1、raising standards worldwideNO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAWBSI Standards PublicationBS 7843-4:2012Acquisition andmanagement ofmeteorological precipitationdata from a raingaugenetworkPart 4: Guide for the estimation ofareal rainfallPublishing and copyright inform
2、ationThe BSI copyright notice displayed in this document indicates when the documentwas last issued. The British Standards Institution 2012Published by BSI Standards Limited 2012ISBN 978 0 580 71312 5ICS 07.060The following BSI references relate to the work on this standard:Committee reference CPI/1
3、13Draft for comment 11/30219647 DCPublication historyBS 7843-1.1 first published July 1996BS 7843-1.2 first published September 1996BS 7843-2.4 first published July 1996First published as BS 7843-4, February 2012Amendments issued since publicationDate Text affectedBS 7843-4:2012 BRITISH STANDARDCont
4、entsForeword ii1 Scope 12 Normative references 13 Terms and definitions 14 General comments on the estimation of areal rainfall 25 Approaches to the estimation of areal rainfall 46 Normalization and the average annual rainfall (AAR) 47 Specific techniques for the estimation of areal rainfall 68 Use
5、of radar data 10Bibliography 13List of figuresFigure1Anexample of Thiessen polygons 7Summary of pagesThis document comprises a front cover, an inside front cover, pages i to iv,pages 1 to 14, an inside back cover and a back cover.BRITISH STANDARD BS 7843-4:2012 The British Standards Institution 2012
6、 iForewordPublishing informationThis part of BS 7843 is published by BSI Standards Limited, under licence fromThe British Standards Institution, and came into effect on 29 February 2012. Itwas prepared by Technical Committee CPI/113, Hydrometry. A list oforganizations represented on this committee c
7、an be obtained on request to itssecretary.SupersessionBS 7843-4:2012 supersedes BS 7843-2.4:1996, which is withdrawn. Together withBS 7843-1:2012, it supersedes BS 7843-1.1:1996 and BS 7843-1.2:1996, which arewithdrawn.Relationship with other publicationsBS 7843, Acquisition and management of meteor
8、ological precipitation data froma raingauge network, comprises four parts. Part 1: Guide for design, development and review of a raingauge network. Part 2: Code of practice for operating raingauges and managingprecipitation data. Part 3: Code of practice for the design and manufacture of storage and
9、automatic collecting raingauges. Part 4: Guide for the estimation of areal rainfall.Taken together the four parts of BS 7843 provide guidance on the acquisitionand management of meteorological precipitation data from a raingaugenetwork. This part of the standard is not directly applicable to the des
10、ign,development and review of a raingauge network; such guidance is given inBS 7843-1. The operation of a raingauge network is detailed in BS 7843-2, whilea code of practice for the design and manufacture of storage and automaticcollecting raingauges is given by BS 7843-3.Information about this docu
11、mentThis is a full revision of the standard.Use of this documentAs a guide, this part of BS 7843 takes the form of guidance andrecommendations. It should not be quoted as if it were a specification andparticular care should be taken to ensure that claims of compliance are notmisleading.Any user clai
12、ming compliance with this part of BS 7843 is expected to be able tojustify any course of action that deviates from its recommendations.Presentational conventionsThe provisions in this standard are presented in roman (i.e. upright) type. Itsrecommendations are expressed in sentences in which the prin
13、cipal auxiliaryverb is “should”.Commentary, explanation and general informative material is presented insmaller italic type, and does not constitute a normative element.BRITISH STANDARDBS 7843-4:2012ii The British Standards Institution 2012Contractual and legal considerationsThis publication does no
14、t purport to include all the necessary provisions of acontract. Users are responsible for its correct application.Compliance with a British Standard cannot confer immunity from legalobligations.BRITISH STANDARD BS 7843-4:2012 The British Standards Institution 2012 iiiBRITISH STANDARDBS 7843-4:2012Th
15、is page deliberately left blankiv The British Standards Institution 20121 ScopeThis Part of BS 7843 gives guidance on the methods commonly used in the UKfor the estimation of areal rainfall. It is applicable to the use of meteorologicalprecipitation data from a raingauge network.This Part of BS 7843
16、 highlights factors to be considered during the estimation ofareal rainfall and presents an outline of methods (full technical details of themethodologies are not provided).It does not cover methods using precipitation data collected by weather radars.However, discussion of their relevance is includ
17、ed.2 Normative referencesThe following referenced documents are indispensable for the application ofthis document. For dated references, only the edition cited applies. For undatedreferences, the latest edition of the referenced document (including anyamendments) applies.BS 7843-1:2012, Guide for de
