Rainfall and River Discharge Trend Analysis: A Case Study of Jajrood Watershed, Iran

Jajrood watershed is located in the southern part of central Alborz mountain range which is located in northern Iran. Latyan dam in Jajrood River is one of the main sources of water for Tehran metropolitan area. This study was carried out in an attempt to determine whether or not there has been any significant change in rainfall and discharge over this watershed during the last four decades. Thus, the time series of monthly and annual values of rainfall and discharge for the period of 1967-2008 were analyzed using statistical methods. Trend analysis of Mann-Kendall test was performed and the best-fitted models were determined using the statistical criteria of Mean Error (ME), Mean Square Error (MSE), Root Mean Square Error (RMSE), Mean Absolute Error (MAE) and Mean Absolute Percent Error (MAPE). Probability distributions were fitted and the Kolmogorov-Smirnov tests were used to select the theoretical distribution, which best fitted the data. The analysis of rainfall data shows an increasing trend upstream but a decreasing trend downstream. While river discharge variations show the strong negative slope in recent years in Latyan station, water storage increased by Latyan dam, and water consumed in Tehran result in the decreasing trend of the river discharge.