Performance evaluation of the SRM model under the influence of different melting months (Case study: Doab Samsami Watershed)

The use of models in snowy basins, due to the effect of snow’s properties, requires quantitative and qualitative knowledge of the factors on the melting and runoff resulting from it. Temperature is one of the factors. The impact of its variation on the performance of model based on the degree-day approach should be explained. For this purpose, in the Samsami catchment, the Longitude of 50, 10, 2.2 seconds to 50, 26, 17.6 seconds East and Latitude 32, 5, 16.5 seconds to 32, 15, 1 seconds North the area of 266 square kilometers evaluated the performance of the SRM model in different melting season in years 1393 to 1394 using a product of maximum eight-day snow cover (MOD10A2) and criteria for assessing EI efficiency and Volume Ratio. Then it was processed using ENVI software and ArcGIS software was provided with digital model, composition and snow cover curves. Determination of snow melting season was carried out using snow cover curves. To determine the beginning of the melting season, changes in snow cover levels between 2008 and 2016 were investigated in the Doab Samsami’ basin. This basin is one of the northern Karun sub basins in Chahar Mahal and Bakhtiari province with an average rainfall of 1,175 millimeters per year. 61 percent of the precipitation is snow and 39 percent is rain. Due to the significant drop of snow at the surface of the basin, there are large permanent and seasonal springs. The most important of these are the Dezdaran and Koufi springs. These springs have a relatively large reservoir and the origin of the formation of two rivers are named for these two. The two rivers joined in the geographical position of 50 degrees, 17 minutes and 15 seconds east longitude and 32 degrees, 10 minutes and 14.1 second northern latitudes in the vicinity of Samsami village at altitudes of altitude of 1993 meters from the sea level, the water of two waters of Samsami is formed. The study basin has the Doab Samsami meteorological station and Safa Abad hydrometric station at the outlet point of the basin. The basin lacked a snowstorm station, which is why the synoptic Kouhrang station, the nearest Synoptic station to the Doab Samsami basin, was assisted. The use and interpretation of model simulation results is subject to the study of temperature’s variations in different altitudes. The performance of the model in different months of melting is different due to the variable state of the structure of snow accumulated due to temperature. The temperature difference in the melting months, with the effect of stacking on the snowpack, causes the snow to ripe. On the other hand, the ripening of snow causes delays in runoff due to melting and, as a result, the difference between observational and simulated discharge. Because the ripening phenomenon is directly related to the temperature, it can be said that the performance of the RMS model decreases with increasing temperature, in proportion to the melting season. Accuracy of model SRM decrease with increasing temperature. For February to May, the Volume Ratio rises from 10 to 40 percent for an increase of 16 ° C. The observed difference between simulated and observational values in May, the end of the melting season, was such that the value of the volume difference increased to 40% and reduced the efficiency by minus 0.55. These results were obtained in March, the second month of melting, with a mean temperature of 4 ° C, the volume difference decreased by 3.5% and the efficiency index increased to 0.81. Hence, due to the very good results of the SRM model in March, its simulation results can be used with acceptable capability to estimate snow flood from the studied basin. However, the use of the model in months with a mean high temperature and the end of the melting season, due to snow structure transformation, has a significant error in the simulation results. In order to estimate the snowflake outflow, more attention should be paid to the role of the springs and the necessary modifications are made in simulation results. Statistical simulations include Volumetric Difference (%) and Efficiency Index (Dimensionless) in each months of February, March, April, May are 17.1, 3.5, 14, 40 and 0.83, 0.81, 0.66, -0.55 respectively. It is therefore seen that, gradually, with the melting months, the accuracy of model estimation decreases. So, the use of model outcomes in the end months of melting requires temperature correction. In March, the best fit and in May, the lowest compliance is observed.