18、sign, development and review of a raingaugenetworkBS EN ISO 772:2001, Hydrometric determinations Vocabulary and symbols3 Terms and definitionsFor the purposes of this part of BS 7843, the terms and definitions given inBS EN ISO 772:2001 and the following apply.3.1 areal precipitationvalue of precipi
19、tation within a specified time interval averaged over a specifiedarea3.2 areal rainfallvalue of rainfall within a specified time interval averaged over a specified areaNOTE UK convention is for calculations of areal rainfall to be based on totalmeasured precipitation, where measured precipitation in
20、cludes both rainfall (asmeasured in a raingauge; see 3.11) and the water equivalent of new snow (seeBS 7843-2).3.3 average annual rainfall (AAR)arithmetic mean of the water equivalent of the annual sums of daily amounts ofmeasured precipitation at a point over a period of time, usually 30 years3.4 d
21、aily precipitationtotal precipitation that accumulates over 24 h ending at 0900 UTCNOTE For general use this total is credited against the previous days date.3.5 domaingeographical area with specific and common characteristics of precipitation3.6 exposureextent of shelter or protection from the weat
22、her3.7 monthly precipitationtotal precipitation from 0900 UTC on the first day of the month to 0900 UTC onthe first day of the following monthBRITISH STANDARD BS 7843-4:2012 The British Standards Institution 2012 13.8 normalizationmathematical procedure for adjusting rainfall data according to the a
23、verageannual rainfall for the locality3.9 precipitationwater or ice derived from the atmosphere and deposited at ground levelNOTE Measured in terms of the depth in mm of its liquid equivalent.3.10 precipitation metadatarecords relating to the site, the equipment and the operating practices thatspeci
24、fy the circumstances under which precipitation measurements are made3.11 rainfalltotal liquid component of precipitation, including condensation from theatmosphere, collected and measured by a raingauge3.12 sitearea of ground where a raingauge is deployed for measurement or has beendeployed in the p
25、astNOTE Other instruments for measuring the environment may also be deployed atthe site.3.13 UTCCoordinated Universal Time4 General comments on the estimation of arealrainfall4.1 Estimates of areal rainfall are required for a range of applications, each withdiffering requirements (e.g. temporal and
26、spatial scales). A range of techniquesexist for the calculation of areal rainfall estimates.NOTE Example application areas and aspects of precipitation data requirements aregiven in BS 7843-1.4.2 Because of the spatial variability of rainfall, the uneven distribution ofraingauges and factors which i
27、nfluence the uncertainty associated withindividual raingauge catches (see 4.6), areal values calculated from a set of pointobservations should be treated as estimates, not as measurements. To becapable of making a good estimate of areal rainfall, the raingauge networkneeds to be capable of capturing
28、 the normal spatial variability in rainfall, and amethod has to be available to estimate the profile between raingauges. Manysuch methods are capable of estimating the distribution of rainfall over an area(e.g. a catchment) via a grid of rainfall estimates (termed a surface) in additionto estimating
29、 the areal average.4.3 Spatial variability in rainfall tends to diminish or become more systematicover longer time periods. Therefore, the choice of method used for arealestimation is most critical for short time periods. This is particularly true when asubstantial proportion of the rainfall is conv
30、ective. Generally, frontal rainfallexhibits considerable spatial coherence, whereas rainfall of convective originoften exhibits very discontinuous behaviour with extreme rainfall gradients.Under such circumstances any interpolation technique will be of limitedaccuracy. Radar-based estimators, where
31、possible incorporating adjustmentsbased on contemporaneous raingauge catches, can be particularly helpful insuch circumstances.BRITISH STANDARDBS 7843-4:20122 The British Standards Institution 20124.4 BS 7843-1 provides guidance for the design of a raingauge network,including an outline of the data
32、requirements that influence its development.Where estimates of areal rainfall are of principal interest the followingrequirements are or particular importance.a) Spatial density. The data requirement for the calculation of areal rainfall isdependent on the type of application and varies according to
33、 the spatialand temporal scale of interest. In some cases this may be expressed in termsof the spatial density of raingauges.b) Spatial representativeness. The degree to which measurements arerepresentative of the local area should be taken into account during thedesign of the network and subsequent
