1.   Purpose and objectives

     Lag and K routing is a storage routing technique.

2.   Description

     Examination of historical flood hydrographs of varying magnitude provides a basis for establishing lag and K relationships for a reach. Constant lag implies that all possible flows are always to be lagged with the same fixed lagged time. A table of points defining a lag versus inflow relationship is needed for use of variable lag. Linear interpolation is used to determine lag for flows which fall between points. Highly variable lag versus inflow relationships may show two or more discharges occurring at the same time on the lagged inflow hydrograph. This "multiple intercept problem" occurs as a result of the hydrological technique, but it does not exist in nature. The problem is handled by adding the area under the lagged inflow curve at each time interval to maintain the correct volume. K can be thought of as the ratio of storage to discharge. It has the dimensions of time, and can be constant or variable. For variable K, a K versus outflow table is constructed. Linear interpolation is used to determine values of K between points. Since K is related to storage and discharge, it can be used to solve the storage routing (continuity) equation. A curve is developed of outflow versus twice the storage per time interval plus outflow values; values for outflow are found by linear interpolation from this curve.

3.   Input

     Line input of inflow time series data; parameters defining the lag and K relationships; and initial inflow, outflow and storage values.

4.   Output

     Printed output. Instantaneous outflow time series are generated.

5.   Operational requirements and restrictions

     a. One person with knowledge and experience in hydrological processes, 
        models, and a grasp of the elements of river forecasting, and basic 
        understanding and familiarity with computers and software can run 
        the program. 

     b. With the qualifications described above, 1-2 days of training is 
        required.

     c. Program is written for use on the HP Unix computer, but could be 
        modified to run on other main frame computers having large storage 
        capabilities and scratch work space.

     d. Max. array sizes are variably dimensioned and are specified by the 
        user in the MCP3 program.

6.   Form of presentation

     Compressed file on the Internet; written documentation is furnished.

7.   Operational experience

     This technique has been used extensively by the National Weather Service with good results; implementation is difficult but not complex.

8.   Originator and technical support

     Originator: George Smith, Hydrologic Research Laboratory, NOAA-NWS.
     Support: Janice Sylvestre, Hydrologic Research Laboratory, NOAA-NWS.

9.   Availability

     From the HOMS National Reference Centre for USA.

10.  Conditions on use