Trend Analysis of Precipitation and Air Temperature in water basins of Iran

In this study, the trend of precipitation and temperature variables in basin scale across Iran was evaluated using ministry of energy weather stations data. Initially, among the total 2,142 climatology and 1,230 evaporation stations corseponding data of those with a minimum of 40 years record and a maximum of 3 months of missing data were selected, resulting in 476 and 116 climatology and evaporation stations, respectively. The study period was from water year 1973-74 to 2013-14. The monthly, seasonal, and annual precipitation and temperature data of the selected stations were spatially interpolated in different catchements across the country using the Thiessen method. The Mann-Kendall test was then employed to detect trends of times series. Considering the significance levels of 90, 95, and 99 percent, the results were analyzed at each station, second-degree basin, first-degree basin, and across the entire country. The findings of this research indicate an increasing trend in autumn precipitation for the northern part of the country (13 percent of stations), a decreasing trend in winter precipitation for 33 percent of stations, no significant trend in spring precipitation, and an increasing trend in summer precipitation. Additionally, an increasing trend was observed in all temperature time series. By comparing the changes over the last 5 and 10 years of study period (ending by 2013-2014) with the long term 40-years average, a significant decrease in precipitation in most regions of the country, was observed, this accompanied by an increase in the mean temperature, suggests a substantial reduction in available water resources in study period.