باد صدوبیست روزه‎ی سیستان

Sistan is located in the southeastern part of Iran downstream of Hirmand River. The Climate of the region is extremely dry and suffering from extensive successive droughts nowadays. Sistan wind is the most powerful regional wind blowing during summer in south eastern parts of Iran and in some parts of Afghanistan and Pakistan. In order to detect the major characteristics of this regional wind, three hourly wind direction and wind speed data at Zabol station has been used. During a 44 years period from 1966 to 2009 about 75354 valid observations is available. A frequency analysis of wind direction showed that the wind direction is north northwest throughout the year. In calm season few winds from different sparse directions may observe but the dominant direction still remains the same and wind blows from north northeast. So the wind direction cannot be seen as a discriminant variable for defining Sistan windy season. In the other word topography dramatically affects the wind direction in the region and configuration of pressure systems has a second hand role in the wind direction.
Meanwhile there is a remarkable difference between wind speed in windy season and the rest of year. Wind speed distribution in Zabol is a triple modal one. The first mode is weak winds with speeds not more than 5 meters per second that are frequent in calm season. The second and the third modes are 10 and 12 meters per second winds respectively. In windy season winds are frequently faster than 10or even 12 meters per seconds. Moreover in some cases wind speeds of 15, 18, 20, 22 and 25 meters per seconds are not rare. On July 15th 1979 wind speed exceeds 41 meters per second. A PCA analysis applied to wind speed data revealed that windy season starts on May 17th and ends at 29th of September. So the windy season lasts about 136 days in Zabol. During the windy season wind direction is dominantly from north northwest and wind speed is frequently about 11 meters per second.
Analysis of smoothed wind speed time series suggests that the wind speed has experienced a sharp jump since 1998. Because of the thermal nature of this regional wind this jump may reflect the same remarkable increase in temperature time series of Iran. In other word global warming means more powerful thermal low over Pakistan and Iran. Although the major factor affecting the Sistan wind is a pressure gradient exists between northeastern part of Iran and southern part of Pakistan, topography via gap wind mechanism plays an important role in Sistan wind. The more powerful the low pressure over Pakistan (Pakistan low) the more powerful the Sistan wind is. Considering the increase of wind speed in Sistan and regarding the position of Margo and Rigestan deserts downstream of Hirmand River it is very obvious that Iran’s share of Hirmand’s water is not just a legal right referring to a treaty between two neighboring countries but it is an environmental issue that will affect Afghans too sooner or later.