The estimation of snowmelt runoff using SRM model (case study: Sahzab Catchment)

Introuduction
Snow reserves of mountain areas considered as one of the most important water resources of country which accurately recognition of quantity of these resources are essential due to the increasing value of freshwater and optimal utilization of resources. Sabalan mountain are one of the important basins of country that the water from melting snow which has fallen in winter, provide water for agriculture and surrounding areas drinking in spring and summer.  In this study for simulations runoff from snowmelt in the Sahzab catchment, (of sub-basin in the southern part of Sabalan) from mod 10a2 product and changes in snow cover of 2010-2011, using SRM model(based on degree-day method), has conducted. Results suggest during the statistical period, the greatest and lowest amounts of snow coverage are for February and may, respectively.
Methodology
 The Snowmelt Runoff Model (SRM) as the most frequently used model in model in prognosticating and simulating runoff in mountainous basins using snow covered areas as an input, was employed in current study to simulate the runoff produced from snowmelt. The S factor (snow cover area) was extracted using 8 day images of MODIS sensor (combining bands number 6 and 4), NDSI index (with the necessary thresholds) ENVI and GIS software. In order to achieve the desired objectives, ground data such as temperature , rainfall and debit were also used.
Results and discussion
Evaluation of SRM model using correlation coefficient and volume subtracting indicator are 81 and 2/3 percent, respectively. R-value indicator acceptability depends on the quality of data and may not be acceptable even by having enough data of 85% value but in a catchment with inappropriate and low data, lower value also is acceptable. With respect to shortage of meteorological and terrestrial data in Sahzab catchment, the SRM results of this study showed acceptable simulations for runoff simulation that was caused by snow melting in sahzab catchment.
Conclusion
Using of hydrological models and satellite images combined with powerful ground-based data can be used as a tool for planning and management of water resources, particularly in the area where snow melting is one of the factors leading to runoff. If studies on snowcovered area, snowmelt and its effects are considered on small scale such as riverside tributaries extraction of snow cover maps for each of the catchment basins, careful planning can be done for each region which will be in line with sustainable development.