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Trends in monthly precipitation over the northwest of Iran (NWI)

机译：伊朗西北（NWI）的月度降水趋势

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Increasing global temperatures during the last century have had their own effects on other climatic conditions, particularly on precipitation characteristics. This study was meant to investigate the spatial and temporal monthly trends of precipitation using the least square error (LSE) approach for the northwest of Iran (NWI). To this end, a database was obtained from 250 measuring stations uniformly scattered all over NWI from 1961 to 2010. The spatial average of annual precipitation in NWI during the period of study was approximately 220.9-726.7 mm. The annual precipitation decreased from southwest to northeast, while the large amount of precipitation was concentrated in the south-west and in the mountainous areas. All over NWI, the maximum and minimum precipitation records occurred from March to May and July to September, respectively. The coefficient of variation (CV) is greater than 44 % in all of NWI and may reach over 76 % in many places. The greatest range of CV, for instance, occurred during July. The spatial variability of precipitation was consistent with a tempo-spatial pattern of precipitation trends. There was a considerable difference between the amounts of change during the months, and the negative trends were mainly attributed to areas concentrated in eastern and southern parts of NWI far from the western mountain ranges. Moreover, limited areas with positive precipitation trends can be found in very small and isolated regions. This is observable particularly in the eastern half of NWI, which is mostly located far from Westerlies. On the other hand, seasonal precipitation trends indicated a slight decrease during winter and spring and a slight increase during summer and autumn. Consequently, there were major changes in average precipitation that occurred negatively in the area under study during the observation period. This finding is in agreement with those findings by recent studies which revealed a decreasing trend of around 2 mm/year over NWI during 1966-2005.
机译：上个世纪全球气温上升对其他气候条件，尤其是降水特征，都有其自身的影响。本研究旨在使用最小平方误差（LSE）方法研究伊朗西北部（NWI）的降水时空月度趋势。为此，从1961年至2010年从250个均匀分布在整个NWI的测量站获得了一个数据库。研究期间，NWI年降水量的空间平均值约为220.9-726.7 mm。年降水量从西南向东北递减，而大量降水则集中在西南和山区。在整个NWI，最大和最小降水记录分别发生在3月至5月和7月至9月。在所有NWI中，变异系数（CV）均大于44％，在许多地方可能达到76％以上。例如，最大的简历范围发生在7月。降水的空间变异性与降水趋势的时空格局一致。在这几个月中，变化量之间存在相当大的差异，而负面趋势主要归因于集中在西北地区东部和南部地区而不是西部山脉地区。此外，在非常小的偏远地区可以发现降水量呈正趋势的有限区域。尤其是在西北偏西地区的东部，这一点尤其明显，该地区大多位于远离西风的地方。另一方面，季节性降水趋势表明，冬季和春季略有减少，夏季和秋季则略有增加。因此，在观测期间，研究区域的平均降水量出现了较大的负变化。这一发现与最近的研究结果相吻合，最近的研究表明在1966-2005年期间，NWI比NWI下降了约2mm /年。

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