Analyzing of Precipitation Trend in Sarab Plain during The Period of 1976-2005

One of the challenges considered in the subject of climate change is proving, recognizing and quantification of precipitation. Hydrological data trend analysis is important tool for understanding climate change effects. Furthermore, trend analysis of precipitation data is necessary for scientific management of water resources with respect to global warming in each region. Precipitation is one the most important element that its quantity and distribution vary over different parts in the earth. Recognition of precipitation trend in each region will be useful for application planning. Method and Material: The aim of this study is to use non-parametric Mann-Kendall test for estimating precipitation trend of stations located in Sarab plain on monthly, seasonal and annual time scales for the period 1976-2005. Also sequential Mann-Kendall diagrams were plotted to consider of precipitation trend date change at seasonal and annual time scale. Results showed that Sarab plain stations had different change date but trend is mostly decreasing. Within studied stations except Sarab synoptic station other stations namely Heris, Mehraban, Sahzab and Mirkooh are consisted of rain gauges. Furthermore except Mirkooh station which has 31- year period other stations have 32 years records. Among studied stations only Mirkooh station had statistical gap which was completed using linear interpolation. For trend computing autocorrelation coefficient in study scales of 31 lag were computed. Then trend analysis was performed with conventional MK method (for insignificant auto correlation) and modified MK (for significant auto correlation). Mann – Kendall method is one of the most commonly used non parametric tests for considering trend in different hydrological and climatical variables. Test statistic Z was obtained from Mann-Kendall equations and compared with normal Z in significance levela. If Mann-Kendall test statistic absolute value Z was more than 1.645 then the trend (increasing or decreasing) could be significant in 90 % confidence level. If it was more than 1.96, trend could be significant in 95% confidence level and if more than 2.33 then confidence level will be 99%. Moreover recognizing trend or no trend, slope of trend line also should be computed for which Sen’s slope estimator was used. Also for recognition of the change year in seasonal and annual time scales, sequential Mann-Kendal was used. Results showed that precipitation decreases in Sarab plain in different time scales. At monthly scale the strongest decreasing trend occurred on May rainfall in Mehraban and most of the other stations showed increasing trend on August and September. In (SCDC) is made using the DEM and snow cover maps to get daily percent values of snow covered area. The SCDC is an important variable for the SRM simulations. Other variables (temperature and precipitation) and parameters (degree-day factor, recession coefficient, runoff coefficients, time lag, critical temperature and temperature lapse rate) are used as input to the SRM model for snowmelt simulation. The results snow runoff modeling showed discharge is tightly related with precipitation. In winter and spring time, with high precipitation, the discharge at outlet arrives peak point. While, in summer and autumn, with little precipitation, the discharge is also very low. it is related with temperature too this is mainly because temperature directly influences snow melting hence determining discharge. What is more, when temperature is higher than 00 Celsius, the precipitation is rain fall, which confluxes to the outlet of this catchment and in snow melt season there would be more discharge peaks. The coefficient of determination (R2) 44and difference in volume agreement (%DV) is used for quality assessment. The results show value of (R2) is 0.5 and the volume difference DV is 0.39 % for the years 2004-2005 and 0.35 and 0.48 % for the years 2005-2006. And there is a good agreement between the observed and computed runoff.