Proceedings on International Conference on Disaster Risk Management, Dhaka, Bangladesh, January 12-14, 2019 Page | 319 CLIMATE CHANGE EFFECT ON POTENTIAL EVAPOTRANSPIRATION IN BANGLADESH Jannatun Nahar Jerin 1 and A.R.M. Towfiqul Islam 1 ABSTRACT Potential evapotranspiration (ETo) is considered one of the key factors under changing climate which plays a pivotal role in irrigation schedule and water resource management. However, how much ETo affected by climate change has been less understood in Bangladesh. This study aimed to estimate monthly, annual and decadal ETo using the FAO-56 Penman-Monteith (FAO-56 PM) model based on daily data of 25 meteorological stations across Bangladesh from 1975-2017. To detect ETo trends, the Mann-Kendall (MK) technique was employed to compare trends with standard. The Sen’s slope estimator was adopted to calculate the magnitude of trend lines. The results show that approximately 82.67% of the monthly ETo time series had declined trends, out of which 39.67%, 16.33% and 31% of monthly ETo time series were shown statistically significant at 0.10, 0.01 and 0.05 levels, respectively. Only 17.33% of the monthly ETo time series had demonstrated a significant increasing trend. The dominant significant increased trend (p<0.01 level) was found in February at the Chittagong station, while the dominant significant decreased trend ( p<0.01) was noticed at Mymensing station. Based on an annual and decadal timescale, over 92% and 73% of the stations had the decreased trend lines; but the ascending trends were observed at Rangamati, Barisal and Bhola stations, respectively. As a whole, ETo are moving on a decline tendency pattern for almost every station over Bangladesh; suggesting a paradoxical shifting under background of the global warming. Keywords: ETo trend, Water use, Bangladesh, Global warming, Penman-Monteith model Introduction Potential evapotranspiration study is crucial for irrigation and water resources development and management in Bangladesh. A better assessment of potential evapotranspiration is important for efficient irrigation management, crop production, environmental assessment, ecosystem modelers and solar energy system. According to the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC), the global warming trend, which is mainly caused by the increasing amount of greenhouse gas emissions, will increase continue in future 2100 (IPCC 2014). Bangladesh is already experiencing the effects of climate variability and climate change (Ali 1996; Mirza 2002; Karim and Mimura 2008; Climate Change Cell 2008,). Different sectors of natural system such as agriculture, hydrology, and ecosystem have been affected by climate change. Potential evapotranspiration is expected to increase due to temperature increases. However, the decrease of observed ETo has been widely detected over recent decades in many areas (Yin et al. 2010;Limjirakan and Limsakul2012; Darshana et al. 2013). This phenomenon of a discrepancy between expected and observed trends in evaporation is known as the ‘‘evaporation paradox’ ’Evapotranspiration (ET), one of the primary elements of the hydrologic cycle that is influenced by climate change. There are a number of climatic parameters that have been identified to change in ETo. The variation of ETo not only influence by the increased air temperature, but also other primary climatic factors such as relative humidity, vapor pressure, wind speed and solar radiation. The changing of ETo at particular station may be influenced by these meteorological parameters. Although different aspects of ETo in different parts of Bangladesh have been investigated to some extent; however, no detailed study has yet been conducted on impact of ETo trend in Bangladesh. This region is known as an agricultural region. Rain-fed cereal production is done most part of the Bangladesh. Unfortunately, some area has suffered from prolonged droughts during summer season in recent years. Moreover, over exploitation of groundwater has depleted underground water reservoirs. Drastic shrinkage of major rivers has created serious problems in its ecosystem and hydrologic balance. In order to meet these challenges, it seems that a study to detect trends in ETo in Bangladesh is needed. The objective of this study was to detect monthly, annual and decadal trends of ETo using the FAO-56 Penman-Monteith (FAO-56 PM) model based on daily 25 meteorological stations data in Bangladesh in the past 4 decades and estimate their magnitudes from 1975-2017 using non-parametric methods, such as MK and Sen’s estimator. 1 Department of Disaster Management, Begum Rokeya University, Rangpur 5400, Bangladesh Author:[email protected], * Corresponding author: [email protected]
