1.   Purpose and objectives

     This chapter establishes procedures for estimating depth and peak rates of runoff from small watersheds for use in designing soil and water conservation measures.

2.   Description

     Tables and graphs are included for a quick and reliable way to estimate peak rates of discharge, and associated runoff volumes for a range of rainfall amounts, soil types, land use, cover conditions. The procedures for determining peak rates are applicable to drainage areas of 5 to 200 acres.

     The graphs were prepared for the solution of general relationships, and are based on 24 hour rain distributions. The data for the peak rate discharge were computed using procedures from the NRCS National Engineering Handbook, Section 4 Hydrology (NEH-4).

3.   Input

     The required data are land use and hydrologic soils group to calculate a runoff curve number, 24 hour rainfall amount, drainage area and an estimate of average watershed slope.

4.   Output

     Estimates of peak rate of discharge and runoff volume for drainage areas from 5 to 2000 acres are read from the graphs.

5.   Operational requirements and restrictions

     a. An engineering technician who can calculate time of concentration 
       (Tc) can easily use this procedure.

     b. One day of training or familiarization with the procedure may be 
        necessary.
  
     c. The data for the graphs were computed using procedures from the 
        NRCS National Engineering Handbook, Section 4, Hydrology (NEH-4)
        Chapter 16.
  
     d. Component K22.3.02 "Computer Program for Project Formulation 
        Hydrology (TR-20)" should be used in lieu of this procedure for
        special situations and areas, or where this procedure is too genera
        to provide good estimates of discharge.

6.   Form of presentation

     Presented as Chapter 2, National Engineering Field Manual for Soil and Water Conservation, Natural Resources Conservation Service, U.S. Department of Agriculture, August 1989.

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.