Study of Long Term Changes in Reference Evapotranspiration in some Selected Warm Climates of Iran

Introduction
Investigating the trends of hydroclimatic components as a result of climate change has been an interesting task for the scientists and water resources managers. So far, numerous case studies have been conducted to exploit the possible trends in temperature and precipitation which imposed by climate change events. Unfortunately, there is a few research works to address the long term variations in some hydrometeorological components such as evaporation and evapotranspiration, perhaps for their complexity. Majority of the recent studies in America, China, India and Australia have reported a decline (reducing) trend in reference evapotranspiration (ET0). The downward trend in pan evaporation and evapotranspiration over most of the United States and former Soviet Union implies that, for large regions of the globe, the terrestrial evaporation component of the hydrological cycle has been decreasing. One explanation is that increased global cloudiness, especially low cloud cover, would be an expected consequence of higher global temperatures. Some increases in annual mean cloudiness have been observed over Europe, Australia, the Indian sub–continent and North America. When cloudiness over the oceans is also considered, it is not possible to be confident that average global cloudiness has really increased. The main purpose of this article is to explore the possible trend in daily ET0 in some selected sites located in warm regions of Iran. The assessment of trend in daily ET0 would be an important tool for the decision makers in water resources engineering and agriculture sectors.

Data and Methods
The authors have used a 50-year (1957-2006) dataset of the observed meteorological variables (mean of daily maximum temperature, mean of daily minimum temperature, relative humidity, wind speed, water vapor pressure, dew point temperature, and air pressure) which recorded in 13 synoptic sites (IRIMO, 2007) during the period of study. Prior to the analysis, all datasets were tested for quality check and gaps. The annual and seasonal trends of ET0 were derived by Mann-Kendall Test and Sen's estimator as non-parametric methods. To compare the daily means of ET0 (50-year and 16-year), Mann-Whitney test was applied. For each selected site, the 50-year ET0 trends were compared against the ET0 trends of 16-year (1991-2006) period. The trend analysis from Sen's method was capable to perform better results, because of eliminating the effect of repeating data in ET0 time series.

Results and Discussion
The either tests (Mann-Kendall and Sen) confirmed that the maximum and minimum significant trends (annual and seasonal) have occurred in summer (47% of total stations) and winter (7% of total stations), respectively. Therefore, the first hypothesis of the study (the existence of significant trend in ET0 values was confirmed. The comparison of the trends in different climates showed that the slope of the ET0 trend during the recent 16 years (1991-2006), which is slightly greater than that of 50-year (1957-2006), has the same overall sign (negative) for most of the stations. As a result, the second hypothesis of the study (the existence of similar trends in recent decades) was also confirmed.

Main Findings and Conclusions
In general, 65% of the stations revealed a negative (decline) trend in ET0 time series at the specified significant level (P< 0.05). The highest and lowest slopes of ET0 were observed for summer (-4 mm/year) and winter (2.56 mm/year), respectively. The comparison of the ETo trends for 50-year period with those of 16-year period presented a good consistency. The results obtained for 16-year analysis showed that 67% of the study sites have experienced negative trends. This suggests that a higher number of sites had the chance to experience reduction in annual ET0 during the recent years. Trend analysis of Mann-Kendall and Sen methods were generally in good agreement. It was shown that 47% of the case studies have no significant trend (positive and negative) in reference evapotranspiration values. In seasonal scale, summer seasons experienced higher number of significant ET0 trends, in comparison to other seasons. This result is in good agreement with most findings from other research works reported from outside Iran. The detection of trends in ET0 for other climate types is required for a national comprehensive work. Further study is also required to find out the reason for different trend signs.