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Proceedings on International Conference on Disaster Risk Management,
Dhaka, Bangladesh, January 12-14, 2019
Page | 319
CLIMATE CHANGE EFFECT ON POTENTIAL EVAPOTRANSPIRATION IN
BANGLADESH
Jannatun Nahar Jerin1and A.R.M. Towfiqul Islam1
ABSTRACT
Potential evapotranspiration (ETo) is considered one of the key factors under changing climate which plays a
pivotal role in irrigation schedule and water resource management. However, how much ETo affected by
climate change has been less understood in Bangladesh. This study aimed to estimate monthly, annual and
decadal ETo using the FAO-56 Penman-Monteith (FAO-56 PM) model based on daily data of 25
meteorological stations across Bangladesh from 1975-2017. To detect ETo trends, the Mann-Kendall (MK)
technique was employed to compare trends with standard. The Sen’s slope estimator was adopted to calculate
the magnitude of trend lines. The results show that approximately 82.67% of the monthly ETo time series had
declined trends, out of which 39.67%, 16.33% and 31% of monthly ETo time series were shown statistically
significant at 0.10, 0.01 and 0.05 levels, respectively. Only 17.33% of the monthly ETo time series had
demonstrated a significant increasing trend. The dominant significant increased trend (p<0.01 level) was
found in February at the Chittagong station, while the dominant significant decreased trend (p<0.01) was
noticed at Mymensing station. Based on an annual and decadal timescale, over 92% and 73% of the stations
had the decreased trend lines; but the ascending trends were observed at Rangamati, Barisal and Bhola
stations, respectively. As a whole, ETo are moving on a decline tendency pattern for almost every station over
Bangladesh; suggesting a paradoxical shifting under background of the global warming.
Keywords: ETo trend, Water use, Bangladesh, Global warming, Penman-Monteith model
Introduction
Potential evapotranspiration study is crucial for irrigation and water resources development and management
in Bangladesh. A better assessment of potential evapotranspiration is important for efficient irrigation
management, crop production, environmental assessment, ecosystem modelers and solar energy system.
According to the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC), the global
warming trend, which is mainly caused by the increasing amount of greenhouse gas emissions, will increase
continue in future 2100 (IPCC 2014). Bangladesh is already experiencing the effects of climate variability
and climate change (Ali 1996; Mirza 2002; Karim and Mimura 2008; Climate Change Cell 2008,). Different
sectors of natural system such as agriculture, hydrology, and ecosystem have been affected by climate change.
Potential evapotranspiration is expected to increase due to temperature increases. However, the decrease of
observed ETo has been widely detected over recent decades in many areas (Yin et al. 2010;Limjirakan and
Limsakul2012; Darshana et al. 2013). This phenomenon of a discrepancy between expected and observed
trends in evaporation is known as the ‘‘evaporation paradox’ ’Evapotranspiration (ET), one of the primary
elements of the hydrologic cycle that is influenced by climate change. There are a number of climatic
parameters that have been identified to change in ETo. The variation of ETo not only influence by the increased
air temperature, but also other primary climatic factors such as relative humidity, vapor pressure, wind speed
and solar radiation. The changing of ETo at particular station may be influenced by these meteorological
parameters. Although different aspects of ETo in different parts of Bangladesh have been investigated to
some extent; however, no detailed study has yet been conducted on impact of ETo trend in Bangladesh. This
region is known as an agricultural region. Rain-fed cereal production is done most part of the Bangladesh.
Unfortunately, some area has suffered from prolonged droughts during summer season in recent years.
Moreover, over exploitation of groundwater has depleted underground water reservoirs. Drastic shrinkage of
major rivers has created serious problems in its ecosystem and hydrologic balance. In order to meet these
challenges, it seems that a study to detect trends in ETo in Bangladesh is needed.
The objective of this study was to detect monthly, annual and decadal trends of ETo using the FAO-56
Penman-Monteith (FAO-56 PM) model based on daily 25 meteorological stations data in Bangladesh in the
past 4 decades and estimate their magnitudes from 1975-2017 using non-parametric methods, such as MK
and Sen’s estimator.
1 Department of Disaster Management, Begum Rokeya University, Rangpur 5400, Bangladesh