1.   Purpose and objectives

     The Sacramento Model is a conceptual soil moisture accounting model. It is an attempt to parameterise soil moisture characteristics in a manner that logically distributes applied moisture in various depths and energy states in the soil and that has rational percolation characteristics.

2.   Description

     The model represents the distribution of soil moisture by an upper and lower zone. Within each zone the moisture is separated into tension water (water held tightly by soil particles) and free water (water which can move within the soil mantle). The movement of water between the two zones is 
controlled by a physically based percolation equation which is controlled by the contents of the upper zone free water and the soil moisture deficiency in the lower zone. The model also includes a representation of the impervious portion of a catchment. The model contains 16 parameters
defining the capacities of the soil zones, the drainage rates of the zones, the shape of the percolation curve, and the size of the impervious areas. The model has a basic approach for simulating the effects of frozen ground. A time series of temperature is required for analysis of frozen ground effects.

3.   Input

     Required are times series data for precipitation (rain plus melt); optional are times series of evapotranspiration and areal extent of snow cover. User supplies values of the model parameters. The model requires proper definition of the input data to produce satisfactory results.

4.   Output

     Times series of channel inflow (runoff), runoff components and soil moisture storages are output.

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 can connect the model 
        execution subroutine to a user designed program.

     b) With the qualification described above, training and experience are 
        required to properly calibrate and implement the model.

     c) Program is written in FORTRAN.  The program runs as part of the
        National Weather Service Forecast System (NWSRFS), but may be
        obtained separately for execution in a user designed program.

     d) Maximum array sizes are variably dimensioned and are specified by
        the user.

6.   Form of presentation

     Source code and written documentation. The main execution routine and documentation are available from the HOMS National Reference Centre (HNRC) of USA. These files can be obtained through ftp procedures.

7.   Operational experience

     The model has been used in humid, arid and semi-arid areas with good results; usage in USA by National Weather Service; implementation is complex.

8.   Technical support

     Dr Michael Smith, Hydrologic Research Laboratory, NOAA-NWS.

9.   Availability

     From the HOMS National Reference Centre for USA.

10.  Conditions on use

     Free.