphere to the earth.  Precipitation is one of the major elements of hydrology after it falls on the ground because it is the source of water on the earth.  Precipitation can fall in either liquid or solid phases, or transition between them at the freezing level. FORMS AND TYPES OF PRECIPITATION  Precipitation occurs in many forms e.g. drizzle, rain, glaze, sleet, snow, hail, dew and frost, depending upon the causes and temperature at the time of formation. Dew is condensation on the ground of atmospheric vapor caused by radiational cooling of the lower layers of atmosphere, usually at night. Frost is dew formed under freezing conditions. Dew and frost are quantitatively unimportant and rarely measured.  Drizzle: Drop size < 0.5 mm in dia. and intensity is usually < 1 mm/hr and generally occurs in conjunction with warm frontal lifting.  Rain: Drop size is between 0.5 to 6 mm in dia. Drops bigger than 6 mm tend to break up as they fell. It is formed by condensation and coalescence of cloud droplets at temperatures above the freezing point.  Glaze: It is the ice coating formed when drizzle or rain freezes as it comes in contact with cold objects on the ground.  Sleet: It is frozen raindrops cooled to ice stage while falling through air at sub-freezing temperature.  Snow: It is a precipitation in the form of ice crystals resulting from sublimation, i.e., directly from water vapor to ice.  Snow Flake: It is made of a number of ice crystals fused to gather.  Hail: It is precipitation in the form of balls or lumps of ice over 5 mm diameter formed by alternate freezing and melting as they are carried up and down in highly turbulent air currents. The following four conditions are necessary for the production of rainfall.:  Mechanism to produce cooling of the air – The pressure reduction due to ascending air from surface to upper levels in the atmosphere is the only known mechanism capable of producing large drops in the temperature.  Mechanism to produce condensation – Condensation in the atmosphere takes place on “hygroscopic nuclei” small particles of substances that have an affinity for water. The following four conditions are necessary for the production of rainfall.:  Mechanism for droplet growth – A tendency for the droplets to remain small and therefore to resist falling is called “colloidal stability”. The most effective processes for droplet enlargement are, The difference in speeds between large and small droplets, and The co-existence of ice crystals and water droplets.  Mechanism to produce accumulation of moisture of sufficient intensity to account for the observed rates of rainfall – Regardless of whether or not the other conditions for precipitation are fulfilled, continuity considerations demand that there must be a good amount of moisture present in the atmosphere so that evaporation losses between ground and cloud be compensated, if there is to be appreciable rain. There are three major types of precipitation: cyclonic, convective, and orographic. Each type represents a different method of lifting an air mass, resulting in cooling and condensation of atmospheric water vapor. It is caused by lifting associated with the horizontal convergence of inflowing atmosphere into an area of low pressure. There are two kinds of cyclonic precipitation. Non-frontal precipitation involves only this convergence and lifting. Frontal precipitation results when one air mass is lifted over another. A front is defined as the boundary between two air masses of different temperatures and densities. A warm front is the result of a warm air mass overriding a cold air mass, causing extensive areas of cloudiness and precipitation. As the warm front approaches a given area, the precipitation becomes more continuous and intense. Warm fronts move at a speed of 15-50 km/h (10-30 mph). A cold front results from a strong push of a cold air mass against and beneath a warm air mass. At the front towering clouds develop together with intense short duration precipitation. Cold fronts move at a speed of 30-80 km/h (20-50 mph). An occluded front occurs when a cold front overtakes a warm front. The precipitation pattern is a combination of both warm and cold frontal distribution. Occluded fronts move at a speed of from 8-50 km/h (5-30 mph). https://youtu.be/UKHXfoKZxE4?si=C0Tw_ImD-U4T1_ry It results when air that is warmer than its surrounding rises and cools. The precipitation is of a shower type, varying from light showers to cloudbursts. The typical thunderstorms resulting from heating of the atmosphere in the afternoon hours is the best example of convective rainfall. Thunderstorms occur throughout the world, especially in the summer. They are the characteristic form of rain in the tropics, wherever cyclonic circulation does not operate. Thunderstorm It is caused when air masses are lifted as they move over mountain barriers. Such orographic barriers tend to increase both cyclonic and orographic precipitation due to the increased lifting involved. Precipitation is generally heavier on the windward slope than on the leeward slope. Orographic Precipitation RAINFALL MEASUREMENT Introduction Mostly estimates of runoff are made based on rainfall data, amount and intensity of rainfall should be known by measurement. Rainfall data is also required for calculating irrigation requirements. Raingage: The purpose of the raingage is to measure the depth and intensity of rain falling on a flat surface without considering infiltration, runoff or evaporation. The problems of measurements include effects of topography, nearby vegetation and the design of gage itself. There are mainly two types of raingages (non-recording and recording). 1. Non-recording Gage The standard raingage, known as Symon’s gage is recommended and installed by the Indian Meteorological Department. This is a vertical, cylindrical container with top opening 127 cm in diameter. A funnel shaped hood is inserted to minimize evaporation losses. The water is funneled into an inner cylinder. Considerations for Installation 1. The site should be an open place, 2. The distance between the raingage and the nearest object should be at least twice the height of the object, 3. As for as possible it should be a level ground, 4. In the hills, the site should be so chosen where it is best shielded from high winds and wind does not cause eddies, and 5. If a fence is erected, it should be atleast at a distance of twice the height. 