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estimate .of the PMF), FDr a givencDmbination .of values fDr L< and <1>, the RT that yielded the largest PMF varies between the various land-cover distributions in most instances. The design engineer ShDUld: (1) Find the .optimum tempDral "iin- fall distribution for the given spatialla~d-cover distribution; . and (2) find the .optimum stDrm-center locatiDn fDr the given temporal rainfall distribution. The ,aforementioned results "in- dicate that selecting the .optimum temporal rainfall distribu' tion is a very important criterion in obtaining -an accurate PMF estimate, otherwise th~ accuracy. used in selecting,the . optimum values for both the storm orientation and the stann center location may be lost. VDlume .of RunDff and Its Spatial and TempDral Distributi.on The following information concerning the runoff is also revealed by this study: (1) The vDlurpe .of ruQDff is mainly a functiDn .of the percent .of fully develDped land-cDver distri- butiDn; (2) the spatial distributiDn .of runDff is .only a functiDn .of tbe spatial distributiDn .of rainfall f.or a spatially hDmDge- r:aequs land-cover distribution, while its mainly a function of the spatial land-cDver distributiDn fDr a spatially nDnhDmO-: geneDus landcover distributiDn; and (3) the tempDraLrunDff ' distributiDn, I, .of the PMF, is mainlya functiDn .of the selected tempDral rainfall distributiDn, It shDuldbe nDted that the tDtal vDlume .of runDff appears tD be the mDst impDrtant factDr .over the spatial distributiDn .of runDff in estimating the PMF, This nDte implies that a change in the percent .of fully de- velDped land-cDver distributiDn has mDre impact on die PMF than the spatial distribution .of a given percent .of fully de' velDped landcDver distributiDn, Furthefl!1Dre, the resulting Ip .of the PMF is directly an effect .of the assumed tempDral 'rainfall distributiDn, ' ' Use .of Guidelines to Estimate PMF The fDregDing guideli,nes may be used tD assess the effects .of each cDntributing factDr .on the PMF and the interactions between the factDrs, Although the assumptiDns made fDr each factDr included a range .of values that' wDuldbe expected in typical design situatiDns, the guidelines presented here are intended fDr lhe fDIIDwing tWD purposes: (1) To .obtain a plaiining estimate .of the 'PMF. using a single HMRS2/HEC-l computer run and the adjustment factDrs; and (2) tD .obtain a design estimate .of the PMF in which the designer must actually conduct a sensitivity analysis guided by the research resuits. Obtaining Planning Estimate .of PMI' TD .obtain a planning estimate .of the PMF, it is recom- mended that the designer make an HMRS2/HEC-l computer run fDr the base point values: 1810 fDr stDrm DrientatiDn (i.e" alDng'the majDraxis .of the drainage area); (0, 0) for storm' center location (i.e... drainage-centered), and a center-peaked tempDral rainfall distributiDn fDr an elliptical iSDhyetal axes ratio 2,5 tD I fDr the design land-cDver distributiDn in the drainage basin, Adjustment factDrs frDm Fig, 4 are applied tD the base pDint PMF tD reflecl changes made in 'the values fDr the cDntributing factDrs. AccDrding tD the results .of the "sensitivity analysis, the base point values for 4>, Le. and Rr , are applicable fDr each .of the five land.CDver distributiDns examined (each land-cover distribution having a different maximum PMF). Therefore, with this assumption (Le., for the base pDint values fDr <1>, L" and Rr), the designer may use the adjustment factDrs from the charts .of Fig" 4 tD de- termine what percentage of the maximum PMF would result for a one-factor. two-factor, or three-factor variation from .' t'lo....,rtn...lon '-' --- ___181 PMF 0.' ~ max 0.8 __- -':_210 0,' 0,' '-' early- peaked eellter- peaked late- peaked b.I.O ..0..... o,..~..<1.... 181' 0,' --:~:-- PHF PH"Fmax 0.' -~210' 0,' -- 0,' 0,' early- peaked center- peahd late- peaked ',0 ...... _1....Uti.... . pJotF 0,' ~ -- lit" -x .,. - " 0,' ,~:=- -- no" 0,' - 0,' early- center- late- peaked peaked peaked Temporal Rainfall Di~tribution FIG.- 4. Variation of PMF for One-Factor, Two-Factor, and Three- Factor Variation from Base Point Combination of Factors for Fol- lowing Slorm Center Locations: (a) Drainage-Centered; (b) Down- stream Subbaslns-centered; (c) Upstream Subbaslns-Centered (Land-Cover . Distributions .'are Denoted as -=-- :;: Downstream Half Fully Developed; -,--,; Upper Middle Half Fully Developed; ---- :;: Homogeneous Fully Undeveloped;.-.-. :;: Homoge- neous Fully Developed; - - ; Upstream Half Fully Developed) 1.06 ---./1l'_09fIlOlllllnclf'O"fIOOf c1o..nst..u. 1I11( full c1fUIOll'c1 -...-"0_""''''0''1.'''111 d,ulop,c1 PMF-, 1.0" ......... --...-- IllaX 1.0C: at 2.5 to LO 1.00 IsobyeU.l Axea 0.98 Ratio. 0.96 ~<~ , --.;,~ 0.9. 1.0 - 2.5 .., Elliptical Isohyetal A."tes Ratio (to LO) AG. 5. Variation of PMFwith-Change In Isohyetal Axes Ratio from 2.5 to 1.0 fo ell her, 1:0 to 1,0 or 4,0 to 1.0 , , the base pDint cDmbinatiDn .of factDrs, Depending upDn the currently existing land-cover distribution, the designer would . use the appropriate curves in each chart in Fig. 4 to determine the percentage of the maximum PMF. To determine what percentage .of the adjusted planning PMF wDuld result fDr a variatiDn in the iSDhyetal axes ratiD frDm 2,5 tD 1 tD 1.010 I or to 4.0 to 1, the designer may use the adjustment factors from Fig,S. In the case in which either the currently existing land-cover distribution differs significantly from the five distributions examined in this research, or if a change in the existing land- cover distribution is anticipated, the designer may use the dashed line .of Fig, 6 tD adjust fDr the appropriate difference 334/ JOURNAL .oF IRRIGATION AND DRAINAGE ENGINEERING / SEPTEMBER/OCTOBER 1995