1.   Purpose and objectives

     The program provides for hydrologic analyses of a watershed under present conditions and with various combinations of land cover, use and structural or channel modifications using single event storm rainfall- frequency data.

2.   Description

     TR-20 generates subarea surface runoff hydrographs from storm rainfall using a dimensionless unit hydrograph, drainage areas, times of concentration, and NRCS runoff curve numbers. Standard control instructions to develop, route, add, store, divert or divide hydrographs are established to convey floodwater from the headwaters to the watershed outlet. Uniform rain depth and distribution are assumed over a subarea, groups of  sequential subareas or the whole watershed. 

3.   Input

     The program uses batch input. The required data are: sequential standard control instructions which include drainage area, time of concentration and the NRCS runoff curve number for each subarea: structure tables - elevation, discharge, and storage data; cross section tables -  elevations, discharge, and end area data or optional routing coefficients; reach lengths; main time increment; dimensionless unit hydrographs; baseflow; storm rainfall tables - depth, duration, distribution, and starting time data; and the soil moisture conditions - wet, average, dry.

4.   Output

     The program provides in tabular form peaks and flood hydrographs, their time of occurrence and water surface elevations at any desired location in the watershed.

5.   Operational requirements and restrictions

     a. One person can use the program with basic knowledge of watershed 
        hydrology and hydraulics and a basic familiarity with computers and 
        computer software. At least one day of training is helpful to use 
        the program plus several days additional if unfamiliar with the 
        NRCS hydrological techniques.

     b. The program was written for personal computer systems in
        FORTRAN IV. Memory requirements are 212k Bytes and two temporary
        files.

     c. In one continuous operation, the program con route through 99 
        reservoirs and 200 stream reaches with unlimited number of 
        variations in each.

     d. Both the reach routing and reservoir routing techniques use the 
        conservation of mass equation and a single-valued rating curve
        as the conservation of momentum equation.

6.   Form of presentation

     The program is on a diskette and a hard copy of the user manual is available.

  
7.   Operational experience

     The technique is used extensively by the U.S. Natural Resources Conservation Service with good results, and is used worldwide.

8.   Originator and technical support

     Water Science Technology Team
     National Water and Climate Center
     Natural Resources Conservation Service
     U.S. Department of Agriculture

9.   Availability

     From the HOMS National Reference Centre for USA.

10.  Conditions on use

     None.