USE AND LIMITATIONS OF UNIT HYDROGRAPHAs the unit hydrographs establish a relationship between the ERH and DRH for a catchment, they are of immense value in the study of the hydrology of a catchment.They are of great use in:The development of flood hydrographs for etreme rainfall magnitudes for use in the design of hydraulic structures,Etension of flood!flow records based on rainfall records, andDevelopment of flood forecasting and warning systems based on rainfall."nit hydrographs assume uniform distribution of rainfall over the catchmentThe intensity is assumed constant for the duration of the rainfall ecess#on!uniform areal distribution and variation in intensity within a storm are very common. "nder such conditions unit hydrographs can still be used if the areal distribution is consistent between different storms.$t is generally felt that about %&&& 'm( is the upper limit for unit!hydrograph use. )lood hydrographs in very large basins can be studied by dividing them into a number of smaller subbasins and developing DRHs by the unit!hydrograph method.There is a lower limit also fortheapplicationofunit hydrographs.Thislimitis usuallyta'enasabout(&& ha. At this level of area, a number of factors affect the rainfall!runoffrelationship andtheunithydrographis notaccurateenoughforthe prediction of DRH.*ther limitations to the use of unit hydrographs are:+recipitation must be from rainfall only. ,now!melt runoff cannot be satisfactory represented by unit hydrograph.The catchment should not have unusually large storages in terms of tan's, ponds, large flood!ban' storages, etc. which affect the linear relationship between storage and discharge.$f the precipitation is decidedly non!uniform, unit hydrographs cannot be epected to give good results.$ntheuseofunit hydrographsveryaccurate reproductionofresults shouldnotbeepected. -ariations in the hydrograph base of as much as .(&/ and inthepea'dischargeby .0&/ are normally considered acceptable.DURATION OF THE UNIT HYDROGRAPHThe choice of the duration oftheunithydrograph dependsontherainfall records.$frecording raingaugedataare available,anyconvenient time depending on the si1e of the basin can be used.Thechoiceisnotmuchifonlydaily rainfallrecordsareavailable.A rough guide for the choice of duration Disthatitshouldnoteceedthe least of:2i3the time of rise,2ii3 the basin lag, and 2iii3the time of concentration. A value of D e4ual to about 056 of the basinlagisaboutthebestchoice. 7enerally, for basins with areas more than 0(&& 'm( a duration D 8 0( hours is preferred.DISTRIBUTION GRAPHThedistribution graphintroduced by 9ernard 20:;%3 is a variation of theunit hydrograph. $t is basicallyaD!h unithydrograph withordinates showingthe percentage of the surfacerun!off occurringin successive periodsofe4ual time intervals of D!h.SYNTHETIC UNIT HYDROGRAPHTo develop unit hydrographs to acatchment,detailed information about the rainfall andtheresultingflood hydrographareneeded. However,suchinformation wouldbeavailableonlyata fewlocationsandina mas studyareasandisconsideredasan indicationoftheretentionand storagecapacityofthewatershed. Ai'eCt,thevaluesofCpalsovary 4uiteconsiderablydependingonthe characteristicsoftheregionand valuesofCpintherange&.;0to &.:; have been reported.I" a non6standard ran"a&& duraton tR ! s adopted7 nstead o" t!e standard %a&ue tr to der%e a unt !ydro'rap! t!e %a&ue o" t!e (asn &a' s a#e$ted+ T!e /od8ed (asn &a' s '%en (y)*+,0-4!ere5 (asn &a' n !ours "or an e#e$t%e duraton o" tR ! and tp s as '%en (y E9+ )*+:- or )*+,;-+ T!e %a&ue o"/ust (e used nstead o" n E9+ )*+,,-+ T!us t!e pea. ds$!ar'e "or a nonstandard ER o" duraton tR s n /01s)*+,3a-The time base of a unit hydrograph is given by ,ynder as2?.063where8 time base. =hile E4. 2?.063 gives reasonable estimates offor large catchments, it may give ecessively large values of the time base for small catchments. Taylor and ,chwart1 recommend2?.0%3with2given in h3 ta'en as the net larger integer value divisible by tR, i.e. Tb is about five times the time!to!pea'.To assist in the sketching of unit hydrographs, the idths of unit hydrographs at !" and #!$ of the peak ha%e &een found for 'S catch(ents &y the 'S )r(y *orps of Engineers+ These idths ,in ti(e units- are corre.ated to the peak discharge intensity and are gi%en &y,/+0/-and ,/+0#-here 1 idth of unit hydrograph in h at !"$ peak discharge 1 idth of unit hydrograph in h at #!$ peak dischargeq 1 Qp2A 1 peak discharge per unit catch(ent area in (32s2k(4,incethecoefficientsCtandCp varyfromregiontoregion,in practicalapplicationsitis advisablethatthevalueofthese coefficientsaredeterminedfrom 'nownunithydrographsofa meteorologicallyhomogeneous catchment and then used in the basin understudy.Thisway,nyder>s e4uations are of use in scaling the hydrographinformationfromone catchmenttoanothersimilar catchment.Cat$!/ent A Cat$!/ent BL 5 3" k( L 5 5! k(Lca 5 0! k( Lca 5 4! k(A 5 4!" k(4A 5 5"" k(4ExampleTwocatchmentsAandBareconsideredmeteorologically similar. Their catchment characteristics are given below.ForcatchmentA,a2-hunithydrographwasdevelopedandwas oundto haveapea! discharge o "#m$%s.The timeto pea! rom thebeginningotherainallexcessinthisunithydrographwas &.#h.'sing(nyder$essn t!sunt!ydro'rap!4as :+;!+Usn'Snyder