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# Methods of measuring rainfall

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### Methods of measuring rainfall

1. 1. Cairo University Faculty of Engineering 4th Year Mechanical Power Engineering Department Hydraulic Power Station Submitted to: Dr Mahmoud khalifa Prepared by Wael Kamal Mohamed Mahmoud SEC:6 B.N:25
2. 2. CONTENTS PART1 Rainfall Measurement.......................................................................... 3 Introduction...................................................................................................... 3 Methods of Measuring Rainfall......................................................................... 3 Methods of Measuring Rainfall (Manual) ........................................................ 3 Methods of Measuring Rainfall (Remote) ........................................................ 3 Rain gauges ...................................................................................................... 4 Principles.......................................................................................................... 4 Types of rain gauges ....................................................................................... 5 1-Ordinary (Standard rain gauge) rain gauge............................................... 5 2-The weighing rain gauge.......................................................................... 5 3-Optical rain gauge.................................................................................... 6 4-Tipping bucket rain gauge........................................................................ 7 Number and Distribution of Gauges.................................................................. 7 Distribution of Gauges...................................................................................... 7 Number of Gauges............................................................................................ 8 Site Specific Rainfall Information..................................................................... 8 The weather station........................................................................................... 9 Ground-based radar equipment ....................................................................... 10 PART 2 Methods OF Measuring Runoff........................................................ 11 Runoff............................................................................................................ 11 Measurement and mathematical modeling ...................................................... 12 Runoff curve number...................................................................................... 13 Infiltration and surface runoff simulating extreme precipitation ...................... 14 Surface runoff with natural precipitation......................................................... 15 Runoff (flowthrough) in catchment areas used for farming ............................. 15 References ......................................................................................................... 16
3. 3. Rainfall Measurement Introduction At climatological and synoptic stations worldwide, it is standard procedure to measure rainfall at each scheduled hour, then calculate the total rainfall during the previous six-hour and twelve-hour period. Regular rainfall measurement is also an essential requirement in many aspects of agriculture, forestry, industry, education and other activities. Rainfall is rarely uniform in intensity or duration across a wide area, so continuous data on local conditions is of particular importance to farmers and those concerned with irrigation, scientists researching into crop performance and soil erosion, and to water and river authorities in respect of reservoir supplies and ground water feeding into rivers. Accurate measurement is also essential for calculating evaporation. The rain gauge is the standard method of collection. This has to be sited in a location, which is representative of the rainfall area to be measured. Often a network of sites will be integrated to cover average rainfall over a large area.Rainfall at any one place against time is expressed in terms of the depth to which it would cover the surface locally, assuming there was no loss by evaporation or run-off. The rain gauge is often used in conjunction with other instruments for a full picture of local climatic conditions. Methods of Measuring Rainfall A. Manual B. Remote Methods of Measuring Rainfall (Manual) Often have a funnel opening into a cylinder gaugeCome in a variety of shapes and sizes Calculate the rainfall (in mm) by dividing the volume of water collected by the area of the opening of the cup. Methods of Measuring Rainfall (Remote) Tipping bucket rain gauge -The bucket tips when precipitation of 0.2 mm, 0.5 mm, 1.0 mm has been collected. Each tip is recorded by a data logger.
4. 4. Weather Station - Records rainfall, but also evaporation, air pressure, air temperature, wind speed and wind direction (so can be used to estimate evapo- transpiration) Radar - Ground-based radar equipment can be used to determine how much rain is falling and where it is the heaviest. Rain gauges A rain gauge (also known as a udometer or a pluviometer [Pluviograph ] or a cup) is a type of instrument used by meteorologists and hydrologists to gather and measure the amount of liquid precipitation (as opposed to solid precipitation that is measured by a snow gauge) over a set period of time. Principles Most rain gauges generally measure the precipitation in millimeters. The level of rainfall is sometimes reported as inches or centimeters. Rain gauge amounts are read either manually or by AWS (Automatic Weather Station). The frequency of readings will depend on the requirements of the collection agency. Some countries will supplement the paid weather observer with a network of volunteers to obtain precipitation data (and other types of weather) for sparsely populated areas. In most cases the precipitation is not retained, however some stations do submit rainfall (and snowfall) for testing, which is done to obtain levels of pollutants. Rain gauges have their limitations. Attempting to collect rain data in a hurricane can be nearly impossible and unreliable (even if the equipment survives) due to wind extremes. Also, rain gauges only indicate rainfall in a localized area. For virtually any gauge, drops will stick to the sides or funnel of the collecting device, such that amounts are very slightly underestimated, and those of .01 inches or .25 mm may be recorded as a trace. Another problem encountered is when the temperature is close to or below freezing. Rain may fall on the funnel and freeze or snow may collect in the gauge and not permit any subsequent rain to pass through.
