1.   Purpose and objectives

     To provide a user-friendly interactive computer package to analyse 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 provides 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 probabilities 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.