1.   Purpose and objectives

     The Snowmelt-Runoff Model (SRM) is a physically-based model designed to be used to simulate and forecast daily streamflow in mountain basins where snowmelt is a major runoff factor. It is intended to be simple to apply and at the same time utilize the new technology of remote sensing.

2.   Description

     The model uses daily temperature to calculate degree days which are used as an index of the complex energy balance leading to snowmelt. Snow- cover extent data obtained from remote sensing is used for applying the snowmelt algorithm to the specific portion of the basin with snow pack.  Daily precipitation is required for calculating the amount of rainfall- derived runoff to be added to the daily snowmelt component. For model operation, the following parameters must be determined: degree-day factor, runoff co-efficient, recession co-efficient, temperature lapse rate, and discharge time lag. Beginning with Version 3.2 of SRM, the model can simulate the effect of climate change on a given basin-year of data, with the changed climate induced through changes to present
temperature/precipitation and model parameters. 

3.   Input

     The model program includes a graphical user interface (GUI), through which the user provides daily values of precipitation, temperature, and snow cover for the entire simulation or forecast time period. The same GUI is used for supplying values of the model parameters. Optionally, the input data and parameter values can be generated externally, and imported into the model. The optimum conditions for accurate simulation of runoff have been identified as follows: temperature and precipitation are recorded at the mean elevation of the basin inside the basin boundaries (or at the zonal mean elevation for large basins); snow cover is available reliably once per week to detect short-term variations in zonal areal extent; several climatological stations are available for large basins, especially  in areas with frequent summer precipitation events; and several years of  daily runoff records have been acquired for the determination of the recession coefficient. Decreases in accuracy will be expected as these optimum conditions are compromised. However, acceptable simulations will result even under the following minimum conditions: temperature and precipitation data are observed outside the basin at a considerable horizontal and vertical distance; snow-cover observations are only available two to three times during the snowmelt season; climatological observations are not possible at multiple stations; and no runoff records are available so that the recession coefficient must be estimated from the 
basin size.

4.   Output

     Daily precipitation contributing to runoff in each elevation zone, daily total depth (precipitation + snowmelt) contributing to runoff in each elevation zone, computed daily stream runoff, and various statistical measures.

5.   Operational requirements and restrictions

     One person with knowledge and experience in hydrological processes, models, and snowmelt with a basic understanding of computers can initiate runs using SRM. With these qualifications, no formal training is required. The program runs as an executable program, on the DOS operating system of 
any IBM-compatible personal computer. The model was developed on QuickBasic, Version 4.5. In situations where manpower is not limited but computer resources may be, any pocket calculator with the function x to the power y is sufficient for day-to-day computation of the discharge.

6.   Form of presentation

     A compressed distribution file containing the executable program, sample data files, and a limited "read-me" file is available via the internet from the Snowmelt Runoff Model home page at: 

http://www.ars.usda.gov/services/software/download.htm?softwareid=7&modecode=12-65-06-00#downloadForm

     The model User’s Manual is available from the same site, in ‘HTML’ and ‘pdf’ formats.

7.   Operational experience

     The model has been tested in mountain basins ranging in climate conditions from humid to semi-arid with satisfactory results. Operational implementation was carried out by hydrological consultants in the 1983 snowmelt season in the USA.

8.   Originator and technical support 

     Dr. Albert Rango, Hydrology Laboratory, USDA-ARS.

9.   Availability

     From the HOMS National Reference Centre for USA.

10.  Conditions on use

     None.