Trends in reference crop evapotranspiration over Iran
Research highlights
► Both upward and downward significant trends observed for annual and monthly ET0. ► The strong positive annual ET0 trend magnitude was about 186 mm/year per decade. ► The strongest positive (negative) monthly trend was found in April (July) in Iran. ► ET0 trends between stations and months were heterogeneous in Iran. ► Wind speed found to be the most dominant variable influencing ET0 in Iran.
Introduction
It is now widely accepted that the concentration of greenhouse gases (GHG) of the Earth’s atmosphere have increased in recent decades. The main effect of increase in the (GHG) concentration has been the increase in air temperature. The increased air temperature over different parts of the world has caused different effects on elements of the hydrologic cycle. Some of the researchers (Tayanc et al., 1997, Zhang et al., 2001, Kahya and Kalaycı, 2004, Lins and Slack, 1999, Lins and Slack, 2005, Partal and Kahya, 2006, Aziz and Burn, 2006, Burn, 2008, Basistha et al., 2009, Jhajharia et al., 2009, Sahoo and Smith, 2009, Deni et al., 2010; and many others) have investigated the trends in different types of hydrological and hydro-meteorological parameters such as air temperature, precipitation, streamflow, water pollutant concentration, drought characteristics, Pan evaporation (Epan) and reference crop evapotranspiration (ET0) over different parts of the world.
ET0 is one of the main elements in the hydrological cycle, which is affected by the changes in air temperature, sunshine duration, and wind speed and so on. Changes in air temperature can alter the saturation vapor pressure, which in turn changes the evaporation and ET0 rate. It is well known that ET0 is a nonlinear complex function of many parameters and change in any one parameter can change the other parameter(s) and therefore, the effect of such changes on ET0 is very difficult to understand. Most of the meteorological variables can influence ET0; however, sunshine duration and wind speed are the two important meteorological parameters controlling ET0 in different regions of the world (Thomas, 2000, Jhajharia et al., 2009).
Many investigators have studied trends in ET0 across the different regions of the world. In contrast to the global warming, most of the investigators have reported decreasing trends in ET0 in several countries. For example, Chattopadhayay and Hulme (1997) studied Epan and potential ET0 trends over different parts of India and reported decreasing trends in both Epan and potential ET0. Gan (1998) reported decreasing trends in ET0 at 13 stations in Alberta. Xu et al. (2006) investigated trends in Epan and ET0 in Changjiang (Yangtze River) catchment and found decreasing trends in both Epan and ET0 for the whole catchment. Bandyopadhayay et al. (2009) also studied ET0 trends in India and reported decreases in ET0 all over India. Tebakari et al. (2005) conducted trend analysis of the Epan in the Chao Phraya River basin located in Thailand and observed decreasing trend in Epan. Jhajharia et al. (2009) reported decreasing Epan trends mainly in pre-monsoon and monsoon seasons over the northeast India. They showed that sunshine duration followed by wind speed strongly influenced the observed Epan changes in the northeast India. Similarly, Shen et al. (2010) found statistically significant decreasing trends in Epan in the arid regions of China.
On the other hand, several researchers also reported increases in ET0 trends. Yu et al. (2002) observed increasing trends in ET at Kao-Hsiung, south Taiwan, using 48 years of data. Hess (1998) reported an increasing trend in ET0, which was caused primarily due to the observed increases in wind speed in the northeast arid zone of Nigeria. Burn and Hesch (2007) analyzed trends in Epan at 48 stations of the Canadian Prairies and found both increasing and decreasing trends in Epan during the period of 1971–2000. They observed that the wind speed (vapor pressure deficit) had more influence on decreasing (increasing) trends in Epan over the Canadian Prairies.
Spatial and temporal homogeneity of trends is an important aspect of trend analysis of any hydro-meteorological parameter in any region. Van Belle and Hughes (1984) developed a method to test the homogeneity of trends by Mann–Kendall (MK) test. Homogeneity test is performed on a data set obtained by combining the data of several stations to obtain a possible single global trend. This test was previously applied for homogeneity of trends in different variables, such as rainfall (Kampata et al., 2008), streamflow (Kahya and Kalayci, 2004), precipitation and air temperature (Gan, 1995 and Gan, 1998), and groundwater levels (Panda et al., 2007). To our best knowledge, no such study concerning homogeneity of trend using ET0 time series is available in the literature.
Iran, mainly an agrarian society is seriously vulnerable to the anthropogenic-induced climate change as most of the geographical area falls under the arid and semi-arid type of climate. It seems very likely that any change in the availability of water will play a key role in the sustainable development of agriculture and environment in Iran. Few studies are available in the literature on the trend analysis of mean annual air temperature (Ghahraman, 2006) and precipitation (Ghahraman and Taghvaeian, 2008; and Modarres and da Silva, 2007) in Iran. However, very little information is available on the trend analysis of ET0, an important component of hydrologic cycle, on temporal and spatial basis over Iran. Therefore, the present study is undertaken with the four objectives, which are as follows: (1) to estimate the monthly and annual ET0 using the Food and Agriculture Organization (FAO-56) Penman Monteith (PM) method, and to detect the monotonic linear trends in the ET0 time series using the MK non-parametric test; (2) to estimate the slopes of trend lines of ET0 times series using the Theil–Sen’s estimator method; (3) to test the homogeneity of trends of ET0 series over Iran; and (4) to identify the most dominating and important meteorological variables affecting the ET0 time series using multiple (stepwise) regression analysis.
