عنوان مقاله [English]
Global warming due to greenhouse gas emissions is leading to changes in the spatial and temporal distribution of water resources on a global scale. On the other hand, as Iran is located in an arid and semi-arid climatic region, about 75% of rainfall in the country is directly returned to the atmosphere through evaporation. Since potential evapotranspiration is one of the most important components of the natural water cycle and is the identification of plant water requirement, its exact estimation plays a key role in the planning related to the type of cultivation and irrigation. Based on the literature, the Mann-Kendall test has been used repeatedly to examine the trend of changes in meteorological components. Considering the importance of determining the trend of evapotranspiration changes in water resources planning, the purpose of this study is its investigation in a monthly timescale using Mann-Kendall test and the Sen’s estimator slope in Iran. In addition, by analyzing the spatial distribution of evapotranspiration using the GIS, the trend of evapotranspiration changes was also examined.
Materials and methods:
The study area in this research includes 79 appropriately distributed synoptic meteorological stations with acceptable data quality which belong to the Irainan Meteorological Organization. Their common statistical period is 22 years (1995 to 2016). From a geographical point of view, Iran is located in the northern hemisphere between 25 and 40 degrees north latitude and between 44 and 63.5 degrees east longitude. It has a dominant arid and semi-arid climate with low rainfall and high evapotranspiration. After selecting synoptic stations, the required data including geographical coordinates of stations, altitude, daily temperature (minimum, maximum, and average), wind speed, and sunshine hours were collected for the common time period (1995 to 2016). Then, the potential evapotranspiration was calculated using Hargreaves-Samani method and the total monthly evapotranspiration was extracted for the desired time period. By performing the Mann-Kendall test, the trend of evapotranspiration variation has been estimated and the significance of this trend has been analyzed using the Sen’s estimator slope.
Results and discussion:
The results of this study obtained by the analysis of the evapotranspiration trend during 1995 to 2016 and implementing the Mann-Kendall test and the Sen’s estimator slope on a monthly time scale show that the trend does not follow the same pattern during different months of the year. In terms of time, in spring and summer, significant increasing trends have been seen in most cases where its highest were in June. In July, different climates of Iran have a significant increasing trend. Due to low rainfall in this month and sensitive conditions, this increasing trend of evaporation can raise the demand for water resources and in some cases reduce the yield of agricultural products. In August and September, despite the oppressive heat, in most of central and southern parts of the country, with a hot and dry climate, there was a decreasing trend of evapotranspiration. At the beginning of autumn and in October, the central parts of Iran had a significant downward trend, but in November and December, the existence of significant trends is much less. In winter, there is no trend in most stations. Generally speaking, the most significant increasing trends have occurred in the warm months of the year. Due to the decrease in precipitation oscillations and increase of temperature in these months, increase of evaporation has raised the water needs for agricultural products. Thus, the failure in its management will lead to a shortage of available water and damage to agriculture by the reduction of crop efficiency and yield. From spatial viewpoint, there is a more increasing trend in the cold regions and mountainous areas of the country and a negative trend has appeared in the tropical, central and eastern regions. In general, the percentage of non-trending stations varies from about 68% in summer to about 86% in autumn, and on average, about 78% of stations have a steady state about the evapotranspiration. Of the remaining 22% of stations, most (about 15%) have an increasing trend, and this goes back to the spring and summer seasons. This means that the plants’ water requirement is increasing when there is less rainfall, which is consistent with the conditions created by climate change, which leads to more severe drought events in arid areas.