Effects of variations in climatic parameters on evapotranspiration in the arid and semi-arid regions (original) (raw)
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Effects of variations in climate parameters on evapotranspiration in the arid and semiarid
The main objective of this study is to investigate the effects of climatic parameters variability on evapotranspiration in five climatologically different regions of Iran. The regions include Tehran, Esfahan, Shiraz, Tabriz and Mashhad. Fifty four-year monthly records of temperature, relative humidity, sunshine duration, wind speed, and precipitation depth from 1951 to 2005 comprise the database. Trend and persistence analyses of the data are performed using the Mann–Kendall test, the Cumulative Deviation test, Linear Regression, and the Autocorrelation Coefficient. A sensitivity analysis of meteorological variables in these five regions is carried out using Penman-Monteith formula. In all of studied regions, temperature and relative humidity are the most sensitive parameters in Penman-Monteith formula respectively. The results of this study indicate that the effective climatic variables in evapotranspiration are changing, though in each region the variables have significant long-term trends and persistence.
2013
Evapotranspiration is one of the most important components of the hydrological cycle which is directly influenced by atmospheric conditions. This study investigated annual and seasonal trends in reference evapotranspiration (ET0) and its key influencing climatic variables during 1966-2005 at 10 stations in southern Iran (with centrality of Fars province). First, multivariate regression analysis was performed to identify the major meteorological variables affecting ET0. Second, annual and seasonal trends in climatic variables as well as ET0 were assessed using the Mann-Kendall test, Spearman's rho, the Pearson correlation and linear regression to evaluate their contribution to the temporal trend in ET0. Results suggested that the more effective variables for ET0 were wind speed (U2), relative humidity (RH) and sunshine hours (n). Also, the majority of trends in seasonal and annual ET0 were non-significant and after that decreasing and increasing trends had higher frequencies. In addition, distributions of relative frequencies of trend types at all considered time-scales were similar for both parametric and non-parametric techniques. Hence, the disagreement between parametric and non-parametric trend results did not depend on the degree of normality in the annual and seasonal ET0 distributions in the study area.
Analysis of spatial and temporal variations of reference evapotranspiration (ETo) is important in arid and semi-arid regions where water resources are limited. The main aim of this study was to analyse the spatial distribution and the annual, seasonal and monthly trends of the Penman–Monteith ETo for 21 stations in the arid and semi-arid regions of Iran. Three statistical tests the Mann-Kendall, Sen's slope estimator and linear regression were used for the analysis. The analysis revealed that ETo increased from January to July and deceased from July to December at almost all stations. Additionally, higher annual ETo values were found in the southeast of the study region and lower values in the northwest of the region. Although the results showed both positive and negative trends in annual ETo series, ETo generally increased, significantly so in six (∼30%) of the stations. Analysis of the impacts of meteorological variables on the temporal trends of ETo indicated that the increasing trend of ETo was most likely due to a significant increase in minimum air temperature, while decreasing trend of ETo was mainly caused by a significant decrease in wind speed. At the sites where increasing ETo trends were statistically significant, the rate of increase varied from (+)8·36 mm/year at Mashhad station to (+)31·68 mm/year at Iranshahr station. On average, an increasing trend of (+)4·42 mm/year was obtained for the whole study area during the last four decades. Seasonal and monthly ETo have also tended to increase at the majority of the stations. The greatest numbers of significant trends were observed in winter on the seasonal time-scale and in September on the monthly time-scale. Copyright © 2011 John Wiley & Sons, Ltd.
Research in Civil and Environmental Engineering, 2013
Evapotranspiration is one of the most important components of hydrologic budget which commonly is calculated from records of climatic data and geographical attributes of a meteorological station. Many scholars have found that climate change can reveal different local effects on climatic variables. In order to handle any possible potential impact of such changes on evapotranspiration, current study attempts to analyze the sensitivity of FAO Penman-Monteith reference evapotranspiration equation to four climatic variables for four stations placed in north-western of Iran. In annual scale through all stations maximum temperature was the most sensitivity inducing variable and Ardabil had the greatest sensitivity so that by a ±30% change in maximum temperature, reference evapotranspiration varied from -32.03% to 38.19%. The second important variable was actual vapor pressure which, also, had the most impact in Ardabil. Wind speed and minimum temperature were in the distant priorities. The sensitivity induced by U2 didn't change much from month to month. The results, although were not accomplished from real changes in the variables, provided good view for future water planners because wide range of climate change scenarios has been discussed in the paper.
Evapotranspiration is one of the most important components of hydrologic budget which commonly is calculated from records of climatic data and geographical attributes of a meteorological station. Many scholars have found that climate change can reveal different local effects on climatic variables. In order to handle any possible potential impact of such changes on evapotranspiration, current study attempts to analyze the sensitivity of FAO Penman-Monteith reference evapotranspiration equation to four climatic variables for four stations placed in north-western of Iran. In annual scale through all stations maximum temperature was the most sensitivity inducing variable and Ardabil had the greatest sensitivity so that by a ±30% change in maximum temperature, reference evapotranspiration varied from -32.03% to 38.19%. The second important variable was actual vapor pressure which, also, had the most impact in Ardabil. Wind speed and minimum temperature were in the distant priorities. The sensitivity induced by U2 didn't change much from month to month. The results, although were not accomplished from real changes in the variables, provided good view for future water planners because wide range of climate change scenarios has been discussed in the paper.