Section snippets
Study area and data
The study area encompasses the entire region of Iran’s geographical area of about 1650,000 km2. Iran is located in Asia, approximately between 25°00′N and 38°39′N latitudes and between 44°00′E and 63°25′E longitudes (Fig. 1). The mean annual precipitation of Iran is about 224 mm. The two main mountain chains in Iran are Alborz and Zagros. Alborz Mountains extends from the north to the west and east Iran, while, Zagros Mountains extends from the northwest to the southern part of Iran. Generally,
FAO-56 Penman–Monteith (PM) method
For estimating ET0, Allen et al. (1998) derived the FAO-56 Penman–Monteith equation from the original Penman–Monteith equation and the equations of aerodynamic and surface resistance. The FAO-56 Penman–Monteith method, designated as PM in the present study, is now universally accepted for calculating ET0 in various climates. The PM method can be expressed as:where ET0 is the reference crop evapotranspiration (mm/day), Δ is the slope of vapor
Temporal Trends in ET0 in Iran
Table 3 shows the ET0 trend results in terms of Z statistic for all months and the year. It is important to emphasize that any Z statistic within the confidence limits (insignificant Z) presents a value due to random fluctuations, meaning not much in inferring existence of a trend from the statistical standpoint. Herein we perceived and treated insignificant Z statistics, particularly for its greater values, just as a tendency for trend-type behavior of the time series under consideration. Fig.
Discussion
Based on the PM estimates (1965–2005), ET0 in annual time scale exhibited both upward and downward trends in different locations of Iran. Most of the positive trends were observed for northwest and northeast parts of Iran, covering large areas of mountainous dry-farming regions of Iran. This implies that crop water requirements for production of various crops in these areas will increase in view of increases in ET0. A large portion of spring rainfall in these regions, originated from the
Conclusions
The ET0 trends in the annual and monthly time scales during the period of 1965–2005 carried out using the Mann–Kendall method over Iran. The effect of significant lag-1 serial correlation removed from data by pre-whitening prior to trend analysis. Results indicated that both positive and negative trends detected in the annual and monthly ET0 series at different stations located in different regions of Iran. Increasing ET0 trends prevailed in rain-fed agricultural areas in northwest and
Acknowledgements
The authors are grateful to the anonymous reviewers for their useful comments and suggestions that have led to improved quality of the paper. We also appreciate Dr. Damadi for his useful comments. This work financially supported by the Office of Vice Chancellor for Research of University of Maragheh, gratefully acknowledged.
References (55)
Climatic influences on streamflow timing in the headwaters of the Mackenzie River basin
Journal of Hydrology
(2008)- et al.
Trends in evaporation for the Canadian Prairies
Journal of Hydrology
(2007) Study of reference crop evapotranspiration in I.R. of Iran
Agricultural Water Management
(2006)- et al.
Selection of variables for the purpose of regionalization of Iran’s precipitation climate using the multivariate methods
Journal of Hydrology
(2004) - et al.
The association between regional and global atmospheric patterns and winter precipitation in Iran
Atmospheric Research
(2008) Sensitivity of evapotranspiration to global warming: a case study of arid zone of Rajasthan (India)
Agricultural Water Management
(2004)Trends in reference evapo-transpiration in the North East Arid Zone of Nigeria, 1961–1991
Journal of Arid Environment
(1998)- et al.
Linear trend analysis: a comparison of methods
Atmospheric Environment
(2001) - et al.
Temporal characteristics of pan evaporation trends under the humid conditions of northeast India
Agricultural Forest Meteorology
(2009) - et al.
Trend analysis of streamflow in Turkey
Journal of Hydrology
(2004)
Trend analysis of rainfall in the headstreams of the Zambezi River in Zambia
Physics and Chemistry of the Earth
Streamflow trends in Indiana: Effects of long term persistence, precipitation and subsurface drains
Journal of Hydrology
Rainfall trends in arid and semi-arid regions of Iran
Journal of Arid Environment
The influence of drought and anthropogenic effects on groundwater levels in Orissa, India
Journal of Hydrology
Analysis of spatial distribution and temporal trend of reference evapotranspiration and pan evaporation in Changjiang (Yangtze River) catchment
Journal of Hydrology
Essentials of Meteorology, An Introduction to The Atmosphere
Variation of 500 hPa flow patterns over Iran and surrounding areas and their relationship with the climate of Iran
Theoretical and Applied Climatology
Trends and variability in the hydrological regime of the Mackenzie River basin
Journal of Hydrology
Temporal trends in estimates of reference evapotranspiration over India
Journal of Hydrologic Engineering
Analysis of historical changes in rainfall in the Indian Himalayas
International Journal of Climatology
Evaporation and potential evapotranspiration in India under conditions of recent and future climate change
Agricultural Forest Meteorology
Climatic change on the Tibetan Plateau: potential evapotranspiration trend from 1961–2006
Climate Change
Spatial trends on dry spells over Peninsular Malaysia during monsoon seasons
Theoretical and Applied Climatology
Trends in air temperature and precipitation for Canada and north-eastern USA
International Journal of Climatology
Hydroclimatic trends and possible climatic warming in the Canadian prairies
Water Resources Research
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Present address: Department of Civil Engineering, American University of Sharjah, Sharjah, United Arab Emirates.