Germi County Seasonal Precipitation Routing and Analysis, Using Holt-Winters Method for Time Series with Non-Seasonal Changes

Iran is an arid land located in an arid and semi-arid region in the world. Precipitation is among the main effective factors on water reserves, especially in warm seasons. To determine the warm region precipitation changes, seasonal and annual precipitation data from synoptic stations during a 13 year statistical period from 1999 to 2011 and precipitation was studied to determine precipitation changes in the region. Holt-Winters exponential smoothing model was used to forecast the time series with non-seasonal and annual changes. Results suggested that annual precipitation forecast trend is an upward trend and it indicates an increase in precipitation in coming years in the region. Considering the forecasts using Holt-Winters model, it was observed that the annual precipitation during years 2012 to 2016 will have a downward trend and it will have a 6.9mm decrease comparing to the year before. Hence, the precipitation trend in coming years will be a downward trend.