Analysis of the trend of time series changes in the maximum daily and annual rainfall of Karkheh and Dez catchment

Precipitation is the most important climatic characteristics of any region. This climatic characteristic is one of the most fluctuating climatic variables that seriously affects the water resources the region. This issue is  important in Iran, where the average rainfall is over 250 mm. The purpose this study is to analyze and explain the heavy rainfall  Karkheh and Dez using statistical methods. For this purpose, precipitation data from 14 synoptic stations and rain gauges period 60 years (1959-2019) were used. In order to achieve the objectives  the research using the probabilistic distribution function the final limit  type 1 Gumble, the heavy rain threshold each station was determined. Then to identify the uniform and non- uniform discharges in the time series maximum daily precipitation and total annual precipitation the ITA method and Man – Kendall non-parametric test were used. The results were obtained from heavy rainfall thresholds, The average rainfall of more than 40/7 mm in Karkheh and the average rainfall  more than 47 mm in Daz  are considered as heavy rainfall. The average rainfall in Karkheh is 118 days and the dose  is 81 days. The flood threshold of Karkheh  is less than Dez catchment. Therefore Karkheh rains occur more suddenly and irregularly while Dez  have more normal time series. As a result Karkheh is more vulnerable to floods than Dez in terms increased heavy rainfall. The results of using Man – Kendal and ITA test showed that Man – Kendal test can only detect uniform trends in time series as the advantage ITA method is that in addition to uniform trends. It is able to detect hidden trends and internal in identify time series. For example, although man – Kendal test did not show a specific trend for maximum rainfall at Khorramabad station, The ITA test detected hidden and non-unform trends for this station.