Understanding the trends of reference evapotranspiration (ET_o) and its influential meteorological variables due to climate change is required for studying the hydrological cycle, vegetation restoration, and regional agricultural production. Although several studies have evaluated these trends, they suffer from a number of drawbacks: (1) they used data series of less than 50 years; (2) they evaluated the individual impact of a few climatic variables on ET_o, and thus could not represent the interactive effects of all forces driving trends of ET_o; (3) they mostly studied trends of ET_o and meteorological variables in similar climate regions; (4) they often did not eliminate the impact of serial correlations on the trends of ET_o and meteorological variables; and finally (5) they did not study the extremum values of meteorological variables and ET_o. This study overcame the abovementioned shortcomings by (1) analyzing the 50-year (1961–2010) annual trends of ET_o and 12 meteorological variables from 18 study sites in contrasting climate types in Iran, (2) removing the effect of serial correlations on the trends analysis via the trend-free pre-whitening approach, (3) determining the most important meteorological variables that control the variations of ET_o, and (4) evaluating the coincidence of annual extremum values of meteorological variables and ET_o. The results showed that ET_o and several meteorological variables (namely wind speed, vapor pressure deficit, cloudy days, minimum relative humidity, and mean, maximum and minimum air temperature) had significant trends at the confidence level of 95% in more than 50% of the study sites. These significant trends were indicative of climate change in many regions of Iran. It was also found that the wind speed (WS) had the most significant influence on the trend of ET_o in most of the study sites, especially in the years with extremum values of ET_o. In 83.3% of the study sites (i.e., all arid, Mediterranean and humid regions and 66.7% of semiarid regions), both ET_o and WS reached their extremum values in the same year. The significant changes in ET_o due to WS and other meteorological variables have made it necessary to optimize cropping patterns in Iran.

中文翻译：

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驱动参考蒸散量趋势和气候变化信号的力量的复杂性

为了研究水文循环，植被恢复和区域农业生产，需要了解参考蒸散量（ET _o）的趋势及其因气候变化而影响气象的变量。尽管有几项研究评估了这些趋势，但它们具有许多缺点：（1）他们使用了不到50年的数据序列；（2）他们评估了一些气候变量对ET _o的个体影响，因此不能代表所有驱动ET _o趋势的力的相互作用。（3）他们主要研究ET _o的趋势和类似气候区域的气象变量；（4）他们经常没有消除序列相关性对ET _o和气象变量趋势的影响；最后（5）他们没有研究气象变量和ET _o的极值。这项研究克服了上述缺点，方法是：（1）分析伊朗18个研究地点的ET _o的50年（1961-2010）年度趋势和12个气象变量，对比伊朗的气候类型，（2）消除序列相关对通过无趋势的预增白方法进行趋势分析，（3）确定控制ET _o变化的最重要的气象变量（4）评估气象变量和ET _o的年极值的一致性。结果表明，ET _o和几个气象变量（即风速，蒸气压赤字，阴天，最小相对湿度以及平均，最高和最低气温）在95％的置信度下（大于50％）具有显着趋势。研究地点。这些重大趋势表明伊朗许多地区的气候变化。还发现在大多数研究地点，风速（WS）对ET _o的趋势影响最大，尤其是在ET _o极值出现的年份_。。在83.3％的研究地点（即所有干旱，地中海和湿润地区以及半干旱地区的66.7％）中，ET _o和WS均在同一年达到极值。由于WS和其他气象变量，ET _o的显着变化使得有必要优化伊朗的种植方式。