عنوان مقاله [English]
Evapotranspiration is one of the most important factors in determining the region's climate and also in hydrological cycle after precipitation , which is important for estimating water demand, especially in agriculture. In this research, evapotranspiration process of the reference crop was investigated by Penman-Monteith method for Razavi Khorasan province. Razavi Khorasan Province is located in north east of Iran. The province, with an area of about 118,000 square kilometers, is the fourth largest province in the country and one of the most important agricultural parts, and it is evident that the proper estimation of evaporation and prediction of its trend for water resource planning and management is of particular importance. In this research, Tissen polygon method has been used to estimate the evapotranspiration of the reference crop in Khorasan Razavi province. In addition, the trend of changes in important climate parameters affecting evapotranspiration including temperature, relative humidity, wind and sunny hours in synoptic stations of Khorasan Razavi province have been investigated in annual, seasonal and monthly time series. For this purpose, the 30 year statistics of ten synoptic stations of Khorasan Razavi province were used during the period of 1988 to 2017.
After collecting data, Their accuracy were controlled. After verification of the accuracy of the data and their completeness during the selection period, which was carried out using accepted scientific methods, the calculated potential evapotranspiration of the reference crop has been calculated. Since the data are abnormal, in order to investigate the trend, the Mann-Kendall test was used at a significant level of 5%. The slope estimator was used to determine the trend slope. The results show that the annual ETo data of all stations in the province have a positive amount of ZMK and is significant in the seven stations located in the north and the center of this province. Also, all stations have an upward slope. The highest slope is for the Sabzevar station, which has grown by an average of 18 mm ETo per year in the last 30 years. In winter, ETo all stations in the province have a meaningful positive trend. Also, ZMK is positive in the spring and autumn season for all stations. Meaningful process is observed in seven and five stations for spring and autumn accordingly. The summer season is only negative at the ZMK Torbat Jam Station, and This downward trend is meaningless.An ETo survey of 10 stations per month shows that in 94% of cases there is an increasing trend for ETo, of which 65% of cases are significant. In only 6% of cases, the The ETo without a meaningful trend is decreasing. Neyshabour and Sabzevar, in all months of the year, Mashhad in 11 months and Golmakan and Torbat Heidariye in 10 months of the year show a meaningful trend. The highest monthly Sen’s slope in total was related to Sabzevar and the lowest incremental Sen’s slope in Torbat Jam. In examining the parameters affecting ETo, it is observed that the minimum and maximum monthly temperatures in the 83% of cases are increasing, which is significant in 46% of the time. The temperature of Mashhad has increased more than other cities. The wind speed, which has a direct effect on the increase of ETo, is rising in 97% of cases, and 63% of them have a significant trend. Sunny hours are also rising for 66 percent of the time, but only 13 percent are significant. But relative humidity, which has an inverse relationship with ETo, is 71% lower, and 18% of it has a significant negative trend. The monthly ETo process in Khorasan Razavi province has been calculated using the weighted average and Arc Gis software, and the results indicate that all months of the year have a positive ZMK and, except for November, every 11 months of this trend is Meaningful. The annual and seasonal ETo of the province also has a significant trend. Also, in the statistical period of 30 years, an average of 8.8 mm per year was increased by ETo level in the province. The results of this study indicate the trend in potential evapotranspiration in Khorasan Razavi province. Considering that this parameter has a direct impact on the water needs of agricultural products and water supply, it is necessary to consider these changes in climate predictions for large-scale water management and agricultural management in the province.