Analysis of the precipitation and drought trend variations using Mann-Kendall and Sen Tests in Tehran Province

Trend analysis of climatic factors, especially precipitation time series can show the climatic situation of a region. Simultaneous comparison of the results of trend and drought indices in a region tends to better regional climatic analysis. To study the precipitation trend in Tehran Province, the historical data 1985-2007) of four main synoptic stations namely, Abali, Dushantpeh, Karaj and Mehrabad were provided in this study. The results of non-parametric Mann-Kendall and Sen Statistics were calculated and compared in monthly, seasonal and annual time scale series in this period. Also, a graphical analysis of the trend was carried out using Kendall statistics. The standardized precipitation index (SPI) in annual time series was used for regional drought analysis. The results of Mann-Kendall and Sen Trend tests showed that there is no particular trend in seasonal time series, but using of the Sen Test was revealed the trend in the annual series data of the Abali and Karaj stations at 99% confidence level. Abali and Karaj stations are located at very high level above sea. This attribute and much precise of Sen Test based on the non-zero data may be the reasons for the trend appearance in these stations. Monthly precipitation analysis obtained by the Kendall test was increasingly trend in the most stations. The graphical analysis was also represented the increasingly monthly precipitation trend in the region. The comparison of the precipitation and drought processes showed for annual scale time series the Sen Test reveals the trend value better than the trend in the Kendall test.