WORLD METEOROLOGICAL ORGANIZATION                            HOMS COMPONENT
                                                                   I80.2.03
                                                                   (AUG 90)

               THE LOW FLOW FREQUENCY ANALYSIS PACKAGE (LFA)

1.   PURPOSE AND OBJECTIVES
     To provide a user-friendly interactive computer package to analyze the
data with respect to the underlying assumptions of frequency analysis and
to perform low flow frequency analyses using the three-parameter Weibull
(Gumbel III) frequency distribution.

2.   DESCRIPTION
     The LFA Package allows the user to:  easily enter flood data for
analysis; modify data sets so as to add, delete, or modify all stored
information; save modified data sets for future use; perform nonparametric
tests for homogeneity, trend, independence and randomness; graphically
display data in various fashions; determine low flow quantiles for the
straightforward case and samples with zero flow values.

     The three-parameter Weibull distribution forms the basis of this
technique.  Parameters are estimated by one of three methods.  They are
maximum likelihood, moments, or smallest observed drought.

     The LFA manual provide the technical background of all analytical procedures
incorporated in the package, as well as program documentation and worked
examples.

3.   INPUT
     Minimum n-day discharge and the year and (start) month of its
occurrence constitute the data required for analysis.

4.   OUTPUT
     Output constitutes results of nonparametric tests for homogeneity,
trend, independence and randomness.  Graphical displays of the data are
also available as a visual aid in assessment of the data regarding the
assumptions of the frequency analysis.  Once a sample passes the screening
process, a frequency analysis may be performed.  Summary statistics are
given for the flow data.  Parameters of the three-parameter Weibull
distribution, tables of flow estimates for various probabiliies of
non-exceedence, and frequency curves (cdf) are available.

5.   OPERATIONAL REQUIREMENTS AND RESTRICTIONS
 The LFA software was written in Fortran 77 to run under DOS 3.1 on IBM-type microcomputers with a math coprocessor.  However, it can also be used on computers running Windows 95 and 98. 

     LFA was designed to be user friendly and interactive, thus no
knowledge of programming is required to operate the package.
Interpretation of results should be performed by analysts having knowledge
of the assumptions and limitations of the approaches, mathematical
treatment, and implications regarding the selection of the low flow
quantiles.

6.   FORM OF PRESENTATION
     The user manual and appendices are in English.  Diskettes contain the
source code, executable module, and a sample data set.  Source code is in
FORTRAN 77.

7.   OPERATIONAL EXPERIENCE
     LFA has been applied to several hundred hydrometric sites in Canada
and is currently used by government agencies and private industry.

8.   ORIGINATOR AND TECHNICAL SUPPORT
      Atmospheric Environment Service      
      Environment Canada
Technical support can be provided in both French and English.

9.   AVAILABILITY
      From the HOMS National Reference Centre for Canada.

10.  CONDITIONS ON USE
     The diskettes and user manual are available free of charge.

(First entered :  24 AUG 90                      Last updated :  14 DEC 1999)

