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<br />~ <br /> <br />TECHNICAL RELEASE 20 <br />COMPUTER PROGRAM FOR PROJECT FORMULATION <br /> <br />HYDROLOGY <br /> <br />CHAPTER 2. PROGRAM DESCRIPTION <br /> <br />This chapter provides an understanding of the TR-20 computer program <br />organization and features. Terminology-in general usage by engineers is <br />used without definition. Terminology and acronyms used solely for this <br />computer program are defined the first time they are used and in the <br />Glossary. <br /> <br />To use this program effectively, an understanding of the procedures <br />described in SCS National Engineering Handbook, Section 4 (hereafter <br />referred to as NEH-4) is suggested. <br /> <br />2.1 METHODOLOGY FOR HYDROLOGY AND HYDRAULICS <br /> <br />~ <br /> <br />Subwatershed Delineation <br />The watershed should be divided into as many subwatersheds as required <br />to define hydrologic and alternative structural effects. Hydrologic <br />effects are influenced by entrance of tributaries, watershed shape, <br />valley slope changes, homogeneity of the runoff curve number and existing <br />or proposed water impoundment structures. Each subwatershed is assumed <br />to be hydrologically homogeneous. See Chapter 6, NEH-4. <br /> <br />Rainfall Distribution <br />Either actual or synthetic cumulative rainfall distributions may be <br />used. Rain~fall distributions (mass time-depth data) must be represen- <br />tative of the watershed area under study. This could be individual <br />subwatersheds or the entire watershed. <br /> <br />. <br /> <br />Runoff Volume <br />A mass curve of runoff is developed for each subwatershed. The runoff <br />curve number (CN), rainfall volume and rainfall distribution are the <br />input variables needed to determine this mass curve. CN's are determined <br />by the user for each subwatershed based on soil, land use, and hydrologic <br />condition information, as described in Chapters 7, 8, and 9, NEH-4. <br />