International Bulletin of Water Resources & Development (IBWRD), Vol. (II)– No. (03)- S.N. (06)- Summer 2014 All rights reserved for IBWRD Available online at: www.waterdevelop.com IBWRD Vol. (II)– No. (03)- S.N. (06)- Summer 2014 International Bulletin of th Water Resources & Development ISSN 2345 - 5012 19 Article– P. VII- XIV Rainfall and River Discharge Trend Analysis: A Case Study of Jajrood Watershed, Iran Mehdi Khaleghi 1* , Hossein Zeinivand 2, Shahin Moradipour 3 1 M.Sc. Student of Watershed Management Engineering, Department of Range and Watershed Management Engineering, Faculty of Agriculture, University of Lorestan, Khoramabad, Iran 2 Assistant Professor, Department of Range and Watershed Management Engineering, Faculty of Agriculture, University of Lorestan, Khoramabad, Iran ([email protected]) 3 M.Sc. Student of Watershed Management Engineering, Department of Range and Watershed Management Engineering, Faculty of Agriculture, University of Lorestan, Khoramabad, Iran ([email protected]) *Corresponding Author ([email protected]) Article History Received: 23 February 2014 Revi ewed: 13 March 2014 Revised: 15 April 2014 Accepted: 28 May 2014 Published: 16 September 2014 Abstract 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. Keywords: Statistical Analysis, Trend Analysis, Jajrood Watershed, Times Series et al., 1966). Many researchers have carried out 1. Introduction similar studies. For instance, Nasri and Modarres (2007) studied the rainfall trend using Mann- Kendall Performance evaluation of the water resources and homogenous tests in the north east of Isfahan, projects depends on the prediction of the Iran. The results showed that homogenous rainfall hydrological processes in the future. Knowledge of had no trend in the study area. Modarres and Silva the data recorded in the past may help researchers (2007) evaluated annual rainfall trend and the achieve statistical parameters by considering which number of days that rainfall may occur in the arid probable events can be predicted in the future lands of Iran. Based on the results, most of the trends (Alizadeh, 2010). There are several nonparametric were local and no particular regional trend exists in statistical techniques for the time series trend rainfall. Germmer et al. (2004) analyzed the analysis, but the parametric statistical techniques are increasing and decreasing trend in different stations also widely used. In rainfall and drought studies, in China and derived the map of the trend. Rostamian complete data series is important to derive rating (1996) studied the climate changes in the Caspian curve of the river flow (Alizadeh, 2010). Hejam et al. Sea coastal areas. The research was analyzed using (2008) assessed seasonal and annual rainfall trend in Mann- Kendall test for the 45-year period of 1951 to central Iran using nonparametric Mann- Kendall 1995. The results did not show the significant trend method and Sen’s Estimator Slope. They concluded in northern part of Iran. However, there were some that due to lack of sufficient series, a particular trend variations in the rainfall in the region and the most was not found in seasonal and annual rainfall in the occurrences were sudden, while there was not any study area. Application of the nonparametric Man- significant trend. Meryanji (2006) investigated the Kendall test was recommended to analyze the time series of temperature and rainfall recorded in 5 hydrometeorological series trend by WMO (Mitchell stations for a 30-year period in Hamedan, Iran. The VII International Bulletin of Water Resources & Development (IBWRD), Vol. (II)– No. (03)- S.N. (06)- Summer 2014 All rights reserved for IBWRD International Bulletin of Water Resources & Development (IBWRD), Vol. (II)– No. (03)- S.N. (06)- Summer 2014 All rights reserved for IBWRD results showed that there is a significant trend in four decades. In statistical terms, this is a some stations. Moreover, Zahedi Kelaki (2004) determination of whether or not the probability studied the effect of the decreasing trend of rainfall distribution from which they arise has changed with and the increasing trend of drought on the quality and time. Moreover, the amount or rate of change, in quantity of water resources. Githui et al. (2005) terms of changes in the median as a central value is analyzed the time series and annual values of rainfall quantified. In this study, the Seasonal Kendall and discharge for the period 1963-1998 of Yala River method, a non-parametric test, is used for trend Basin, Kenya. Their results showed a decreasing analysis, for there are few underlying assumptions trend for the rainfall data, a decreasing slope in the about the structure of the data which can make them upstream station, and an increasing trend in the robust against departures from normality (Helsel and downstream station for the discharge data. Hirsch, 1991; Githui et al. 2005). Moreover, the use Hydrologic time series almost always exhibit of ranks rather than actual values makes them seasonality due to the periodicity of the weather. In insensitive to outliers and missing values. Hirsch et Iran, this arises greatly from seasonal variations in al. (1982) proposed that the Seasonal Kendall, a non- precipitation volume, as well as the rate of parametric test, is preferred to the simple or seasonal evapotranspiration. For the observed data that exhibit regression tests when data are skewed, cyclic, and high seasonality, the methods used to analyze trends serially correlated. should incorporate a seasonal component. Spatial differences in trends can occur as a result of spatial 2. The Study Area differences in the changes in rainfall and temperature, and spatial differences in the catchment Jajrood River is located in the southern part of central characteristics that translate meteorological inputs Alborz mountain range which almost entirely covers into hydrological response (Burn and Elnur, 2002). the northern part of Iran (Fig. 1). This river with the length of 40 km drains the basin area of 710 km 2, and The present study aimed to investigate the trends there are two dams on the river, Latyan and of river discharges and rainfall in Jajrood Watershed Mamloon. This study examines (1) the time series of in Iran. Two river gauging stations, Roodak monthly values of discharge and rainfall at the (upstream) and Latyan (downstream), and 6 rainfall selected stations, (2) the existence of trends and the stations including Latyan, Roodak, Ahar, Shemshak, evaluation of the best fitted trend models, and (3) Lavasan, and Garmabdarre were selected. This study their probability distributions. seeks to determine if the discharge and rainfall values have generally increased or decreased during the last Fig. 1. Location of Jajrood River and the gauging stations. 3. Methodology between hydrological variables and meteorological variables. The statistical trend test selected was the Trends and variability in hydrological variables were Mann-Kendall non-parametric test for trend. The assessed using a statistical test for trend as well as Mann-Kendall test is a rank-based method that has further data analysis to examine the relationship been applied in many previous studies for identifying VIII International Bulletin of Water Resources & Development (IBWRD), Vol. (II)– No. (03)- S.N. (06)- Summer 2014 All rights reserved for IBWRD .