Here, three periods are detected of 6, 12, toward and 18 months. The 12-month period is more pronounced than the others, with the power value (y axis) being about 10 times larger. Apparently, the 6- and 12-month periods are related to the seasonal variation of rainfall during the year typically observed in climatological data. The third period of 18 months indicates a variation in rainfall pattern of a longer time. The reason for the existence of this period in the data is unclear and may need further investigation to establish any relation.Figure 3Periodograms for the monthly total rainfall data obtained from the weather stations in urban areas for the time duration from January 1994 to December 2005.3.

Rainfall Temporal VariabilityOwing to the similar seasonal mean and pattern, rainfall data collected for a point estimate can be considered spatially representative over the urban areas of Kuwait. The monthly total data of Kuwait International Airport can thus be considered representative and employed to examine the temporal variability of rainfall in a larger time scale. A full range of data is available with time duration from January 1965 to December 2009. It should be mentioned that some measurements have not been taken in this data from August 1990 to June 1991, corresponding to the Iraqi invasion of Kuwait. To maintain data continuity in terms of time, this lack of information has been handled here by averaging the data by considering the seasonal mean resulting from adding the value of the same month but for the year before and after and then dividing by two.

The monthly total data of Kuwait International Airport for the time from January 1965 to December 2009 are shown in Figure 4(a). Here, a distinct rainfall pattern can be noticed dividing the data into three time intervals of 180 months. The rainfall incidents for the first and third intervals are larger than that for the second. This is evident, given that the corresponding total rainfall amounts through the intervals are 1870, 1420, and 2160mm, and the seasonal means estimated from (1) are 124.47, 94.93, and 144.25mm. This pattern suggests the presence of a large period of 360 months that may not appear in the periodogram as its size is so large relative to Batimastat the size of the data by which there would be no sufficient oscillating movement repetitive over the time interval.