Sustainable Water Resources Management, 2019
Evapotranspiration, as the main component of the hydrologic cycle, will affect plant water demands and water resources plan. The aims of present research were to evaluate the tendency in reference evapotranspiration (ET 0) by the Mann-Kendall test, the Spearman's Rho test and the linear regression analysis at the 5% significant level using meteorological data of 16 synoptic stations at Southern Iran during period 1980-2010. Results revealed that 1, 3, 6, and 12 monthly ET 0 in most regions of study area showed increasing trends. Only, Shiraz station exhibited the significant decreasing trend in all time scales based on statistical methods. The values of the significant raising tendencies in annual ET 0 varied among − 0.501 at Shiraz and + 3.194 mm/year at Bam. Based on seasonal time scale, also Bam station showed the maximum significant increasing tendency (in summer and spring) based on Mann-Kendall and Spearman's Rho techniques. The monthly analysis of trend in ET 0 series indicated that the highest amounts of stations with significant increase tendency discovered in May. In general, this study result indicates a serious warning about warming and climate change of southern Iran. This subject can cause different effects including increment of water consumption in agricultural sector, increase of water rain waste, enhancement of water waste in dams, reservoirs, lakes, etc., decrease of surface water quality, and food crisis.
Trend analysis of reference evapotranspiration in the western half of Iran
Agricultural and Forest Meteorology, 2011
Reference evapotranspiration (ET o ) is an important element of the hydrological cycle, and changes in ET o are of great significance for agricultural water use planning, irrigation system design and management. In this study, annual, seasonal and monthly trends in the Penman-Monteith ET o at 20 meteorological stations during 1966-2005 in the western half of Iran were examined using the Mann-Kendall test, the Sen's slope estimator and the linear regression. Annual analysis of the ET o series indicated a positive trend in 70% of the stations according to the Mann-Kendall test and the Sen's slope estimator and in 75% of the stations according to the linear regression. The magnitude of significant positive trends in annual ET o varied from (+)11.28 to (+)2.30 mm/year. On the seasonal scale, stronger increasing trends were identified in ET o data in winter and summer compared with those in autumn and spring. Meanwhile, the highest numbers of stations with significant trends were found in the monthly ET o series in February, while the lowest numbers of stations with significant trends were observed in November. Analysis of the impact of climatic variables on the significant increasing trend in ET o showed that the increasing trend was mainly caused by a significant increase in air temperature during the study period.
2012
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.
Sensitivity analysis of reference crop evapotranspiration (ET 0 ) in Iran is an important necessity due to better management of water in such an arid and semi-arid country. It is important to analyze the sensitivity of ET 0 to weather parameters as climate seems to be changed to some degree everywhere. In this study, sensitivity of ET 0 to climatic variables at the eight selected stations of Iran was investigated. To estimate the ET 0 , the most recommended form of the FAO-56 Penman-Monteith method was used. Sensitivity of ET 0 was analyzed in terms of change in mean air temperature (T mean ), actual vapor pressure (e a ), wind speed (u 2 ) and net solar radiation (R n ) within a range of ±20 %, in 5 % steps, from their corresponding normal long-term values . Results showed that ET 0 was most sensitive to T mean at the six of the stations in annual time scale. Increase in T mean by +20 %, caused the most increase in ET 0 at Chabahar. In contrast ET 0 was least sensitive to e a in most selected stations. In this study, we investigated the interaction parameters in different scenarios on ET 0 at all stations in monthly and annual time scale. Results showed in annual time scale increasing T mean and WS by 20 % and decreasing e a by 20 % yielded ET 0 increasing 36.4 % at Chabahar. But, in monthly time scale 10 % increase in T mean and WS as well as 10 % decrease in R n yielded ET 0 to be increase to about 30.5 % in December at Bandar Anzali.
Agricultural Water Management, 2017
Reference evapotranspiration (ET 0) has a significant role in agricultural water management. Commonly, FAO56 Penman-Monteith as standard method is used to calculate ET 0. To apply this method, meteorological data recorded at reference stations are required. The use of non-reference stations data in ET 0 calculation can affect trend of this parameter. In the present study, the effect of reference-conditionbased temperature modification on ET 0 trend was evaluated. For this purpose, the collected data from 27 non-reference meteorological stations during 1960-2014 were applied. The mentioned stations are located in arid or semi-arid regions of Iran. The trends of unchanged and modified values of ET 0 in studied stations were calculated using the Mann-Kendall method. Trend results showed that unchanged values of ET 0 had a positive trend, while the modified ones had a negative trend in more than 50% of stations at seasonal and annual scales. According to R 2 and RMSE results, there was no correlation between the trends of unchanged and modified values of ET 0 and temperature modification based on the reference condition can change the trend values of ET 0. Also, the results of T-statistic showed that the difference between two values of trend in arid regions was more than that of semi-arid ones. This difference in stations with arid climate was significant at 95% confidence level.