2. Recording or Automatic Raingage.  Weighting Bucket Type Raingage - This gage weighs the rain, which falls into a bucket set on a platform of a spring or level balance. The increasing weight of bucket and its counts are recorded on the chart held by a clock driven drum. The record shows the accumulation of precipitation with time in the shape of a mass curve of precipitation. The gage must be serviced about once a week when the clock is re-wound and the chart is replaced. For high rainfall, the recording mechanism reverses the direction of record immediately on reaching the upper edge of the recording chart. Recorded mass curve of precipitation in weighing bucket type rain gage. 2. Recording or Automatic Raingage.  Tipping Bucket Type Raingage - The tipping bucket raingage consists of a 30 cm diasharp edge receiver. At the end of the receiver a funnel is provided. A pair of buckets are pivoted under the funnel in such a way that when one bucket receives 0.25 mm of rainfall it tips, discharging its contents in to a tank bringing the other bucket under the funnel. Tipping of the bucket completes an electric circuit causing the movement of a pen to mark on a clock driven revolving drum which carries a record sheet. 2. Recording or Automatic Raingage.  Siphon Type Automatic Rainfall Recorder – In the siphon gage, also known as the float type of recording raingage, the rain is fed into a float chamber containing a light, hallow float. The vertical movement of the float, as the level of water rises, is transmitted by a suitable mechanism in to the movement of the pen on a revolving chart. By suitably adjusting the dimensions of the receiving funnel, float and float chamber, any desired scale value on the chart can be obtained. Siphoning arrangement is provided for emptying the float chamber quickly whenever it becomes full, the pen returns to the bottom of the chart. Errors in Rainfall Measurements There are three main sources of errors in rainfall measurements a) Instrumental defects, b) Improper sitting (location) of the gage, and c) Human errors Raingage Network There is no single answer to determining the mean areal rainfall because it is affected by so many factors. However, the denser the gage network, the more accurate is the representation. Gages are not evenly spaced, high variability areas have more gages and relatively uniform rainfall areas have fewer gages. In addition, costs of installation, maintenance of the network, as well as its accessibility to the observer, are also important consideration. In general, the sampling errors of rainfall tends to increase with increasing mean areal rainfall, and decrease with increasing network density, duration of rainfall, and areal extend. Accordingly, larger average errors are produced by a particular network for storm rainfall than for monthly, seasonal or annual rainfall. The adequacy of an existing rain-gage network of a watershed is assessed statistically. The optimum number of raingages corresponding to an assigned percentage of error in estimation of mean areal rainfall can be obtained as: Where, N is the optimum number of raingages, CV is the coefficient of variation of the rainfall values of the gages, and is the assigned percentage of error in estimation of mean areal rainfall. In which is the mean rainfall defined as and S is the standard deviation of rainfall computed as Where, m is the number of raingages in the watershed recording P1, P2… Pm values of rainfall for fixed time interval. Generally, value of P is taken as 10%. Sample Problem A catchment has six raingage stations. In a year, the annual rainfalls recorded by the gages are as follows: Stations A B C D E F Rainfall (cm) 82.6 102.9 180.3 110.3 98.8 136.7 For a 10% error in the estimation of mean rainfall, calculate optimum number of stations in the catchment. Standard Deviation of 35.04 Answer: N = 8.7 = 9 stations PRESENTATION OF RAINFALL DATA Mass Curve of Rainfall The mass curve of rainfall is a plot of the accumulated precipitation against time, plotted in chronological order. Records of float type and weighing-bucket type gauges are of this form. Mass curves of rainfall are very useful in extracting the information on the duration and magnitude of a storm. Also, intensities at various time intervals in a storm can be obtained by the slope of the curve. For non-recording rain gauges, mass curves are prepared from knowledge of the approximate beginning and end of a storm and by using the mass curves of adjacent recording gauge stations as a guide Mass curve of rainfall Hyetograph A hyetograph is a plot of the intensity of rainfall against the time in the hyetograph is derived from the mass curve and is usually represented as a bar chart. It is a very convenient way of representing characteristics of a storm and is particularly important in the development of a design storms to predict extreme floods. The area under a hyetograph represents the total precipitation received in that period. The time used depends on the purpose; in urban-drainage problems small durations are used while in flood-flow computations in larger catchments the intervals are about 6 hours. Hyetograph of a storm Depth-Area Relation For a rainfall of a given duration, the average depth decreases with the area in an exponential function given by: Where, is average depth in cms over an area A km²,is highest amount ofrainfall in cm at the storm centre and K and n are constant for a given region. Maximum Depth-Area-Duration Curves In many hydrological studies involving estimation of severe floods, it is necessary to have information on the maximum amount of rainfall of various duration occurring over various sizes of areas. The development of relationship, between maximum depth-area-duration for a region is known as DAD analysis and forms an important aspect of hydro- meteorological study. First the severe most rainstorms that have occurred in the region in the question are considered. Isohyetal maps and mass curves of