1.   Purpose and objectives

     BLTM is a computer code that can be used to simulate the fate of any number of conservative or chemically interacting pollutants through a network of one-dimensional open channels that may have reversing flow.

2.   Description

     The BLTM model solves the convective-dispersion equation by using a Lagrangian reference frame in which the computational nodes move with the flow. The one-dimensional equations are solved for any number of  constituents moving in any number of branches which are connected at junctions. The unsteady hydraulics (discharge, area, top width, and tributary inflow) must be supplied to the model and constituent concentrations are assumed to have no effect on the hydraulics. Reaction kinetics of the constituents can be supplied by the user through a subroutine but subroutines are available for simple first order decay such as coliform, temperature, dissolved oxygen, and the nitrogen cycle.

     The advantages of the Lagrangian approach are: a) the scheme is very accurate in modelling the convection and dispersion terms, b) the model is stable for any time step size, c) the model is economical to run, d) the model output includes information which is helpful in calibration and 
interpretation of the system.

3.   Input

     BLTM requires 5 types of input:

     - Flow hydraulics including Discharge, cross-sectional area, top 
       width, and tributary inflow for each grid point at each time step of 
       the simulation. Tributaries can occur at any interior grid.
     - Time series of concentrations of all routed constituents at each 
       external boundary.
     - Initial values of all constituents in each subreach of the model.
     - Data describing the physical size and configuration of the system.
     - Reaction kinetic coefficients for interacting constituents.

     Both DAFLOW (HOMS component K35.3.14) and BRANCH (K35.3.13) can provide the hydraulic input required by BLTM.

4.   Output

     Tables or graphs of concentrations of any constituent at any point as a function of time, or at any time as a function of distance along the channel. Tables of the travel times of individual parcels of water. Tables of the influence of any physical process (dispersion, tributary inflow, production from a specified constituent, decay to a specified decay product) on the concentration of a constituent at a particular point and time.

5.   Operational requirements and restrictions

     The program is written in FORTRAN 77 and can be used on any microcomputer with MS-DOS as well as on larger machines. A mathematical co-processor is highly recommended. As executable code is provided a FORTRAN compiler is not necessary.

     The system must be one-dimensional so no stratification or lateral concentration gradients should be present. All constituents are assumed to be dissolved, so sediment can not be handled.

6.   Form of presentation

     Documentation (USGS Water Resources Investigation Report 87-4163) and programs with dialogue are written in English.  Both source and executable code are provided on a 3 1/2" floppy diskette or via the Internet.

7.   Operational experience

     BLTM has been applied in a number of river and estuary systems in the United States.

8.   Originator and technical support

     U.S. Geological Survey, Office of Surface Water.

9.   Availability

     From the HOMS National Reference Centre for USA for 3 1/2" floppy or printed documentation; or electronically on the Internet at:
http://water.usgs.gov/software/ or http://www.geogr.uni-jena.de/software/

10.  Conditions on use

     Program and electronic version of the documentation are available free of charge; there is a charge for printed documentation.