Siti Aisah Shamsuddin, Maryam Jamilah Mohamed Amran


Reference evapotranspiration (ETₒ) was determined using the Penman-Monteith (P-M) combination equation established in the CROPWAT software v.8 (FAO 1992). The objective of the study is to determine the ETₒ at Bukit Tarek Experimental Watershed, Kerling, Selangor (BTEW), Matang Mangrove at Kuala Sepetang, Perak (MMP) and FRIM Mata Ayer Research Station, Perlis (MAP) using five-year periods of daily weather secondary data. Results showed that ETₒ of 4.10 mm/day at MMP was the highest compared to MAP (3.70 mm/day) and BTEW (3.58 mm/day). The geographical latitude influenced the ETₒ values. The ETo at BTEW was compared with the evapotranspiration (ET) calculated from water balance method which is also at the same location for data period from 1990 to 1994. Results showed that annual average ET was 1552 mm (56.1% of rainfall) while ETo was 43.9% of rainfall only. The annual averages of ET ranged from 3.92 to 4.69 mm day-1 with an average of 4.27 mm day-1. The annual average of ETo was underestimated ET from water balance method ranging from 16.6 to 26.2%. It is expected that the ETo found was underestimated as the actual evapotranspiration requires changes in canopy resistance as function of water availability


Tropical lowland forest; evapotranspiration; water balance; Penman-Monteith model

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