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Climate-induced Historical Drift of Reference Evapotranspiration in Mymensingh Region of Bangladesh

Khadiza Akhter Mousumi
Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh–2202, Bangladesh

Md. Abdul Mojid
Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh–2202, Bangladesh

Tanvir Ahmad
Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh–2202, Bangladesh

Md. Zamil Uddin
Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh–2202, Bangladesh

Md. Ferdous Parvez
Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh–2202, Bangladesh

Abstract/ Executive Summary:

Reference crop evapotranspiration (ETo) is essential for planning and management of irrigation to ensure optimum utilization of a region’s available water resources. ETo being an indicator of atmospheric evaporative demand provides a measure of the integrated effect of climatic parameters like solar radiation, wind, temperature and humidity. Variation of these climatic parameters over long period of time alters ETo. The modified ETo is crucial for periodic adjustment of irrigation planning and management. This study evaluated variation of ETo and contribution of the climatic parameters to ETo-variation in Mymensingh region of Bangladesh by analyzing climatic data of 28 years (1990–2017). ETo was determined by FAO Penman-Monteith method and trends of ETo and its governing climatic parameters were evaluated by MAKESENS trend model. The ETo-governing climatic parameters revealed contrasting trends, which also varied in different months of the year. Net radiation and wind speed showed decreasing trend, while temperature and saturation vapor pressure deficit showed increasing trend. In spite of contrasting contributions of the climatic parameters, their combined effect reduced ETo with a resulting decreasing trend of the monthly average daily ETo over the months of the year except July. These results enhance our understanding of the effects of climate change on ETo and can help correct-planning of water resources for irrigated agriculture.

Keywords: Climate change, Evaporative demand, Trend

Location: Bangladesh Agricultural University (BAU) Research Farm

Start Date: 00-00-1990

End Date: 00-00-2017

Program Area: Knowledge Management

Non-commodity: Weather/Climate

Objectives:

This study aimed (i) to detect and estimate trend of ETo over the years from 1990 to 2017 and (ii) to identify contribution of the climatic parameters in the variation of ETo for Mymensingh region.

Methodology:

Data collection Bangladesh has been divided into seven Hydrological Regions considering surface water flow processes and major rivers as boundaries. The north-central hydrological region comprises 11 administrative districts of which Dhaka, Mymensingh and Tangail have weather stations for recording climatic data. The weather station of Mymensingh (24°38′3″ north latitude and 90°16′4″ east longitude), called Agro-Meteorology cum Pilot Balloon Observatory Station, is located at Bangladesh Agricultural University (BAU) Research Farm. Weather data, recorded at this station, is preserved both at the Bangladesh Meteorological Department in Dhaka and BAU. This study analyzed climatic data of Mymensingh weather station. The daily climatic parameters: maximum and minimum air temperature, dew point temperature, maximum and minimum relative humidity, rainfall, wind speed, sunshine duration and solar radiation for a period of 28 years (1990–2017) were collected from BAU. Determination of reference crop evapotranspiration: With formula

Trend analysis of ETo and ETo-governing factors The monthly average of daily ETo and pertinent daily climatic parameters: net radiation, average temperature, saturation vapor pressure deficit and wind speed were determined for each of the study years (1990–2017). The trends of ETo and climatic parameters were detected and estimated by MAKESENS trend model. This model utilized Mann-Kendall test (Mann, 1945; Kendall, 1975), which is a non-parametric method developed for analyzing trend in time series. The MAKESENS is a software package developed in Microsoft Excel97 and the macros were coded with Microsoft Visual Basic (Salmi et al., 2002). The ETo and its governing climatic parameters were tested for the presence of any monotonic increasing or decreasing trend with the Mann-Kendall test and then slope of the linear trend, if present, was estimated with non-parametric method of Sen as explained by Gilbert (1987). Estimation of climatic parameters’ contribution to ETo Step-wise multiple linear regression analysis was done for ETo-governing climatic parameters to evaluate their relative contribution to ETo. This analysis was done following Draper and Smith (2014) and the significant impact-generating parameters were identified. In this technique, one additional climatic variable was added to the regression equation in the consecutive regression analysis. The coefficient of determination (r2) in each step of regression analysis revealed relative contribution of the climatic parameters (s) in generating ETo. The probability values (p-values) obtained in the regression analyses provided the significance level at p < 0.10, 0.05, 0.01 and 0.001; in this study, these probability values were categorized as p < 0.10: fairly significant, p < 0.05: significant, p < 0.01: highly significant and p < 0.001: very highly significant.

Source of Information: J Bangladesh Agril Univ 17(2): 258–264, 2019 ISSN 1810-3030 (Print) 2408-8684 (Online)

Article Link (Online Journal): https://doi.org/10.3329/jbau.v17i2.41991

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Net radiation and wind speed decreased but air temperature and saturation vapor pressure deficit increased, all at different rates, in different months of the year during 1990 to 2017 in Mymensingh region of Bangladesh. The net radiation and wind speed played the most dominant role in the variation of reference crop evapotranspiration, ETo, over air temperature and saturation vapor pressure deficit. Consequently, the pooled effect of the climatic parameters provided a declining trend of the monthly average daily ETo in different months of the year except July, when air temperature and saturation vapor pressure deficit exerted the most dominant role over net radiation and wind speed in ETo-variation. The climatic parameters differed over the months of the year, and hence any fixed set of parameters did not exert similar impact on ETo in every month of the year. If the current climatic trend continues, it is anticipated that ETo would continue decreasing in the future in spite of the much expected increased temperature in future, as predicted by most climatic models; this anticipation is due to the lessdominant role of temperature in ETo. The decreasing trend of ETo indicates reduced crop-water demand since it is a direct function of ETo. The trend of climatic parameters, observed during 1990–2017, if continues in the future, would therefore reduce irrigation requirement in the study area. So, the results of this study need to be considered in planning irrigation development and management based on available water resources.

Knowledge Management: Journal