Trends Analysis of Five Important Daily Rainfall Characteristics at Urmia Lake Basin

Urmia Lake is one of the largest hypersaline Lake in Iran which its capacity is about 32 MCM in its normal condition. In recent years, climate change and decreasing rainfall led to falling water level in the Lake. Although, due to its high salt concentration this fall in water level led to extending areas of saline lands around the Lake. Furthermore, drought occurrence and rainfall decreasing trend cause solute concentration to be increase in the Urmia Lake water. For logical and scientific management of fresh water in Urmia Lake Basin it is necessary to investigate the trends of different characteristics of daily rainfall. The aim of this study is to detect trends in the number of days with different rain depths in five distinct classes in Urmia Lake basin. In order to detect trends the information of the twenty five stations having at least 10 years consecutive daily rainfall data were used. Trends of time series detected using the non-parametric Mann-Kendall method. Moreover, slope of trend lines estimated by using the Sen's estimator approach. In order to analyze the spatial trends for each of the time series some relevant maps prepared in which, the stations having significant trends were shown by markers. This facilitate the interpretation of trends in the area under study. The results showed that the slope of trend lines for days having rainfall less than 1 mm is often positive. About 84% of the stations had upward trend line. The five stations located in the southern part of the basin experienced (in level of 10% or less) significant upward trend in the number of days with precipitation less than 1 mm. The slope of trend lines in the case of the days having rainfall depth between 1 and 5 mm were almost negative. In the case of days with rain between 1 and 5 mm, the 15 stations had downward trend lines, which five of them showed significant trend, whereas the remaining stations had upward trend line slops (insignificant at 10% level). On the other hand, the slope of trend lines for the number of days with rainfall between 5 to 10 (mm) in 44% of the stations were negative in which, only four of them were significant (in 10% level). Trend line slopes in 40% of the station were positive, in which only one station experienced significant trend at ten percent level. The steepest positive trend magnitude belonged to the station namely Herris with β = 1. 633 (day/year). For the number of days with precipitation between 10 and 15 mm about 32% of the stations had negative trend line slopes, in which only three stations experienced negative significant trend. About 26% of stations had positive trend line, in which only one section (namely Maraghe) exhibited significant trend in 1% level. The steepest slope of the trend lines was related to station Shahindezh with β = 0.585 (days/year). The slope of trend lines for the number of days with rainfall more than 15 mm were positive for 52% of the stations, in which only one of them experienced significant trend (in 10% level). Moreover, 24% of the stations had negative trend in the mentioned class. Out of the total 25 selected stations only one section (namely Maraghe) had significant negative trend. The steepest slope of the trend lines belonged to the station Herris with β = 0.464 (days/year). In general trends of the number of days having rain depth less equal one mm in Urmia Lake basin was upward. But trends for the time series of the number of days having rainfall in the classes of 1-5, 5-10 and 10 -15 mm were downward. This means that light (shower) storms events in the study area is going to be occur frequently but heavy rains is going to be rare. It can be concluded that the number of light raining days had increasing trend. This implies that such light rains can not to be used for crops and trees. Because such rains immediately lost by evaporation proses after ceasing the rain. On the other hand, heavy rains which showed increasing trends for most of the selected station might be crate huge floods in the rivers flowing into the Urmia Lake Basin.