5. 5. Rain gauges, like most meteorological instruments, should be placed far enough away from structures and trees to ensure that any effects caused are minimised. Types of rain gauges 1-Ordinary (Standard rain gauge) rain gauge The most common rain gauge is the ordinary rain gauge, which simply consists of a collector place above a funnel that leads into measuring cylinder, where the rainwater is stored between observations. The measuring cylinder is specially graded to give the rainfall measurement in mm. Alternatively, where rainfall can be particularly heavy, a large container is used to collect the rainwater. Readings are made by pouring the rainwater out of the container into measuring cylinder so that the rainfall could be measured. 2-The weighing rain gauge Another way of determining how much rainwater has been collected is to weigh the water inside the container. In these instruments, the container sits on top of a scale and this weighs the container together with the rainwater inside continuously. Figure 1 Standard rain gauge
6. 6. The scales may be adjusted for the container making the reading to be only the collected rainwater. The measurements are usually recorded onto charts by pen and ink but later systems uses digital scales that could record the measurements electronically. Typically, this type of rain gauge does not empty itself and requires routine attention. 3-Optical rain gauge These have a row of collection funnels. In an enclosed space below each is a laser diode and a phototransistor detector. When enough water is collected to make a single drop, it drips from the bottom, falling into the laser beam path. The sensor is set at right angles to the laser so that enough light is scattered to be detected as a sudden flash of light. The flashes from these photodetectors are then read and transmitted or recorded. Figure 2 The weighing rain gauge Figure 3 Optical rain gauge
7. 7. 4-Tipping bucket rain gauge A known volume of water collects in a calibrated bucket, which tips on its pivot whenever it becomes full. Each tipping motion of the bucket closes a switch. The number of times the switch is closed in a given period provides a measure of rainfall rate. Number and Distribution of Gauges o Need to consider: size of area prevailing storm type form of precipitation topography aspect season Distribution of Gauges The distribution of gauges should not be random. Figure 4 Tipping bucket rain gauge
8. 8. o only fixed characteristics of areas can be sampled randomly. Random events must be sampled by a systematic arrangement of sampling points Practical considerations of access and exposure mean that some pragmatism is required in designing a network. o It is useful to locate gauges so that isohyetal maps can be drawn. Some gauges need to be near, or outside the catchment boundary in order to cover the catchment completely. Number of Gauges Based ONLY on area considerations the following tabulation has been suggested: Site Specific Rainfall Information Wagin – annual rainfall distribution Size of area Number of gauges 2 16 hectares 3 40 hectares 10 8 km2 15 16 km2 50 160 km2 300 1600 km2 1000+ 16,000 km2 Figure 5 Wagin – annual rainfall distribution
9. 9. The weather station A weather station is a facility with instruments and equipment to make observations of atmospheric conditions in order to provide information to make weather forecasts and to study the weather and climate. The measurements taken include temperature, barometric pressure, humidity, wind speed, wind direction, and precipitation amounts. Wind measurements are taken as free of other obstructions as possible, while temperature and humidity measurements are kept free from direct solar radiation, or insolation. Manual observations are taken at least once daily, while automated observations are taken at least once an hour Figure 7 weather station Figure 6 Climate Chart for Cairo
10. 10. Ground-based radar equipment can be used to determine how much rain is falling and where it is the heaviest Figure 8 ground based radar
11. 11. Part 2 Methods OF Measuring Runoff Runoff Rainfall that is not absorbed directly into the soil, through the roots and leaves of plants, or accumulated into existing bodies of water such as lakes or rivers is called runoff. In areas where the underlying geologic formation is impervious to water, as in the case of clay, runoff is a natural process, directing water in sheet flow, into lakes, rivers, wetlands, and the ocean. The design of the measuring instruments is important in order to get accurate measurement results. Our experience with run-off measurements in torrents suggests as follows: The measuring profile should be trapezoid in torrents. This design allows for an accurate measurement of low water level recording at the same time peak Figure 1
12. 12. floods many times over this value. The non-structured transverse profile allows for a simple determination of the gauge key as the velocity distribution in the channel is balanced which does not apply to structure transverse profiles (low flow channel....). A stilling basin should be upstream in order to reduce the influence of the washing of the vagues. At the end of the measuring channel should be a transverse dike with complete overfall. The establishment and calibration of the gauge key curve (which is used for the conversion of the measured water levels in terms of flow quantities) should, if possible, foresee continued measuring of the rate of flow. The usual random sample measurement of the rate of flow is often impossible in torrents. High water levels during floods which are of short duration consist a problem as nobody can foresee them to pick the right moment. The mesurement of the flow velocity should take place by radar as in case of a flood episode bedload and wooden logs cause measurement errors and may even destroy the instruments. Measurement and mathematical modeling Runoff is analyzed by using mathematical models in combination with various water quality sampling