Enhancements and Modifications to the Full Equations Utilities (FEQUTL) Model, March 1995 to August 1999.
Note: This document is separate from the U.S. Geological Survey report by Franz and Melching (1997). This description of enhancements and modifications to the Full Equations Utilities Model has not been approved by the Director of the U.S. Geological Survey.

Input description update for section 5.3, CHANRAT command, Franz and Melching (1997b), p. 87

Section 5.3 CHANRAT Command

To RELEASE.TXT

LINE 7
 

 Explanation: Descriptive label for subsequent input.

LINE 8
 

LINE 9
 

for

LINE 9b
 

Notes on automatic head selection: The following considerations apply when LIPREC and MINPFD are used:

1. When LIPREC and MINPFD are used, NFRAC should have a value between 30 and 60. It is used to define a series of upstream heads as well as partial free drops to use in defining the final spacing to be used in the computations.

2. POWER is not now used if automatic head selection is requested. However, it may be used in the future and is retained for consistency with past input.

3. The desired interpolation precision may not be met in all cases. Rapid changes in table value may not be detected.

4. The default integration error tolerance in CHANRAT is reduced to 0.05 if automatic head selection is requested. If you explicitly set the integration error tolerance to a value different than the default, that value will be used. The smaller tolerance is used to get greater consistency in error estimates. Some of the lack of meeting the error tolerance is a result of the other tolerances in the computations. If they are too loose, erratic results, on the order of a fraction of a per cent, result. However, this is often enough to be a large part of a small relative error. Requesting relative errors smaller than 1 per cent is not wise and may not work.

5. MINPFD should not be too small. 0.01 is probably small enough. This often gives a factor of four or so between the largest and smallest flow tabulated for a given upstream head. A MINPFD of 0.005 or less will probably show many locations with LIPREC exceeded. This is primarily the result of the difficulty of computing reliable flows when the drop is small, sometimes less than one-ten thousandth of a foot!

6. Using small minimum heads can result in there being many upstream heads because free flow tends to increase with head roughly proportional to the cube of the head. This requires close spacing at small heads in order to maintain a small uniform relative error.

7. If the errors reported in the output are larger than LIPREC by significant amounts, try increasing NFRAC to 60 or so. Do not go much larger because internal space may be exhausted. This can sometimes improve the results.