1.   Purpose and objectives

     The National Weather Service River Forecast System (NWSRFS) snow accumulation and ablation model is a conceptual model of the physical processes affecting snow accumulation and snowmelt using air temperature as the sole index of energy exchange at the snow-air interface.

2.   Description

     The model separates snowmelt during rain-on-snow periods from melt during non-rain periods to account for differences in the magnitudes of the energy transfer processes. During rain-on-snow periods melt is computed by an energy balance equation after making several simplifying assumptions. 
During non-rain periods melt is computed based on the difference between the temperature of the air and a base temperature, and a proportionality factor or melt factor.  The seasonal variation in the melt factor is represented by a sine function with maximum and minimum values occurring on 21 June and 21 December respectively. Other model parameters are used to determine both the form and amount of snowfall, the surface energy exchange during non-melt periods, snow cover heat storage, areal extent of the snow cover, retention and transmission of liquid-water, and heat exchange at the snow-soil interface.

3.   Input

     Aside from user specified parameters and program options, time series of precipitation and temperature are needed. Time series of observed water equivalents and areal snow cover can also be used. Input format is disk file.

4.   Output

     Printer output. Rain plus melt time series data are produced, as are simulated water-equivalent and areal snow cover values. Time series are written to a file.

5.   Operation 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-week training is required.
     c) Program is written for use on HP workstations; but could be modified
        to run on other platforms having large storage capabilities and
        scratch work space. The program runs as part of NWSRFS, but may be
        obtained as a separate subroutine for execution in a user designed
        program.
     d) Maximum array sizes are variably dimensioned and are specified by
        the user in the MCP3 program.
     e) Program limitations: the sinusoidal melt factor variation was
        developed for use throughout the contiguous U.S. An adjustment
        factor was determined for Alaska and applied to the melt factor.

6.   Form of presentation

     The main execution routine and documentation is available from the HNRC of USA. These files can be obtained through ftp procedures.

7.   Operational experience

     Has been applied to a wide range of snow cover and climatic conditions throughout the USA with good results; regular usage in U.S. by National Weather Service; implementation is complex.

8.   Technical support

     Dr. Victor Koren, Hydrologic Research Laboratory, NOAA-NWS.

9.   Availability

     From the HOMS National Reference Centre for USA.

10.  Conditions on use

     Free.