Changes in reference evapotranspiration and its driving factors in peninsular Malaysia

Trends in reference evapotranspiration (ETo) have been found highly diverse in different regions of the globe due to the contradictory changes in the meteorological variables that define ETo. Despite a significant impact of ETo in water resources and ecology, knowledge on the changes and the cause of the changes in ETo is very limited in tropical regions. The trends in ETo, the factors influencing the changes in ETo and the change point (year) that made the trend significant were evaluated in this study for tropical peninsular Malaysia. The modified version of Mann-Kendall (MK) WA was used for the assessment of unidirectional changes in ETo and the driving meteorological variables. The innovative trend analysis (ITA) was conducted to understand the variations in change with time. Sobol's method was used to measure the sensitivity of ETo to different meteorological factors and the Sequential MK WA was employed to identify the change point. The study revealed an increase in annual (0.009-0.026 mm/year) and seasonal (0.014-0.027 mm/year during southwest monsoon and 0.015-0.074 during northeast monsoon) ETo in peninsular Malaysia which contradicts to evapotranspiration paradox found in many regions. The minimum temperature (31.5-48.2%) was found as the most influencing factor followed by wind speed (15.1-32.8%.) in defining ETo in peninsular Malaysia. Analysis of ITA and sequential MK WA results revealed that the rise in minimum temperature is the major cause of the increase in ETo in peninsular Malaysia. A faster rise in minimum temperature after 1981-1985 caused an increase in ETo after 1993-1996 in most of the locations. The minimum temperature in the region was noticed to rise much faster compared to the global average which indicates a large and continuous increase in ETo due to global warming and thus, reduction in atmospheric water balance in peninsular Malaysia.