Changes in reference evapotranspiration and its driving factors in peninsular Malaysia
Introduction
Evapotranspiration (ET) is the only link between the water and energy cycle and the major contributor of water vapour to the atmosphere. It has importance similar to precipitation in determining atmospheric moisture balance. Reference ET (ETo) is a decisive factor for irrigation management, catchment water equilibrium, water resources planning, ecosystem health and reservoir operation. It is also considered as one of the most crucial factors for any hydrological and climatic study (Roudier et al., 2014; Pour et al., 2020a). Understanding the changes in evapotranspiration is imperative for water resources management and devising climate change adaptation strategies (Wang et al., 2016).
ETo depends on several meteorological variables including solar radiation (SR), temperature, vapour pressure gradient, relative humidity (RH) and wind speed (WS). The changes in ET depends on those meteorological variables and their interactions. Earth mean temperature has increased by 0.8 °C since 1980, while the surface SR has decreased by 10–20% during 1950–1990. The RH over the land surface has changed small and insignificant (0.6%) during 1976–2004, but the average terrestrial WS has decreased by 0.5 km/h in every decade after 1960 (Dodgshun, 2017). The contradictory changes in thermal, radiative and aerodynamic components of ETo have caused an inconsistency in ETo trend over the globe. Therefore, in spite of a sharp rise in temperature, ETo has been found to decline in most parts of the earth in last fifty years (Fu et al., 2009; Pan et al., 2019). Declining SR and WS have been considered as the major cause of evapotranspiration paradox in many regions.
The changes in meteorological variables are not occurring evenly over the earth. For example, though RH change is insignificant at the global scale, it is found to increase by 0.5–2.0% per decade over a major portion of the United States, India and China during 1976–2004. Different changes in meteorological factors have caused a significant rise in ETo in cold and arid steppe sites of Northeast Asia (You et al., 2019), Argentina (D'Andrea et al., 2019), east part of Pakistan (Ahmad et al., 2019), South Korea (Ghafouri-Azar et al., 2018; Hwang et al., 2019), Romania (Prăvălie et al., 2019) and central India (Kundu et al., 2017), while a decrease in semi-arid region of Brazil (Cabral Júnior et al., 2019), Bangladesh (Rahman et al., 2019), humid areas of Iran (Nouri and Bannayan, 2019) and majority of China (Xu et al., 2018) including Yangtze River Delta (Xu et al., 2017) and Hexi Corridor (Yang et al., 2020). No change in ETo has also been reported in some regions including great Britain (Blyth et al., 2019), Southeast region of Pakistan (Ahmad et al., 2019) and Qilian Mountains of China (Lin et al., 2018). Both increase and decrease in ETo have also been noticed in some regions like Tibetan Highland (Zhang et al., 2019), the yellow river source region (Liu et al., 2017) and the West Liao River basin of China (Gao et al., 2017).
Few studies attempted to assess time-varying trends in ETo and reported contrast in trend directions for different sub-periods. Szilagyi and Jozsa (2018) reported declining trend during 1981–2007, but increase during 2007–2015 in the Central Valley of California. Stephens et al. (2018) noticed a reduction in pan evaporation in Australia during 1975–1994, while an increase during 1994–2016. Guo et al. (2019) reported different trends in evaporation in three different periods during 1961–2015 over Siling Co Lake on the Tibetan Plateau, China. The studies reported changes in the global climate have changed the driving factors of ETo and thus the mean of ETo. The studies highlighted the needs for analysis of time-varying trends in ETo to apprehend the recent changes and the possible forthcoming changes in ETo.
The implication ETo changes are much higher in tropical region, particularly in humid tropical region. The humid tropical region hosts the most diverse ecosystems and a high concentration of species. The species have a narrow climatic niche and therefore, highly subtle to climate variability. A small change in ETo can have a large impact on biodiversity. However, studies related to changes in ETo and the causative factors of such changes are still very limited in tropical region. Only a few studies assessed the historical changes in ETo in tropical humid region which also indicated inconsistencies in ETo trends. Rahman et al. (2019) noticed a declining trend in ETo in most of the regions of Bangladesh. Pandey and Khare (2018) reported increasing ETo in the tropical Narmada river basin of India. Nouri and Bannayan (2019) also showed a rising trend in ETo in humid climate zone of Iran. The large inconsistency in reported ETo trends in tropical region in previous studies urges the need for investigation of the causes of such inconsistency in tropical region.
