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Abstract of the Article

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Analysis of Variation of Precipitation Beginning Dates and their Trends in Tehran during the Period 1951-98

Author:
[ Hedayati ] -

Abstract:

Precipitation is one of the most important climatic elements. The important features of arid and semiarid regions are low precipitation and highly fluctuating precipitation. Success of many big national plans or projects depends mainly on precipitation patterns in the country. The purpose of this study is to analyze precipitation-beginning dates for thresholds of 0.1, 1, 5. 10 and 20 mm and determination of treads for annual and seasonal precipitations in Tehran during period 195 1-1998. Results showed negative trends for low thresholds like 0.1 and 1 mm and on the other hand, their beginning dates shifted to the early autumn. We found a significant positive trend for winter precipitation with 49.3mm increase during period 1951-1998. We also found negative trends for spring and summer precipitation with 6 and 5mm decrease during the above period. There was no marked trend for autumn's precipitation. The analysis of ratio of maximum precipitation to total precipitation in each month in the period showed negative trends for later months of autumn and months ofwinter and positive trends for other months of the year.

Keywords:

Precipitation beginning dates, Precipitation threshold.