the storm are compiled. A depth area curve of a given duration of the storm is prepared. Then from a study of the mass curve of rainfall, various durations and the maximum depth of rainfall in these durations are noted. The maximum depth-area curve for a given duration D is prepared by assuming the area distribution of rainfall for smaller duration to be similar to the total storm. The procedure is then repeated for different storms and the envelope curve of maximum depth-area for duration D is obtained. A similar procedure for various values of D results in a family of envelopes curve of maximum depth vs area, with duration as the third parameter.These curves are called DAD curves. In this the average depth denotes the depth averaged over the area under consideration. It may be seen that the maximum depth for a given storm decreases with the area; for a given area the maximum depth increases with the duration. Preparation of DAD curves involves considerable computational effort and requires meteorological and topographical information of the region. Detailed data on severe most storms in the past are needed. DAD curves are essential to develop design storms for use in computing the design flood in the hydrological design of major structures such as dams. Typical DAD Curves Interpolation of rainfall records Sometimes records may be lost from the measuring or monitoring station for a specific day or several days because of the absence of station operator (observer) or because of instrumental failure or malfunction or damage in the recording devices, for any other reason. In order not to lose information, it is best to use an appropriate way to estimate the amount of rain in these days in calculating monthly and annual totals. Procedure for these estimates are based depending on simultaneous records for three stations close to and as evenly spaced around the station with missing records as possible. Interpolation of rainfall records This station should be equi-distant from the three stations and the following conditions should be achieved: If the normal annual precipitation at each of the these stations is within ten percent of that of the station with missing records, a simple arithmetic average of the precipitation at the three stations is used for estimating missing record of the station. If the normal annual precipitation at any one of the three stations differs from that of the station with missing records by more than ten percent, the normal ratio method is used Simple Arithmetic Average Normal Ratio Method Methods used to find the height of rainfall Methods used to find the height of rainfall Include: Arithmetic mean method, Thiessen polygon method and isohyets method. Arithmetic Average Method This method consists of computing the arithmetic average of the values of the precipitation for all stations within the area. Since this method assigns equal weight to all stations irrespective of their relative location and other factors, it should be adopted in area where rainfall is uniformly distributed. Where average precipitation is over an area, P is the precipitations at individual station i, and n is the number of stations. Thiessen Polygon Method This is a graphical technique which calculates station weights based on the relative areas of each measurement station in the Thiessen polygon network.Rainfall varies in intensity and duration from one place to other; hence rainfall recorded by each station should be weighed according to the area (polygons) it is assumed to influence. The individual weights are multiplied by the station observation and the values are summed to obtain the areal average precipitation.This method is useful for areas, which are more or less plain and are of intermediate size (500 to 5000 km2). This method is also used when there are a few raingage stations compared to size. The polygons are formed as follows: 1. The stations are plotted on a map of the area drawn to a scale. 2. The adjoining stations are connected by the dashed lines. 3. Perpendicular bisectors are constructed on each of these dashed lines. 4. These bisectors form polygons around each station (effective area for the station within the polygon). For stations close to the boundary, the boundary lines form the closing limit of the polygons. Area of each polygon (Ai)is determined and the average precipitation is calculated using the following equation:imit of the polygons. Where A is the total area of the watershed. Thiessen Polygon Method Isohyetal Method This is a graphical technique which involves drawing estimated lines of equal rainfall over an area based on point measurements. Then multiply the area between each contour by the average precipitation in the area to get the rainfall volume in the area. Sum these volumes to get the total rainfall volume, and then divide the total rainfall volume by the area of the watershed to get the average areal precipitation in the watershed. Let’s take it step by step: Step1: Determine what contours of equal precipitation (called isohytes) you will use. This varies from situation to situation, but you want to have as many contours as necessary to get an accurate model, but not so many that your construction becomes cluttered. Step2: Draw a line between gauges that will be separated by isohyets. Step3: Plot points on those lines that correspond to the isohyets determined in Step 2. Step4: Now sketch the isohyets. Step5: Redraw the construction onto graph paper with the isohyetal lines. Then count the boxes between each of the isohyetal lines. Step6: Find the actual watershed area between each isohyet. These areas will be lettered starting with A at the top and moving alphabetically toward the bottom of the construction. Step7: Multiply the areas found in Step 6 by the average precipitation in the area. Step8: Divide the sum of the values found in Step 7 by the total area of the watershed to get the average rainfall in the area. SAMPLE PROBLEMS Problem No. 1 Answer : Px = 42.7mm Problem No. 2 Answer = 52mm Problem No. 3 Answer : Px = 212.5 mm  Problem No. 4 Answer : Average Precipitation = 121.8 mm Problem No. 5 Answer : X = 115.5 mm Problem No. 6 Answer : P = 3.63 in Problem No. 7 Answer : P= 3.18 in