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Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,Precipitation Gauge Network,Precipitation varies both in time and space,Sound hydrologic/hydraulic designs require adequate,estimation of temporal/spatial precipitation patterns.,The density of rain gauge network depends on,(1)purpose of the study;,(2)geographic configuration of the study region;,(3)economic consideration.,Rain Gauge Density in HK,Rain gauge density in HK is:,Daily 13.6 km,2,/gauge,Autographic/Automatic 11.0 km,2,/gauge,Rain gauge density is significant higher in Hong Kong Island and much sparse relatively in New Territory(see figure).,Type,Number of Stations,Location Map,HKO Automatic Weather Station Rain Gauges,18,Figure 2,GEO Rain Gauge Stations-Telemetered,86,Figure 2,DSD Rain Gauge Stations-Telemetered,9,Figure 2,HKO Conventional Rain Gauge Stations,51,Figure 3,HKO Automatic Reporting Rain Gauges,21,Figure 2,Conventional Raingauge Locations in HK,Telemetered Raingauge Network in HK,World Meteorological Organization(WMO)Suggestion,A minimum density for precipitation gauge network:(at least 10%are automatic recording gauges),I:Flat region of temperature,Mediterranean,IIa:Mountain region of temperate,Mediterranean&tropical zones,IIb:Small mountains island with very irregular precipitation requiring,very dense hydrographic network,III:Arid and polar zones,Errors Precipitation Measurement,1.,Human Error,:scale reading&water displacement(if a,dip stick is used),2.,Instrumental Defect,:water to moisten the gauge;speed at,which mechanical devices work(such as tipping bucket,gages);&inadequate use of wind shield,3.,Improper Siting,:height above ground of the gage orifice;,exposure angle;®ionalization techniques,(Ref:“Uncertainties in Estimating the Water Balance of Lakes,”by T.C.Winter,Water Resources Bulletin,AWRA,17(1),1981),Effect on Wind on Precipitation Measurement,Analysis of Temporal Distribution of Rainstorm Event,-Only feasible for data obtained from recording gauges.,-,Rainfall Mass Curve,累積曲線,:,A plot showing the cumulative rainfall,depth over the storm duration,-,Rainfall Hyetogragh,(,組体圖,/,過程線,),:,A plot of rainfall depth or,intensity with respect to time,-,Instantaneous Rainfall Intensity,(slope of the mass curve),-,Average Intensity,in(t,t+,t)is,Time,Depth,Time,Depth or,Intensity,Rainfall Mass Curve&Hyetograph,Autographic Chart,Clock-Time vs.Rolling-Time Max Rainfall,Example(GEO Raingage N17 on 5 November 1993),Time15-min5-min,Rainfall(mm)Rainfall(mm),3:45,3:50 9.0,3:5512.5,4:0035.0,13.5,4:05,17.0,4:10,14.5,4:15,37.5,6.0,4:20 5.0,4:25 5.0,4:3014.5 4.5,Clock-time 15-min maximum rainfall depth=,37.5,mm,Rolling-time 15-min maximum rainfall depth=,45.0mm,Example of Rainfall Analysis,Double Mass Analysis,Changes in gage location,exposure,instrumentation,or observational procedures may cause relative change in the precipitation catch.This information is not usually included in the published records.,Doublemass curve analysis tests the,consistency,of the record at a gage by comparing its accumulated annual or seasonal precipitation with the concurrent cumulated values of mean precipitation for a group of surrounding stations.,Abrupt changes or discontinuities in the resulting mass curve reflect some changes at the target gage.Gradual changes in the slope of the mass curve reflect progressive changes in the vicinity of the target gage,such as the growth of trees around a rain gage.,The slopes of different portions of the mass curve can be used as a basis for correcting the record of the target gage.,Operation of Double Mass Analysis,P,i,t,or,P,i,t,/n,P,x,t,Adjustment factor for data,after 1916=S,1,/S,2,i.e.,P,x,t,=P,x,t,S,1,/S,2,t 1916,S,2,S,1,1916,A change of slope should not be considered significant unless it persists for at least 5 years.,Due to the fact that the data may have some scatter,an indicated change in slope should be confirmed by other evidence unless the change in slope is substantial(say,greater than 10%).,Example Double Mass Analysis,Point Rainfall Analysis,Purposes:,To transfer rainfall amounts observed from nearby index,stations to ungauged location or gauge with missing data,Methods,:,-Arithmetic average method,-Normal ratio method,-Inverse distance method(&modified versions),-Linear programming&other optimization methods,-Isohyetal,等雨線,method,-Kriging method,General philosophy:,where ;P,x,=rainfall amount to be estimated;P,i,=rainfall,amount at index station i;a,i,=weighting factor for index station i.,Sometimes,we may want to impose a,i,0 for all i=1,2,n,P,x,?,P,2,P,1,P,3,P,4,Arithmetic Average/Normal Ratio Methods,Arithmetic Average Method,:,Normal Ratio Method:,or,where N,i,=Average annual total rainfall at station i.,Inverse Distance Method,Inverse Distance Method:,i=1,2,n,where,D,i,=distance from index station i to the,point of estimation.,Issue:How to determine the best value for b?,P,x,?,P,2,P,1,P,3,P,4,Modified Methods,Modified Normal Rati
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