Ahmad et al. / Malaysian Journal of Fundamental and Applied Sciences Special Issue on Some Advances in Industrial and Applied Mathematics (2017) 400-404 400 Rainfall, evapotranspiration and rainfall deficit trend in Alor Setar, Malaysia Aimi Athirah Ahmad a, b , Fadhilah Yusof b,* , Muhamad Radzali Mispan c , Hasliana Kamaruddin c a Economic and Social Science Research Centre, Malaysian Agriculture Research and Development Institute, 43400 Serdang Selangor, Malaysia b Department of Mathematical Sciences, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia c Agrobiodiversity and Environment Research Centre, Malaysian Agriculture Research and Development Institute, 43400 Serdang Selangor, Malaysia * Corresponding author: fadhilahy@utm.my Article history Received 16 October 2017 Accepted 8 November 2017 Abstract Rainfall and potential evapotranspiration are important variables in water balance study. Rainfall data were obtained from Malaysian Meteorological Department while estimates of potential evapotranspiration were calculated using Penman-Monteith method. Trend analysis of monthly and annual rainfall, potential evapotranspiration and rainfall deficit are essential to manage irrigation system in agricultural systems. This is because changes in trend of these parameters may affect the water cycle and ecosystem. Annual and monthly values of these variables were analysed from 1980- 2009. Results indicated increasing trends of 16.2mm yr -1 and 3.01 mm yr -1 for both annual rainfall and potential evapotranspiration, respectively. Consequently, these trends resulted in annual rainfall deficit of 1.69mm per year. Keywords: Rainfall, potential evapotranspiration,rainfall deficit, Penman-Monteith © 2017Penerbit UTM Press. All rights reserved INTRODUCTION Rainfall is an important source of water for agricultural production. Rainfall is characterized by its amount, intensity and distribution within a period of time. Rainfall is often expressed in millimeters per day (mm/day). Informations on the characterization of monthly or annual rainfall pattern in most parts of the country are generally available. From the agricultural perspective, rainfall is an important component because all plants need water to survive. While a regular rainfall pattern is essential to healthy crop to optimize its production, too much or too little rainfall can be harmful. Limitations in water availability are frequently a restrictive factor in plant development. Water is essential for the maintenance of physiological and chemical processes within the plant, acting as an energy exchanger and carrier of nutrient food supply in solution (Schulze et al., 1997). As a component of rainfall, evapotranspiration is also a major climate variable affecting agricultural production and it is an important criteria of water management. The rate of potential evapotranspiration ( ! ) is the amount of water that might be potentially lost due to evaporation over a vegetation surface. Evapotranspiration is correlated to solar radiation, air temperature, humidity and wind speed. Estimates of potential evapotranspiration was performed using Penman-Monteith (FAO-56 method) (Allen et al., 1998). Studies in rainfall,  ! and rainfall deficit () trends at different time interval and region are very important for agriculture water balance, irrigation scheduling and cropping system management (Gary et al., 2016). Great concern has arrived in the past few decades on analyses of rainfall, precipitation and rainfall deficit trend because of the attention given to climate change from scientific community (Antonia and Paolo, 2009). Many studies have been conducted to address spatial and temporal trends both globally and locally. For example, Mohtar et al., (2014) studied recent changes in extremes of monthly mean rainfall distribution in the state of Perlis and Johore of Peninsular Malaysia over the period of 1970 to 1972 and 2010 to 2012, and made comparison between northern and southern regions, respectively. Their results showed that northern regions received heavier rainfall in 2010 to 2012 as compared to during 1970 to 1972. In another study, Syafarina et al., (2015) analyzed and compared hourly trends of rainfall during northeast and southwest monsoons of peninsula Malaysia between the years of 1975 to 2010. Their results showed that in general, the hourly extreme rainfall events in peninsular Malaysia showed an increasing trends in short temporal rainfall during inter-monsoon season. The rainfall,  ! and  has high spatial variability and therefore it is essential to conduct in local temporal characteristics, patterns and trends. In this study, data from years 1980 to 2009 at the Alor Setar station were used to analyze rainfall,  ! and  trends. Alor Setar is the state capital of Kedah and encompassed an area of 424 km 2 . Kedah is also known as rice bowl state of Malaysia because of the large paddy granary areas. As most of other parts of Malaysia, Alor Setar features a tropical monsoon climate under the Köppen climate classification. Alor Setar has a very long wet season especially during August to September. During these period the rainfall received were normally 5 to 10mm/day higher than other months (Tukimat and Harun, 2011). Similar to several other regions with this particular climate, precipitations are commonly seen even during the short dry season. Temperatures are relatively consistent throughout the year, with average high and low temperatures around RESEARCH ARTICLE