34、 use of raingauge data for arealrainfall estimation. A raingauge network should capture the spatial andtemporal variability of the precipitation of the region of interest.4.5 Generally, areal assessments incorporate all available raingauge dataincluding, on occasions, non-standard raingauges (e.g. t
35、o help assess extremerainfall events). However, changes in network density over time are inevitableand where homogeneity is a primary requirement (e.g. in the detection andquantification of rainfall trends) areal time series may rely on a representativesubset of long-term, well-sited raingauges with
36、 time series subjected to rigorousquality control. Where a robust association has been established between theareal rainfall estimates thought to be most accurate (usually based upon aperiod for which a more detailed network was available) and the long-termhomogeneous subset of the raingauge network
37、s, this may be used as acorrection factor for the long-term dataset.4.6 The calculation and use of areal rainfall estimates should consider theuncertainty in the raingauge measurements used. BS 7843-1 providesinformation on the systematic and random errors in precipitation measurementby raingauge ne
38、tworks. In particular, areal rainfall is normally assessed using UKstandard raingauges (as recommended in BS 7843-3). These systematicallyunderestimate the rainfall reaching the ground (due largely to wind effects).Typically, the underestimation is in the range of 3-6%. Where snow forms asignificant
39、 component of total precipitation the underestimation is likely to besubstantially greater. To ensure consistency across the area of interest, whenusing areal rainfall estimates it is important to consider whether any correctionshave been applied to raingauge measurements (BS 7843-2 recommends thatv
40、alues of both measured and estimated precipitation should be recorded in thesource data). For some applications (e.g. water resources assessments andcatchment water balances) where corrections have not been applied to datafrom individual raingauges, there might be justification for applying correcti
41、onfactors to the derived estimates of areal rainfall.NOTE When storing estimates of areal rainfall for future use it is important thatdetails of contributing raingauges (or raingauge selection criteria), calculationmethods and, where appropriate, any correction factors applied are recorded in themet
42、adata/data flags to aid future interpretation (see BS 7843-1).BRITISH STANDARD BS 7843-4:2012 The British Standards Institution 2012 35 Approaches to the estimation of areal rainfall5.1 Grid-based approachesA common approach to the estimation of areal rainfall over a particulargeographical area is t
43、o use an interpolation procedure to derive a regularsquare grid of rainfall estimates from the raingauge observations. Grids mayconsist of rainfall estimates at each discrete grid point location or estimates ofthe average rainfall over each particular grid square. Areal rainfall estimates arethen de
44、rived by overlaying the boundary of the area of interest on the grid andcomputing the average of all those grid points/squares that fall within theboundary. Compared with other approaches, this brings a number of benefits:a) the gridded values may be reused;b) the gridded values may be readily combi
45、ned or compared with othergridded datasets, particularly where the grid intervals are the same(e.g. to show rainfall for a period as a percentage of the long termaverage);c) the grid may be displayed in two or three dimensions using standardsoftware;d) grids produced using different interpolation pr
46、ocedures can be compared;e) grids can be exported in standard formats;f) quality control procedures can be applied at the gridding stage.In applying a grid-based approach, the user should select a suitable grid interval.This is normally one that is capable of adequately representing the informationt
47、hat has been recorded by the raingauges.Care should be taken when estimating an areal average for an area thatcontains a low number of grid points (in the order of 50) or where the widthof the area in question spans only a few grid cells (in the order of 5). In suchcases, where it is not appropriate
48、/practical to recalculate a smaller spatialinterval grid, it is recommended that the existing grid is first subdivided, usingbilinear interpolation, to a suitable finer interval in the vicinity of the area.5.2 Non-grid-based approachesA number of approaches to the estimation of areal rainfall do not
49、 make use ofa grid. These include the following.a) Manual graphical methods.b) Methods where, for a given area, a weight is allocated to each of a set ofraingauges.c) Methods where the rainfall surface is represented by an equation that canbe integrated over the area of interest.6 Normalization and the average annual rainfall(AAR)6.1 GeneralPrior to implementing techniques for the estimation of areal rainfall (Clause 7),it is necessary to consider whether or not the raingauge data should first benormalized with respect to the long-term average annual rainfall
