PRECIPITATIONIntroductionPrecipitation variabilityRainfall measurements techniquesDesign of precipitation gauging network Consistency of rain record Filling up of missing record Estimation of mean areal rainfall IDF and DD analysis !now measurement and determination of snow melt1.0INTRODUCTIONPrecipitationdenotesallformsofwaterthatreachtheearthfromthe atmosphere" #he usual forms are rainfall$ snowfall$ hail$ frost and dew"The essential requirements for precipitation to occur are #he atmosphere must have moisture$ Presence of nucleii around which condensation of vapor takes place%eatherconditionsmustbegoodforcondensationofwatervapourto take place$ and#he products of condensation must reach the earth"CondensationCondensationisthechangeofwaterfromitsgaseousform&watervapor' intoliquidwater"Condensationgenerallyoccursintheatmospherewhen warmairrises$coolsandloosesitscapacitytoholdwatervapor"sa result$ e(cess water vapor condenses to form cloud droplets" Forms of PrecipitationRI) Drop si*e + ,"- mm to . mm"Rain is considered as light rain intensity / 0"- mm1h moderateintensity caries from 0"- to2"- mm1h heavyintensity 3 2"- mm1h!)4%in the form of ice crystals$he(agonal in shape5 density of snow 6 ,"7 g1cm8DRI99:Edroplets of si*e / ,"- mm5 Intensity / 7 mm1hr;:9E it is the dri**le$ which free*es immediately in contact with cold obowever$thedurationof precipitation is usually longer and the areal e(tent also is larger"Anti Cyclones#heseareregionsofhighpressure$usuallyoflargeareale(tent"#heweatheris usuallycalmatthecentre" nticyclonescauseclockwisewindcirculationsinthe northernhemisphere"%indsareofmoderatespeed$andattheouteredges$ cloudy and precipitation conditions e(ist".#RECI#IT"TION V"RI"(I'IT&!"N!" (")IN#he annual hydrographcharacterised by low flows during post+ and pre+monsoon seasons and e(tremely high flows during the monsoon season#he annual variability in ma(imum flood discharge rates and volumes of the ;anga River for 7B yearsRainfall recorded on h*dro+raphic ,asins#he rainfall recorded on a hydrographic basin represents values obtained by takingintoaccountthehypothesesregardingthespatialdistributionofthe uneven distribution of the precipitations" #herainfallfalleninatimeunitisma(imuminthecentreoftherainand decreases in a non+linear manner towards the outskirts of the area"#he following methods are used for the calculation of the average rainfall for a basinCarithmetic mean5 Cthe method of the #hiessen polygons5 Cthe isohyets method5 Cthe square grid method" %hentheareaisphysicallyandclimaticallyhomogenousandtherequired accuracyissmall$theaveragerainfallforabasincanbeobtainedasthe arithmetic mean of the hi values recorded at stations"#he method of #hiessen polygons #he method of #hiessen polygons consists of attributing to each station an influence *one in which it is considered that the rainfall is equivalent to that ofthestation"#heinfluence*onesarerepresentedbyconve(polygons" #hesepolygonsareobtainedusingthemediatorsofthesegmentswhich link each station to the closest neighbouring stations &figure 8"B'" #he ratio can be written as@ Wi=Fi/Fwhere@ Fi represents the area of the #hiessen included partially or wholly in the considered basin" Fis the area of the basin#he water volume attributed to each area is@ Vi=Fi hiwhere hi represents the rainfall recorded at station i" #he average value of the rainfall is the ratio between the total volume of the rainfall &as the sum of the partial volumes' over the area of the basin@#heweightsWiaredirectlyproportionaltotheareaofthecorresponding #hiessen polygon"Figure 3.11 - The Thiessen polygons method If the surfaces Fi are equal and the number of the stations is n$ thenand the ratio Fi/F$ which represents the weight Wi$ equals to 1/n"%henthesurfacesofthe#hiessenpolygonsareequal$themethod consists in calculating of the arithmetic mean for the respective basin"#he#hiessenpolygonsmethodtakesintoaccounttherainfallatstations notincludedintheconsideredbasin"#herelationdescribingtheaverage rainfall can be written as follows@ where@ irefers to stations in the considered basin$