Tropical belt in Southeast Asia, a major constituent of maritime continent has significant importance in both local and global climate system (Chen et al., 2019). The region acts as a major source of the convective activity in the globe (Lan et al., 2016). Assessment of ETo in the region is therefore highly important for understanding regional and global climate. However, knowledge on the changes in ETo and its driving factors for Southeast Asia is very limited. Malaysia, located in the west of tropical maritime continent has experienced changes in climate in recent years. The daily temperature has increased significantly in the whole peninsula which is also projected to be continuous in future (Shaaban et al., 2011; Paterson et al., 2015; Khan et al., 2019; Noor et al., 2018, Noor et al., 2019). Climate models also projected a monotonic increase in future temperature in the region. However, the changes in rainfall are still not noticeable. Therefore, most looming impacts of global warming in Malaysia would be from rising temperature and other hydrological factors directly linked to temperature such as ETo. Though vast studies have been conducted to understand the changes in rainfall and temperature in peninsular Malaysia, assessment of the changes in ETo and the driving forces behind those changes has not been explored yet.
The major objective of this study is to evaluate the changes in ETo and the driving factors behind the changes in ETo in peninsular Malaysia. Modified Mann-Kendall (MMK) test (Hamed, 2008) which can separate natural climate variability from global-warming-induced climate change (Shiru et al., 2019) was used to assess the secular trends in ETo and other meteorological variables. The innovation trend analysis (ITA) (Şen, 2012) was used to assess the variations in trends during different parts of the study period. Sobol's approach was employed to assess the sensitivity of ETo to different meteorological variables including maximum temperature (Tmx), mean temperature (Tav), minimum temperature (Tmn), SR, RH and WS. The influence of different meteorological factors on annual and seasonal ETo was also quantified. The sequential MK (SQ-MK) test was used to detect the starting year of change in ETo and the other meteorological variables. Obtained results were collectively analyzed to understand the driving factors and the timing of ETo changes in tropical peninsular Malaysia.
Section snippets
The climate and water resources in peninsular Malaysia
Peninsular Malaysia (Fig. 1), located in tropics has a warm and moist climate which is mainly driven by monsoons, undulating topography and intricate land-sea interfaces. The daily average temperature in the peninsula varies from 21 °C to 32 °C while the annual rainfall ranges between 2000 and 4000 mm (Tangang, 2001). The climate of the peninsula is dominated by two monsoons, northeast monsoon (NEM) (May–August) and southwest monsoon (SWM) (November–February). The climate during SWM is
Estimation of reference evapotranspiration
Observed pan ET data was not available in peninsular Malaysia for a longer period. Penman-Monteith (PM) method (Allen et al., 1998) was adopted in the present study for the estimation of ETo from meteorological variables and then validated for the period for which the pan ET data are available. Previous studies found a high correlation between monthly pan evaporation and the PM estimated ETo in peninsular Malaysia (Ali et al., 2000; Ali and Shui, 2008; Lee et al., 2004; Tukimat et al., 2012;
Characteristics of meteorological factors in peninsular Malaysia
The daily average series of different meteorological variables for all days over a year at all the stations are presented in Fig. 4 in order to show the seasonal variability of meteorological variables in different stations of the country.
The daily Tmx in the study area ranges between 31.1 °C in January and 33 °C in April, while Tav and Tmn vary from 26.6–28.1 °C and 23.3–24.4 °C respectively between January and May. The temperature of peninsular Malaysia is more or less uniform throughout the
Discussion
Changes in climate are found to affect ETo in the most part of the globe. However, the changes are found contradictory in different regions, even having a same climate. Compared to other climatic regions, assessment of ETo trends is very limited in tropical region. Knowledge on the changes in ETo and its driving factors for Southeast Asia is very limited. Therefore, it was not possible to compare the results obtained in this study with the existing literature. However, a large number of studies
Conclusion
The changes in reference evapotranspiration and the driving factors of its changes are evaluated for tropical peninsular Malaysia in this study. The ETo and six meteorological variables used to derive the ETo at eight locations of peninsular Malaysia was analyzed for this purpose. A modified version of MK test was used to assess the monotonic trend, the robust ITA was used to understand the trends in different periods, Sobol's method was employed to understand the influences of meteorological
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are grateful to the Universiti Teknologi Malaysia for providing financial support for this research through Post-Doctoral Fellowship Scheme of the Universiti Teknologi Malaysia, Grant Number Q.J130000.21A2.04E38.
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