methods. Measurements can be made using continuous automated water quality analysis instruments targeted on pollutants such as specific organic or inorganic chemicals, pH, turbidity etc. or targeted on secondary indicators such as dissolved oxygen. Measurements can also be made in batch form by extracting a single water sample and conducting any number of chemical or physical tests on that sample. In the 1950s or earlier hydrology transport models appeared to calculate quantities of runoff, primarily for flood forecasting. Beginning in the early 1970s computer models were developed to analyze the transport of runoff carrying water pollutants, which considered dissolution rates of various chemicals, infiltration into soils and ultimate pollutant load delivered to receiving waters. One of the earliest models addressing chemical dissolution in runoff and resulting transport was developed in the early 1970s under contract to the United States Environmental Protection Agency. This
13. 13. computer model formed the basis of much of the mitigation study that led to strategies for land use and chemical handling controls. Other computer models have been developed (such as the DSSAM Model) that allow surface runoff to be tracked through a river course as reactive water pollutants. In this case the surface runoff may be considered to be a line source of water pollution to the receiving waters. Runoff curve number The runoff curve number (also called a curve number or simply CN) is an empirical parameter used in hydrology for predicting direct runoff or infiltration from rainfall excess.[1] The curve number method was developed by the USDA Natural Resources Conservation Service, which was formerly called the Soil Conservation Service or SCS — the number is still popularly known as a "SCS runoff curve number" in the literature. The runoff curve number was developed from an empirical analysis of runoff from small catchments and hillslope plots monitored by the USDA. It is widely used and efficient method for determining the approximate amount of direct runoff from a rainfall event in a particular area. The runoff curve number is based on the area's hydrologic soil group, land use, treatment and hydrologic condition. References, such as from USDA[1] indicate the runoff curve numbers for characteristic land cover descriptions and a hydrologic soil group. The runoff equation is where Q is runoff ([L]; in) P is rainfall ([L]; in) S is the potential maximum soil moisture retention after runoff begins ([L]; in) Ia is the initial abstraction ([L]; in), or the amount of water before runoff, such as infiltration, or rainfall interception by vegetation; and Ia = 0.2S
14. 14. The runoff curve number, CN, is then related CN has a range from 30 to 100; lower numbers indicate low runoff potential while larger numbers are for increasing runoff potential. Infiltration and surface runoff simulating extreme precipitation A portable precipitation simulator that is equipped with a flat swivelling jet nozzle (2.5 m above the sloping soil surface) is used for measuring infiltration. It simulates almost natural precipitation in terms of distribution, the drop spectrum and the kinetic energy of the droplet when impacting the soil surface. This device can track water infiltration or surface runoff and the soil erosion it causes in relation to soil sealing and macropore flow on arable land. This means that this equipment (Figure 2) can simulate extreme precipitation events. It records the surface runoff from an area of 1 m2 and the soil material eroded during irrigation. Prior measurements were made with this simulator to study the infiltration and soil erosion on slopes with different types of tillage. The results generated form the basis for recommendations for cultivation methods to reduce erosion and promote water retention (such as conservation tillage). In addition, the readings provide the foundations for the EROSION 2D/3D model used in Saxony for modelling erosion and precipitation runoff on farming water catchment areas (also refer to the findings of "Preventative Flood Protection with Conservation Soil Cultivation in the Catchment Area of the Pließnitz River"). Figure 2 Small irrigator
15. 15. Surface runoff with natural precipitation It was not only extreme precipitation that was simulated with an irrigator on a relatively small area. Surface runoff and erosion was also recorded for natural precipitation and snow melt while testing the test irrigator presented in Figure 3. A runoff and sediment catching trough catches the surface runoff and eroded soil material from an area measuring 7 m2. They are conducted to a collecting tank through a runoff pipe (also refer to "The Impact that Agricultural Usage has on the Soil Water Reservoir"). Precipitation and temperature were also measured. Runoff (flowthrough) in catchment areas used for farming The flow-through of a stream that drains a mesoscaled catchment area is recorded on a continual basis. This not only measures flow-through, but also calculates qualitative water parameters at the measuring channel (refer to Figure 4). This measuring station is being operated together with the Saxon State Office for the Environment and Geology. Since the catchment area is primarily used for agricultural purposes and the soils consist of loess, these measurements should prove, among other things, that the greater the area cultivated with techniques for conserving soil and water, the less flood runoff there is (also refer to "The Impact that Agricultural Usage has on the Soil Water Reservoir"). Figure 3 Device for recording surface runoff and erosion due to natural precipitation
16. 16. References http://www courses.nres.uiuc.edu/nres401/rainfall_measurements.ppt http://www.africaguide.com/country/egypt/info.htm http://ccc.atmos.colostate.edu/~hail/index.php http://www en.wikipedia.org/wiki/Rainfall http://www.landwirtschaft.sachsen.de/de/wu/Landwirtschaft/lfl/inhalt/10182_10194.h tm Figure 4 Defined channel for a continuous flow-through measurement