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1、http:/ 留学生论文专业定制代写网站留学生Essay写作供水和管理降水投入Abstract:One of the key issues in flood management is knowledge of the precipitation input into catchments for hydrologists knowledge of this serves to mitigate hazardous and environmental catastrophes,it is thus imperative to adequately determine precipitation
2、 input with appropriate and applicable statistical tools.The objective of this study is to determine the actual precipitation input and suggest the most appropriate method of determining precipitation input for the model catchment provided.Standard and commonly used methods of obtaining the areal pr
3、ecipitation input over a catchment area from rain gauge measurements at the precipitation stations are the Arithmetic mean,Thiessen Polygon,Isohyetal,and the Hypsometric methods.These methods serve as good approximations where the topography of a catchment is flat,if the gauges are uniformly distrib
4、uted and the individual gauge catches do not differ extensively from the mean.Arithmetic mean:This is the simplest form of giving a value of the average rainfall over a certain area,and works well under the following conditions:1. When the catchment area is sampled by many uniformly spaced rain gaug
5、es2. When the area has no marked diversity in topography(Davie,2008)Applying this measurement tool to the arithmetic mean:There are 7 rain gauges with the mean value being 27.14The total catchment area is=456kml 456 million square meters,l 27mm=0.027 metersl So 456,000,000 x 0.027m=12,312,000 m3Thie
6、ssen Polygons:The method was devised by an American engineer,the method provides for the non-uniform distribution of gauges by determining a weighting factor for each gauge.This factor is based on the size of the area within the drainage basin that is closest to a given rain gauge.These areas are ot
7、herwise known as irregular polygons.The method is straightforward and easy to use:l The catchment is divided into polygons by lines that are equidistant between pair of adjacent stationsl The lines/polygons are bisectedl Workout the area of each polygon by counting the squares within eachl Sums up t
8、he areasl Compare to arithmetic method to confirm the two are the samel Convert the individual polygonal areas to million sq meters and multiply by the converted precipitation rain gauges for example:o 178,000,000 x0.055=9,790,000Once this is done add them altogether to derive the total volume of pr
9、ecipitation input within the catchment.Isohyetal methodThis considered one of the most accurate methods;however as one will often find the method is subject to individual abilities and the knowledge of the general catchment.(Shaw,1994)The method is more complicated than the first two:l To derive of
10、an accurate estimation of the rainfall input one must first find the distance between two rain gauges in mm and eventually interpolate and extrapolate the line to give the adjacent rainfall levels,which can later be plotted back onto the catchment sheet.l i.e.method of summation:get the equidistant
11、line between the two rain gaugestake for example the distance in mm between gauge A and B8.5cm-convert to mm-85mml find the difference between the two rainfall gauges 55-30=25l now to work out the aof 85,one would divide 85/100 and multiply this by 25=21.25l Which is subsequently aof the equidistant
12、 line between the two rainfall gaugesl This figure can be used to derive the 2/4 point,thepoint etc.By simply doubling the 21.25 figure you arrive at the 2/4 or 50%point and then to get the 75%point adds 21.25 to the 50%point.One must now expand on the quartiles between the rainfall gauges:l This is
13、 done by using the difference(25)calculated earlier.l Half of this gives 12.5 which when added to the first gauge,or gauge B(30mm)you get 42.5.l Half of 12.5 gives 6.25,which when added to 30 gives 36.25,and so on until it matches against the adjacent measuring line.l (*see supplementary sheets to s
14、ee for techniques and further explanation)-once this is done plot the rainfall values using the adjacent measurements and join lines of equal rainfallThen progress to count the areas between the isohyets and find the average the two.Convert the individual areas to million sq meters and multiply by t
15、he converted average precipitation values for example:l 31,000,000 x 0.059=1,829,000 cm3l Do the same with all the values;add them to get the total volume of precipitation input.Hypsometric MethodThe method uses catchment topography and the rainfall measurements to derive of a total weighted precipi
16、tation input.It fairly accurate however is also dependant on the abilities of an individual,whilst drawing the hypsometric curve.The hypsometric curve allows for adjacent precipitation values to read from the graph.The area underneath the curve of precipitation gives the area of an individual gauge,and c
