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In this research, temporal trends of reference evapotranspiration (ET0) values were investigated in arid regions of Iran. For this purpose, meteorological observations collected from 11 high quality meteorological sites over a 41-year period (1965-2005) were used and statistically significant ET0 trends in the monthly, seasonal and annual time basis were detected using nonparametric Mann-Kendall (MK) and Spearman's Rho (SR) tests at the 5% significant level. To eliminate the effect of serial correlation on the test results, the Trend Free Pre-Whitening (TFPW) approach was applied. The results of this study indicated that the ET0 trends for some cities were increasing (positive) however, for some sites, they showed decreasing (negative) trends. The most number of significant ET0 trends on a monthly time scale occurred at Birjand but no significant trend was observed at the Bandarabbas, Sabzevar and Semnan sites. On the annual time scale, Mashhad revealed the highest positive trend (7.5 mm per year). On the other hand, Esfahan showed the highest decreasing (negative) ET0 trend of about −6.38 mm per year. In general, the performances and abilities of the MK and SR tests were consistent at the verified significant level.

Evaporation is an important component of the hydrological cycle and its change would be of great significance for water resources planning, irrigation control and agricultural production. The main purpose of this study was to investigate temporal variations in pan evaporation (E pan ) and the associated changes in maximum (T max ), mean (T mean ) and minimum (T min ) air temperatures and precipitation (P) for 12 stations in Hamedan province in western Iran for the period 1982-2003. Significant trends were identified using the Mann-Kendall test, the Sen's slope estimator and the linear regression. Analysis of the E pan data revealed a significant increasing trend in 67% of the stations at the 95% and 99% confidence levels.To put the changes in perspective, the trend in E pan averaged over all 12 stations was (+)160 mm per decade. Trend analysis of the meteorological variables for examination of causal mechanisms for E pan changes showed warming trends in T max , T mean and T min series in almost all the stations, which were significant over half of the total stations. On the contrary, no significant changes in precipitation were found approximately at all of the stations. Furthermore, a moderate positive correlation was observed between E pan and T max , T mean and T min , while a inverse correlation was found between E pan and P data. The results indicated that the study area has become warmer and drier over the last 22 years, hence the evaporative demands of the atmosphere and thereby crop water requirements have increased.

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Sixteen water quality parameters have been monitored at four stations located along the Maroon River during 1989-2008. The trend analysis was performed on seasonal and annual time-scales using the Mann-Kendall test, the Sen's slope estimator and the linear regression. The relationships of the water quality parameters to river discharge were also investigated. The statistical methods showed both positive and negative trends in annual water quality data. However, significant trends were detected by the statistical methods only in calcium, magnesium, sodium absorption ratio, pH, and turbidity series. The results indicated that the concentrations of the water quality parameters increased in spring and winter seasons, while the concentrations were diluted in summer and autumn seasons in the last two decades. Moreover, the highest numbers of significant trends were found in the spring and summer series, respectively. According to the regression analysis, most of the water quality parameters were negatively correlated with river discharge.

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