The estimation of Probable Maximum Precipitation (PMP) with Flood Forecast Approach in Urmia Lake Basin

Flood is one of the natural that causes many damages each year and always has been an attraction for experts in the field of hydrology. According to the Intergovernmental Panel on Climate Change (IPCC), the temperature of the Earth's atmosphere will increase in the next century, and one of the major impacts is the increasing in climatic extremes, such as droughts and floods. The intensity of the rainfall has a logical limit that is known as Probable Maximum Precipitation (PMP). The probable maximum precipitation is the highest rainfall that occurs over a specified period in a given area due to climatic and topographic conditions. Due to the climatic conditions in the Urmia Lake basin and mountainous area, as well as the significant changes in rainfall and temperature in recent years in the basin, it is necessary to forecast and control flood beforehand. Therefore, the aim of this research is estimating of the homogeneous climatic regions in terms of the characteristics and calculates the probable maximum precipitation for those regions and determining the maximum annual flood and flood points in the basin.

The probable precipitation is investigated by using synoptic stations data in Urmia Lake basin.. Two statistical and synoptic methods are used for comprehensive study over the region. In method, atmospheric information of upper layers such as, relative humidity, temperature, storms, wind, dew point have been used. In statistical methods, the probable maximum precipitation is calculated according to the climatic characteristics of the area. The statistical methods presented in this study are Hershfield and the maximum probability methods. By definition of the maximum probability method, the maximum probable precipitation is obtained by maximizing the maximum 24-hour precipitation at each station. By using probabilistic distribution, the probable distribution of the maximum 24-hour precipitation rates of the station is fitted and the best probability distribution for these data is determined.In this method, by using probabilistic distribution, the probable distribution of the maximum 24-hour precipitation rates of the station is fitted and the best probability distribution is determined for these data.

In the Hershfield statistical method, the frequency factor K and the probable maximum precipitation for24-hour were calculated for each station. According to the results, the probable maximum precipitation rates of the stations vary from 35 mm for Sarab to 89 mm for Oshnavieh. In the maximum probability method, the most suitable probability distribution is fitted to the maximum precipitation data for 24-hour of each station. The purpose is to use the fitted distribution to obtain the maximum values of the maximum rainfall. According to the results of this method, the probable maximum precipitation values vary from 45 mm for Sarab to 134 mm for Oshnavieh. In addition the spatial pattern of probable maximum precipitation is presented by the values obtained from each station. In the method of estimating the maximum probable rainfall for 24-hour, first the storms were identified and maximized in Urmia Lake basin. In order to determine the maximum rainfall for 24-hour, the amount of precipitation occurred on the corresponding date at the same station is multiplied to maximizing factor. Maximum of these values in total dates is concerned the probable maximum precipitation for each stations. According to the results, the probability maximum precipitation is 118.8 mm, which can be considered as the maximum probable rainfall of the basin.

The results show the consistency of statistical and synoptic methods. The maximum probability has the best estimation than Hershfield and synoptic methods. Distribution of probable maximum precipitation obtained from the statistical methods and the results of the probable maximum precipitation of the stations, shows that the eastern basin of Urmia Lake has a lower probable maximum precipitationthan the other areas. The maximum amount of probable maximum precipitation is in the southwest of the basin in Saqqez, Mahabad and Oshnavieh.The maximum probable precipitation is obtained 102 mm by storm estimation method calculated for Tabriz station where located in Aji Chai basin for the 100-year continuity period. This reserch is consistent with the previous studies (Azizi and Hanafi (2010)) which used synoptic methods to estimate the maximum probable precipitation in Aji Chai basin and showed the maximum probable 24-hour precipitation 84.5 and 103.9 mm respectively for the 50 and 100 year continuity periods.